# Seminars (CFM)

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Event When Speaker Title Presentation Material
CFM 9th May 2013
14:00 to 15:00
Evolution equations for films of mixtures and suspensions - the case of dewetting mixtures
CFM 9th May 2013
15:00 to 16:00
M Pushkin Stirring in active media
CFM 14th May 2013
11:00 to 12:00
Transport of charged macromolecules under applied electric fields
CFM 14th May 2013
12:00 to 13:00
M Fontelos Contact line instabilities in electrowetting
CFM 16th May 2013
14:00 to 15:00
N Yoshinaga Spontaneous motion and deformation of a droplet
CFM 16th May 2013
15:00 to 16:00
T Ratiu What does geometric mechanics have to say about the dynamics of complex fluids?
CFM 21st May 2013
11:00 to 12:00
S Wilson A rigid or elastic plate floating on the free surface of a viscous film: a coupled free boundary and fluid-structure interaction problem
CFM 21st May 2013
12:00 to 13:00
D Holm What does the dynamics of complex fluids have to say about geometric mechanics?
Complex fluid equations are rewritten equivalently in a form that connects with soliton equations.
CFM 23rd May 2013
14:00 to 15:00
K Kruse Hydrodynamic description of the actin cortex
CFM 23rd May 2013
15:00 to 16:00
Motor action in semiflexible networks
CFM 28th May 2013
11:00 to 11:30
R Borcia Phase field models for two-fluid systems
CFM 28th May 2013
11:30 to 12:00
Lubrication approaches for binary mixtures
CFM 28th May 2013
12:00 to 13:00
A Alexeev Modeling swelling kinetics and transport in hydrogels using mesoscale simulations
CFM 30th May 2013
14:00 to 15:00
B Wagner On effective slip for an upper convected Maxwell fluid
CFM 30th May 2013
15:00 to 16:00
Structure formation in thin films of polymer mixtures
CFM 4th June 2013
11:00 to 12:00
On diffuse-interface models for two-phase flow with different mass densities: thermodynamic consistency, simulations, and numerical analysis
CFM 4th June 2013
12:00 to 13:00
Crawling motility
CFM 6th June 2013
14:00 to 15:00
E Virga Aggregation and mean-field for ferrofluid monolayers
CFM 6th June 2013
15:00 to 16:00
N Uchida Orientational order, synchronization and defect turbulence in arrays of active microfluidic rotors
CFM 10th June 2013
17:00 to 18:00
Nonlocal evolution equations (Rothschild Distinguished Visiting Fellow lecture)
I will describe results concerning evolution equations involving nonlocal terms. Particular examples will include the Surface Quasi-Geostrophic equation (SQG) and its generalizations. I will discuss a nonlinear maximum principle for linear nonlocal operators and applications to questions of regularity of solutions, long time dynamics and absence of anomalous dissipation in 2D SQG.
CFM 11th June 2013
11:00 to 12:00
Hydrodynamic vorticity and helicity of conservative liquid crystal flows
CFM 11th June 2013
12:00 to 13:00
Continuum Models of Two-Phase Flow in Porous Media
I discuss two models of two-phase fluid flow in which undercompressive shock waves have been discovered recently. In the first part of the talk, the focus is on two-phase flow in porous media. Plane waves are modeled by the one-dimensional Buckley-Leverett equation, a scalar conservation law. The Gray-Hassanizadeh model for rate-dependent capillary pressure adds dissipation and a BBM-type dispersion, giving rise to undercompressive waves. Two-phase flow in porous media is notoriously subject to fingering instabilities, related to the classic Saffman-Taylor instability. However, a two dimensional linear stability analysis of sharp planar interfaces reveals a criterion predicting that weak Lax shocks may be stable or unstable to long-wave two-dimensional perturbations. This surprising result is related to the hyperbolic-elliptic nature of the system of linearized equations. Numerical simulations of the full nonlinear system of equations, including dissipation and dispersion, verify the stability predictions at the hyperbolic level. In the second part of the talk, I describe a phase field model of a resident fluid being displaced by injected air in a thin tube (a microscopic pore). PDE simulations reveal the appearance of a rarefaction wave together with a faster undercompressive wave that terminates at the spherical cap tip of the injected air. Preliminary analysis and ODE simulations help to explain this structure.
CFM 13th June 2013
14:00 to 15:00
Problems and surprises in geometric mechanics
I will review a number of problems in mechanics where the geometric approach has been fundamental in arriving at a deeper understanding of the problem. Depending on the interest of the participants, topics may include vortex dynamics, long-term numerical integration of mechanics, and mechanical problems in imaging.
CFM 17th June 2013
14:00 to 15:00
Discussion of motion of defects in nematics
CFM 18th June 2013
11:00 to 12:00
Dynamics and rectification of active Brownian particles
CFM 18th June 2013
12:00 to 13:00
Thin-film flows with mass transfer
CFM 18th June 2013
14:00 to 16:00
K Kruse An introduction into the hydrodynamics of active polar gels - part 1
CFM 19th June 2013
11:00 to 12:00
Do particle methods for fluids make sense?
CFM 20th June 2013
11:00 to 13:00
K Kruse An introduction into the hydrodynamics of active polar gels - part 2
CFM 20th June 2013
14:00 to 15:00
R Craster Surfactant driven thin-film flows
CFM 20th June 2013
15:00 to 16:00
Solvent free computer simulation of nanoparticle-cell membrane interactions
CFMW01 24th June 2013
09:45 to 10:30
R Goldstein Spontaneous Circulation of Confined Active Suspensions
Many active fluid systems encountered in biology are set in total geometric confinement. Cytoplasmic streaming in plant cells is a prominent and ubiquitous example, in which cargo-carrying molecular motors move along polymer filaments and generate coherent cell-scale flow. In this talk I will summarize theoretical and experimental work in my group that addresses the possibility that the ordered patterns of streaming seen in nature can arise from a process of self-organization.
CFMW01 24th June 2013
11:00 to 11:45
Active motion: under external fields and collective dynamics
Active motion of microorganisms or artificial microswimmers, such as active colloids, is an appealing subject which has attracted much attention recently. Since these swimmers move constantly in non-equilibrium, they give rise to novel phenomena which, in particular, occur when external fields are applied or when they move collectively.

The talk reviews our recent work on how active Brownian particles behave in external fields and in confinement. For example, they develop orientational order in a gravitational field [1] and exhibit an interesting instability in dense suspensions when they are bottom-heavy. They also create active fluid pumps in a harmonic trap. Finally, under Poiseuille flow they show nonlinear dynamics reminiscent of the nonlinear pendulum where the bounding walls introduce "dissipation" [2].

We also study the collective motion of so-called squirmers in a quasi 2D geometry by means of multi-particle collision dynamics. We observe dynamical clustering, phase separation, and active jamming which is strongly influenced by hydrodynamic near-field interactions. In dense suspensions rotational diffusion is greatly enhanced and the critical volume fraction for crystallisation is different for pushers and pullers.

[1] M. Enculescu and H. Stark, Phys. Rev. Lett. 106, 208103 (2011). [2] A. Zoettl and H. Stark, Phys. Rev. Lett. 108, 218104 (2012).

Co-authors: Marc Hennes (Institute of Theoretical Physics, Technische Universität Berlin), Katrin Wolff (Institute of Theoretical Physics, Technische Universität Berlin), Andreas Zoettl (Institute of Theoretical Physics, Technische Universität Berlin)

CFMW01 24th June 2013
11:45 to 12:30
E Keaveny Optimisation of chiral structures for micro-scale propulsion
In micron-scale hydrodynamics, shape and geometry play a strong role in determining the speed at which a body can move through fluid. This shape dependence is particularly important to the design of many microfluidic devices, including magnetically actuated micro-structures fabricated and studied for biomedical applications. In this talk, I will discuss several important experimentally-realisable micro-structures whose shapes couple their rotations and translations. I will address the optimal design of these devices through an infinite-dimensional optimisation problem, obtaining geometries that maximise speed for a given applied torque. Our optimisations show that attached payloads have a significant effect on optimal micro-structure shapes and current designs can be improved by upwards of 450%.
CFMW01 24th June 2013
14:00 to 14:30
M Koepf A continuum model of epithelial spreading
We present a continuum model of unconstrained epithelial spreading. The tissue is described as a polarizable and chemo-mechanically interacting layer with neo-Hookean elasticity. Our model reproduces the spontaneous formation of finger-like protrusions commonly observed in experiment. Statistics of velocity orientation obtained from numerical simulation show strong alignment in the fingers opposed to an isotropic distribution in the bulk, as has been measured by Reffay et al. (Reffay et al., Biophysical Journal, 2011). The results faithfully reproduce faster relative advance of cells close to the leading edge of the tissue, as well as spatial velocity correlations and stress accumulation within the tissue, which proceeds in form of a "mechanical wave", traveling from the wound edge inwards (cf. Serra-Picamal et al., Nature Physics, 2012).

M H Koepf, L M Pismen: A continuum model of epithelial spreading (2013) submitted

M H Koepf, L M Pismen: Non-equilibrium patterns in polarizable active layers (2013) submitted

Co-author: Leonid M. Pismen (Department of Chemical Engineering, Technion - Israel Institute of Technology, 32000 Haifa, Israel)

CFMW01 24th June 2013
14:30 to 15:00
Traveling and resting crystals in crowds of self-propelled particles
When the density within a crowd of self-propelled particles is high enough and when the interactions between these particles are strong enough, then it is plausible to expect that crystallization will occur. We are interested in the formation and in the behavior of such active crystals that are composed of self-propelled particles. To study this kind of materials using a field approach, we combine the classical phase field crystal model by Elder and Grant with the Toner-Tu theory for active media. In this way we obtain an active phase field crystal model. Our approach can further be justified from dynamic density functional theory. The active crystals that we identify can be classified into two groups: either the crystal is resting, meaning that no net density flux is observed, or it is traveling, meaning that the lattice peaks collectively migrate into one direction. As a central result we find that a transition from a resting to a traveling crystal can occur at a threshold value of the active drive. Consequently a variety of different crystalline phases can be identified: resting hexagonal, traveling hexagonal, swinging hexagonal, traveling rhombic, traveling quadratic, resting lamellar, traveling lamellar, resting honeycomb, and traveling honeycomb. Upon quenching from the fluid phase, the traveling crystals emerge through a coarse-graining process from domains of different directions of collective motion. Qualitatively we also studied the impact of additional hydrodynamic interactions between the lattice peaks. Since the properties and response of active crystals can be very different from their equilibrium counterparts, the knowledge of, classification of, and control of the different crystalline states can provide a starting point for the design of new active materials.

Co-author: Hartmut Lowen (Heinrich Heine University Dusseldorf, Germany)

CFMW01 24th June 2013
15:00 to 15:30
S Banerjee Collective mechanics of epithelial cell colonies on elastic substrates
Crosstalk between cell-cell and cell-matrix adhesions plays an essential role in the mechanical function of tissues. The traction stresses exerted by cohesive keratinocyte colonies with strong cell-cell adhesions are mostly concentrated at the colony periphery. In contrast, for weak cadherin-based intercellular adhesions, individual cells in a colony interact with their matrix independently, with disorganized distribution of traction stresses extending throughout the colony. In this talk I will present a minimal physical model of the colony as adherent contractile elastic media coupled to an elastic substrate. The model captures the spatial distribution of traction forces seen in experiments. For cell colonies with strong cell-cell adhesions, the total traction force of the colony measured in experiments is found to scale with the colony’s geometrical size. This scaling suggests the emergence of an effective surface tension of magnitude comparable to that measured fo r non-adherent, three-dimensional cell aggregates. The physical model supports the scaling and indicates that the surface tension may be controlled by acto-myosin contractility.

Co-authors: Aaron F. Mertz (Yale University), M. Cristina Marchetti (Syracuse University), Eric R. Dufresne (Yale University), Valerie Horsley (Yale University)

CFMW01 24th June 2013
16:00 to 17:00
Rigidity, Zero Modes, States of Self Stress, and Surface Phonons in Periodic and Diluted Periodic Networks near their Instability Limit
Frames consisting of nodes connected pairwise by rigid rods or central-force springs, possibly with preferred relative angles controlled by bending forces, are useful models for systems as diverse as architectural structures, crystalline and amorphous solids, sphere packings and granular matter, networks of semi-flexible polymers, and protein structure. The rigidity of these networks depends on the average coordination number z of the nodes: If z is small enough, the frames have internal zero-frequency modes, and they are "floppy"; if z is large enough, they have no internal zero modes and they are rigid. The critical point separating these two regimes occurs at a rigidity threshold, which corresponds closely to what is often referred to as the isostatic point, that for central forces in d-dimensions occurs at coordination number zc = 2d. At and near the rigidity threshold, elastic frames exhibit unique and interesting properties, including extreme sensitivity to boundary conditions, power-law scaling of elastic moduli with (z- zc), and diverging length and time scales. This talk will explore elastic and mechanical properties and mode structures of model periodic and diluted periodic lattices, such as the square and kagome lattices with central-force springs, that are just on verge of mechanical instability, and 4-coordinated lattices in two and three dimensions that are stabilized by bending forces. It will discuss the origin and nature of zero modes of these structures under both periodic (PBC) and free boundary conditions (FBC), and it will derive general conditions under which (a) the zero modes under the two boundary conditions are essentially identical and (b) under which zero modes do not appear in the periodic spectrum but do appear as surface Rayleigh waves in the free spectrum. In the former situation, lattices are generally in a type of critical state that admits states of self-stress in which there can be tension in bars with zero force on any node, and distortions away from that state give rise to surface modes under free boundary conditions whose degree of penetration into the bulk diverges at the critical state. This general phenomenon also occurs in sub-isostatic lattices like the honeycomb lattice. The talk will also explore diluted 4-coordinated lattices as models for networks of semi-flexible polymers, discuss the special properties that result when constituent polymers adopt strictly straight configurations.
CFMW01 25th June 2013
09:00 to 09:45
Models of low-Reynolds-number swimmers and colloidal particles in confined domains
The talk will survey some simple mathematical models to gain insights into the dynamics of particles or swimmers of various kinds moving at zero Reynolds numbers in geometrically complex domains bounded by no-slip walls and/or free surfaces.
CFMW01 25th June 2013
09:45 to 10:30
Locomotion of helical bodies in viscoelastic fluids
Many microorganisms swim by rotating one or many helical flagella, often propelling themselves through fluids that exhibit both viscous and elastic qualities in response to deformations. In an effort to better understand the complex interaction between the fluid and body in such systems, we have studied numerically the force-free swimming of a rotating helix in a viscoelastic (Oldroyd-B) fluid. The introduction of viscoelasticity can either enhance or retard the swimming speed depending on the body geometry and the properties of the fluid (through a dimensionless Deborah number). The numerical results show how small-amplitude theoretical calculations connect smoothly to large-amplitude experimental measurements.

Co-authors: Bin Liu (Brown University), Thomas R. Powers (Brown University)

CFMW01 25th June 2013
11:00 to 11:45
Concentration fluctuations in a bacterial suspension
Recent analyses and simulations have identified an instability of a quiescent bacterial suspension above a threshold concentration, (nL3)crit = (5/C)(L/U\tau), where n is the bacterium number density, L and U the bacterium length and swimming speed, t the mean interval between tumbles, and C a measure of the intrinsic force-dipole. This instability is thought to underlie the large-scale coherent motions observed in experiments. There, however, remains a discrepancy between theory and simulations. While the former predicts a spatially homogeneous instability with coupled orientation and velocity fluctuations, simulations have observed large-scale concentration fluctuations. Even in the stable regime, solutions of the linearized equations reveal significant concentration fluctuations.

We will formulate an analytical solution that illustrates the linearized evolution of the velocity, orientation and concentration fields in a bacterial suspension starting from an arbitrary initial condition. The analysis relies on a remarkable correspondence between orientation fluctuations in a bacterial suspension and vorticity fluctuations in an inviscid fluid. The governing operators in both cases possess singular continuous spectra in addition to discrete modes. The dynamics of the singular orientation modes leads to transient growth of concentration fluctuations in the manner that the singular vorticity modes lead to kinetic energy growth in high-Reynolds-number shearing flows. We will discuss the velocity, orientation and stress correlations, emerging from an uncorrelated Poisson field, both below and above the critical concentration.

We also analyze the role of tumbling as a source of fluctuations. Regarding a tumble as a ‘linear collision’ governed by Poisson statistics allows one to write down the orientation-space noise, and this in turn leads to the analog of the fluctuating hydrodynamic equations for a bacterial suspension.

Co-author: Donald Koch (Chemical and bio-molecular engineering, Cornell University, NY, USA.)

CFMW01 25th June 2013
11:45 to 12:30
Hydrodynamic coordination of bacterial motions: from bundles to biomixing
Many bacteria propel themselves though their fluid environment by means of multiple rotating flagella that self-assemble to form bundles. At a larger scale, the fluid motion generated by an individual microbe as it swims affects the motions of its neighbors. Experimental observations indicate the presence of long-range order and enhanced transport in suspensions of bacteria -- these phenomena may be important in many aspects of bacterial dynamics including chemotaxis and development of biofilms. This talk focuses on the role of fluid dynamics in the bundling of flagella and the interactions between swimming organisms.

We first describe theory and simulations of hydrodynamically interacting microorganisms, using very simple models of the individual organisms. In the dilute limit, simple arguments reveal the dependence of swimmer and tracer velocities and diffusivities on concentration. As concentration increases, we show that cases exist in which the swimming motion generates large-scale flows and dramatically enhanced transport in the fluid. A physical argument supported by a mean field theory sheds light on the origin of these effects.

The second part of the talk focuses on the dynamics of the flagellar bundling process, using a mathematical model that incorporates the fluid motion generated by each flagellum as well as the finite flexibility of the flagella. The initial stage of bundling is driven purely by hydrodynamics, while the final state of the bundle is determined by a nontrivial and delicate balance between hydrodynamics and elasticity. As the flexibility of the flagella increases a regime is found where, depending on initial conditions, one finds bundles that are either tight, with the flagella in mechanical contact, or loose, with the flagella intertwined but not touching. That is, multiple coexisting states of bundling are found. The parameter regime at which this multiplicity occurs is comparable to the parameters for a number of bacteria.

CFMW01 25th June 2013
14:00 to 14:45
J Yeomans Active Nematics
Active systems, such as the cytoskeleton and bacterial suspensions, provide their own energy and hence operate out of thermodynamic equilibrium. Continuum models describing active systems are closely related to those describing liquid crystal hydrodynamics, together with an additional ‘active’ stress term. We discuss how the behaviour of the active continuum models depends on model parameters, such as the strength of the activity and the liquid crystal tumbling parameter, and we compare our results to recent experiments on cytoskeletal gels.
CFMW01 25th June 2013
14:45 to 15:30
Hierarchical active matter: from extensible bundles to active gels, streaming liquid crystals and motile emulsions
The emerging field of active matter promises an entirely new category of materials, with highly sought after properties such as autonomous motility and internally generated flows. In this vein, I will describe recent experiments that have focused on reconstituting dynamical structures from purified biochemical components. In particular I will describe recent advances that include: (1) assembly of a minimal model of synthetic cilia capable of generating periodic beating patterns, and conditions under which they exhibit metachronal traveling waves, (2) study of 2D active nematic liquid crystals whose streaming flows are determined by internal fractures and self-healing as well as spontaneous unbinding and recombination of oppositely charged disclination defects, (3) reconstitution of active gels characterized by highly tunable and controllable spontaneous internal flows, and (4) assembly of active emulsions in which aqueous droplets spontaneously crawl when in contact with a ha rd wall.
CFMW01 25th June 2013
16:00 to 16:45
J Toner Rice, Locusts and Chemical Waves: A Hydrodynamic Theory of Polar Active Smectics
We present a hydrodynamic theory of polar active smectics, by which we mean active striped systemsactive systems, both with and without number conservation. For the latter, we find quasi long-ranged smectic order in $d=2$ and long-ranged smectic order in $d=3$. In $d=2$ there is a Kosterlitz-Thouless type phase transition from the smectic phase to the ordered fluid phase driven by increasing the noise strength. For the number conserving case, we find that giant number fluctuations are greatly suppressed by the smectic order; that smectic order is long-ranged in $d=3$; and that nonlinear effects become important in $d=2$.

Co-author: Leiming Chen (The China University of Mining and Technology)

CFMW01 26th June 2013
09:00 to 09:45
The Five S's: Chemical Swimming, Sailing, Surfing, Squirming and Swarming
The design of nanoengines that can convert stored chemical energy into motion is an important challenge of nanotechnology, especially for engines that can operate autonomously. Recent experiments have demonstrated that it is possible to power the motion of nanoscale and microscale objects by using surface catalytic reactions -- so-called catalytic nanomotors. The precise mechanism(s) responsible for this motion is(are) still debated, although a number of ideas have been put forth. Here, a very simple mechanism is discussed: A surface chemical reaction creates local concentration gradients of the reactant (the fuel) and product species. As these species diffuse in an attempt to re-establish equilibrium, they entrain the motor causing it to move. This process can be viewed either as osmotic propulsion or as self-diffusiophoresis. The simplest way to break symmetry and achieve motion is by an asymmetric reactivity on the motor surface. The mathematical description of suc h motion is analogous to that used to describe the swimming of microorganisms, hence the name 'chemical swimming.' However, symmetry can also be broken by the motor's shape and, even for uniform reactivity, propulsion can be achieved -- 'chemical sailing.' A motor particle at an air-water interface can change the local concentration of surface-active agents and propel itself -- 'chemical surfing.' And even local variations of hydrodynamic mobility and interactive potential between the motor and the fuel can lead to net motion, a form of 'chemical squirming.' The implications of these mechanisms on the attainable propulsive speeds as a function of reaction rate and fuel concentration will be discussed and compared with Brownian dynamics simulations. It will also be shown that chemically active particles can attract or repel each other through long-range 'Coulomb-like' interactions. And suspensions of active particles can exhibit Debye-like screening leading to 'chemical swarming.'
CFMW01 26th June 2013
09:45 to 10:30
R Lipowsky Remodelling of membrane compartments
Biomembranes undergo continuous remodelling by budding and fission processes, which create new membrane compartments in the form of vesicles, as well as by adhesion and fusion, which combine two such compartments into a single one. The vesicles are transported by teams of molecular motors from donor membranes, where they are created, to acceptor membranes,into which they are incorporated by membrane fusion. In this talk, I will address the energetics and kinetics of these remodelling processes as well as their relevance for intracellular transport.
CFMW01 26th June 2013
11:00 to 11:45
Physarum Polycephalum Percolation as a Paradigm for Topological Phase Transitions in Transportation Networks
We study the formation of transportation networks of the true slime mold Physarum polycephalum after fragmentation by shear. Small fragments, called microplasmodia, fuse to form macroplasmodia in a percolation transition. At this topological phase transition, one single giant component forms, connecting most of the previously isolated microplasmodia. Employing the configuration model of graph theory for small link degree, we have found analytically an exact solution for the phase transition.

The universality of percolation may be used as a general gauge in the analysis of transportation networks. Some malignant tissues derive their blood vessels not by angiogenesis, i.e., remodeling of existing vessels, but rather by denovo vascularization like embryos. Since topologically, percolation is independent from detailed mechanisms and even space dimensions, i.e., 2D versus 3D growth, it may serve as a reference point in space and time when comparing the dynamics of network formation in tumors of varying size and shape. Since restricting blood supply via hindering vessel percolation is paramount for suppressing tumor growth, this may foster development of antiangiogenic therapy.

CFMW01 26th June 2013
11:45 to 12:30
N Yoshinaga Spontaneous motion and deformation of a droplet driven by chemical reaction
Spontaneous motion has been attracting lots of attention in last decades in nonlinear and nonequilibrium physics partially for its potential application to biological problems such as cell motility. Recently several model experiments showing spontaneous motion have been proposed in order to elucidate underlying mechanism of the motion. The systems in these works consist of relatively simple ingredients, for instance oil droplets in water, but nevertheless the results show rich motion and deformation of the droplet. Importantly, the system breaks symmetry and chooses one direction of motion. In this work, we theoretically derive a set of nonlinear equations exhibiting a transition between stationary and motile states starting from advection-reaction-diffusion equation driven away from an equilibrium state due to chemical reactions. A particular focus is on how hydrodynamic flow destabilizes an isotropic distribution of a concentration of chemicals. We also discuss a shape of the droplet. Due to self-propulsive motion and flow around the droplet, a spherical shape becomes unstable and it elongates perpendicular to the direction of motion. This fact would imply that the self-propulsion driven by chemical reaction is characterized as a pusher in terms of a flow field.
CFMW01 26th June 2013
14:00 to 14:30
Data mining in swarming models
CFMW01 27th June 2013
09:00 to 09:45
F MacKintosh Active stresses and mechanics of intra/extracellular networks
Much like the bones in our bodies, the cytoskeleton consisting of filamentous proteins largely determines the mechanical response and stability of cells. Similarly, extracellular networks contribute to tissue mechanics. Unlike passive materials, however, living cells are kept far out of equilibrium by metabolic processes and energy-consuming molecular motors that generate forces to drive the machinery behind various cellular processes. We describe recent advances both in theoretical modeling of such active gels, as well as experiments on reconstituted in vitro acto-myosin networks and living cells. We show how such internal force generation by motors can lead to dramatic mechanical effects, including strong mechanical stiffening. Furthermore, stochastic motor activity can lead to non-equilibrium diffusive-like motion in cells.
CFMW01 27th June 2013
09:45 to 10:30
Cytoskeletal pattern formation: Self organization of driven filaments
Living cells rely on the self organization mechanisms of cytoskeleton to adapt to their requirements. Especially in processes such as cell division, intracellular transport or cellular motility the controlled self assembly to well defined structures, which still allow a dynamic reorganization on different time scales are of outstanding importance. Thereby, the intricate interplay of cytoskeletal filaments, crosslinking proteins and molecular motors a central role. One important and promising strategy to identify the underlying governing principles is to quantify the physical process in model systems mimicking the functional units of living cells. Here I will present in vitro minimal model systems consisting of actin filaments, crosslinking molecules and myosin II exhibiting collective long range order and dynamics. I will discuss how a balance of local force exertion, alignment interactions, crosslinking and hydrodynamics affect the evolving dynamic structures.
CFMW01 27th June 2013
11:00 to 11:45
L Truskinovsky Passive cooperative behavior in striated acto-myosin systems
Passive mechanical response of skeletal muscles at fast time scales is dominated by long range interactions inducing cooperative behavior without breaking the detailed balance. This leads to such unusual 'material properties' as negative equilibrium stiffness and different behavior in force and displacement controlled loading conditions. Careful fitting of experimental data suggests that 'muscle material' is finely tuned to perform close to a critical point.
CFMW01 27th June 2013
11:45 to 12:30
A Alexeev Hydrodynamics of an elastic swimmer at low Reynolds number
We use fully-coupled three-dimensional computer simulations to examine the hydrodynamics of an elastic swimmer that swims in a viscous Newtonian fluid, and to probe how elasticity and resonance oscillations can be harnessed for efficient locomotion in a low-Reynolds-number environment. Our simulation approach is based on the lattice Boltzmann method. The swimmer is modeled as a rectangular elastic plate. We examine two types of swimmer actuation. In the first case, the elastic swimmer is actuated by imposing sinusoidal oscillations at its root. We also examine internally actuated swimmers that are driven by time varying internal moment producing swimmer bending. We probe the hydrodynamic forces and fluid structures generated by the swimmers and compare different actuation regimes. In particular, we show that the resonance actuation leads by the fastest swimmer propulsion velocity. This fast swimming, however, is inefficient, whereas an efficient swimming can be obtained for o ff-resonance frequencies. Furthermore, we compare our simulations of internally actuated swimmers with the experimental results for swimmers made of piezoelectric macro-fiber composites. The results are useful for designing efficient self-propelling fish-like robots driven by internally powered fins
CFMW01 27th June 2013
14:00 to 14:45
Growth and instabilities of healthy and cancerous tissues
During development or during tumor growth, cells organize collectively by cell division and apoptosis in a tissue. The aim of our work is to build up theoretical tools to describe the mechanical properties of tissues and to apply them to various biologically relevant situations.

We first show that because of the coupling between cell division and the local stress, a tissue can be considered as a visco-elastic liquid at time scales larger than the cell division time. We then show recent model experiments on cell aggregates showing the effect of mechanical stress on tissue growth.

Finally, we use the hydrodynamic description to discuss the steady state structure of villis which are the protrusions of the surface of the intestine. We describe the formation of villis as a buckling instability of a polar cell monolayer. In addition to the mechanical properties, we also consider the role of stem cells and their differentiation.

CFMW01 27th June 2013
14:45 to 15:30
A Balazs Reconfigurable assemblies of active, auto-chemotactic gels
Using computational modeling, we show that self-oscillating Belousov-Zhabotinsky (BZ) gels can both emit and sense a chemical signal and thus, drive neighboring gel pieces to spontaneously self-aggregate, so that the system exhibits auto-chemotaxis. To date, this is the closest system to the ultimate self-recombining material, which can be divided into separated parts and the parts move autonomously to assemble into a structure resembling the original, uncut sample. We also show that the gels’ coordinated motion can be controlled by light, allowing us to achieve selective self-aggregation and control over the shape of the gel aggregates. By exposing the BZ gels to specific patterns of light and dark, we design a BZ gel “train” that leads the movement of its “cargo”. Our findings pave the way for creating reconfigurable materials from self-propelled elements, which autonomously communicate with neighboring units and thereby actively participate in cons tructing the final structure. In essence, the BZ gels resemble pieces of a construction toy that can be reused to build multiple structures and thus, provide a new route for creating dynamically reconfigurable materials.
CFMW01 27th June 2013
16:00 to 16:45
E Farge Mechanogenetic reciprocal coupling in early embryonic differentiation and morphogenesis, and evolutionary involvement in primitive organisms emergence
Biochemical patterning and morphogenetic movements coordinate the design of embryonic development. The molecular processes through which differentiation patterning closely controls the development of morphogenetic movements are today becoming well understood. Recent experimental evidence demonstrates that mechanical cues generated by morphogenesis activate mechano-transduction pathways, which conversely regulate the tissue differentiation and acto-myosin dependent active morphogenesis of embryonic development (1). Such mechanotransduction processes was discovered at Drosophila embryos gastrulation (2). These include the Armadillo/β-catenin dependent mechanical activation of the master differentiation patterning protein Twist (2,3) and the Fog dependent mechanical activation of the master morphogenetic patterning protein Myosin-II (4). Experiments combining genetics and biomechanics physiological perturbations, with theoretical analysis and simulation of the mechanoge netic control of early drosophila development, showed that these mechanotransduction processes are required for the physiological functions of mid-gut differentiation and mesoderm invagination, respectively (2,3,4). They also indicate the evolutionary involvement of such mechanotransductive processes in the emergence of primitive animals first morphogenetic and differentiation patterns (1,2,5).

1 Farge, E. Curr Top Dev Biol 95, 243-265, doi:10.1016/B978-0-12-385065-2.00008-6 (2011). 2 Farge, E. Curr Biol 13, 1365-1377 (2003). 3 Desprat, e al., E. Dev Cell 15, 470-477, doi:S1534-5807(08)00288-8 [pii] 10.1016/j.devcel.2008.07.009 (2008). 4 Pouille, P. A., Ahmadi, et al. Science signaling 2, ra16, doi:scisignal.2000098 [pii] 10.1126/scisignal.2000098 (2009). B. Driquez, A. Bouclet et al , Phys. Biol, Dec;8(6):066007. doi: 10.1088/1478-3975/8/6/066007. Epub 2011 Nov 25. (2011). 5 Brunet1, A. Bouclet, A. et al , in Euro Evo-Devo 2012 (Lisboa).

CFMW01 27th June 2013
16:45 to 17:15
D Head Microscopic simulation of active gels: The controlling role of end detachment
We detail results of two microscopic models for active media integrated numerically. In the first, point particles lacking orientational degrees of freedom are driven individually through random force noise, and dissipate energy collectively through inelastic interactions. This leads to large length-scale density fluctuations (aka 'giant number fluctuations') and super-diffusion, and we provide a simple scaling argument that links the two, suggestion a degree of commonality between these two features. A simple mechanism relating this model to those with orientational degrees of freedom is postulated. The second model is of apolar filaments driven by two-headed motile springs mimicking motor proteins, in quasi-2D systems with and without lateral confinement. With confinement, a range of structure formation is observed akin to in vitro experiments and in vivo visualisation of dividing cells. Without confinement, layers, asters and bundled states are found that broadly a gree with the predictions of a simple model. Super-diffusion is also observed for less strongly-bound states, as is a suggestion of giant number fluctuations. It is hoped this bottoms-up approach may lead to analytical theories for active gels valid on length scales on the filaments, relevant to some biological situations.

Co-authors: G. Gompper (Juelich, Germany), W. J. Briels (Twente, The Netherlands), H. Tanaka (Tokyo University)

CFMW01 27th June 2013
17:15 to 17:45
Phase transitions and solitons in self-propelled particles: kinetic theory and diagrammatic approach
In this talk, I will summarize our recent progress on the kinetic theory of collective motion. The theory starts with an exact Markov chain for the Vicsek model in phase space and is made tractable by the mean-field-like Molecular Chaos approximation. This leads to an Enskog-like equation which we solve numerically as well as analytically in certain limits. The kinetic equation was also used to rigorously derive the hydrodynamic equations from the microscopic collision rules. We show that our results on the phase diagram and the formation of soliton-like waves agree quantitatively with direct simulations for large particle velocities. We find that the solitons modify the character of the flocking transition from continuous to discontinuous. To understand the behavior in the small velocity limit where mean-field theory is invalid, we developed a diagrammatic approach to systematically include particle correlations and show how they shift the flocking transition.
CFMW01 28th June 2013
09:00 to 09:45
D Needleman The Metaphase Spindle as an Active Liquid Crystal
The spindle is a complex assembly of microtubules, motors, and other associated proteins, which segregates chromosomes during cell division. In metaphase, the spindle exists in a steady-state with a constant flux of molecules and energy continuously modifying and maintaining its architecture. While the self-organization of systems of microtubules and motors have been investigated using theory and experiments, there have been few attempts to test if the proposed theories can be used to understand the dynamics and structure of complex biological systems in vivo. Here we use polarized light microscopy, 3D time-lapse spinning disk confocal microscopy, single molecule imaging, second harmonic generation microscopy, and mechanical measurements to test the validity of continuum models of metaphase spindles. Our results show that a simple continuum model can quantitatively explain spindle structure and dynamics, demonstrate that rigorous physical theories can be used to quantitat ively describe complex subcellular systems, and provide a framework for understanding the structure of the spindle and its response to physical and molecular perturbations.

Co-author: Jan Brugues (Harvard University)

CFMW01 28th June 2013
09:45 to 10:30
Modeling dorsal closure during embryonic development of the fruit fly
This talk concerns "dorsal closure", a dynamic and complex process during the embryogenesis of Drosophila. Experiments have documented distinct phases of dorsal closure, each with rich and sometimes contradictory observations. We build a mathematical model to rationalize the results and test various hypotheses put forth by experimenters. The cells are coupled mechanically through the position of the nodes and the elastic forces on the edges. Besides, each cell has radial spokes on which myosin motors can attach and exert contractile forces on the nodes, the myosin dynamics itself being controlled by a signaling molecule. This simple model successfully reproduces the cell and tissue pulsation in the early phase of dorsal closure, as well as the consistent contraction in the slow phase through a cellular ratcheting mechanism.

Co-authors: Qiming Wang (Univ of British Columbia), Len Pismen (Technion)

CFMW01 28th June 2013
11:00 to 11:45
I Aronson Modeling of Cell Movement on Adhesive Substrates
Modeling the movement of living motile cells on substrates is a formidable challenge; regulatory pathways are intertwined and forces that influence cell motion on adhesive substrates are not fully quantified. Here, we present a mathematical model coupling cell shape dynamics, treated in the framework of the Ginzburg-Landau-type equation for auxiliary mass density (phase field), to a partial differential equation describing the mean orientation (polarization of actin filaments) of the cell's cytoskeletal network. In order to maintain the total area of the cell, the phase field equation is subject to a global conservation constraint. Correspondingly, the equation for mean polarization incorporates key elements of cell mechanics: directed polymerization of actin network at the cell membrane, decay of polarization in the bulk of the cell, and formation of actin bundles (stress fibers) in the rear. The model successfully reproduces the primary phenomenology of cell motil ity: discontinuous onset of motion, diversity of cell shapes and shape oscillations, as well as distribution of traction on the surface. The results are in qualitative agreement with recent experiments on the motility of keratocyte cells and cell fragments. The asymmetry of the shapes is captured to a large extent in this simple model, which may prove useful for the interpretation of recent experiments and predictions of cell dynamics under various conditions. We also investigate effects of adhesion and substrate elasticity on the shape and dynamics of moving cells. We demonstrate that on hard adhesive substrates the cells exhibit steady-state motion. A transition to stick-slip motion is observed on soft and weakly adhesive surfaces.

CFMW01 28th June 2013
11:45 to 12:30
A poroelastic model for mechanochemical waves and pattern formation in Physarum polycephalum
Many processes in living cells are controlled by biochemical substances regulating active stresses. The cytoplasm is an active material with both viscoelastic and liquid properties. First, we incorporate the active stress into a two-phase model of the cytoplasm which accounts for the spatiotemporal dynamics of the cytoskeleton and the cytosol. The cytoskeleton is described as a solid matrix that together with the cytosol as interstitial fluid constitutes a poroelastic material. We find different forms of mechanochemical waves including traveling, standing and rotating waves by employing linear stability analysis and numerical simulations in one and two spatial dimensions.

In a second step, we expand the chemo-mechanical model in order to model the manifold contraction patterns observed experimentally in protoplasmic droplets of Physarum polycephalum. To achieve this, we combine a biophysically realistic model of a calcium oscillator with the poroelastic model derived in the first part of the talk and assume that the active tension is regulated by calcium. With the help of two-dimensional simulations the model is shown to reproduce the contraction patterns observed in in protoplasmic droplets as well as a number of other traveling and standing wave patterns.

Co-authors: Markus Radszuweit (PTB Berlin), Sergio Alonso (PTB Berlin), Harald Engel (TU Berlin )

CFMW01 28th June 2013
14:00 to 14:45
Collective behaviour of phoretically active colloids
Interfacial phoretic transport mechanisms can be used to design self-propelled active colloids, due to their force-free nature. In my talk, I will discuss the generic properties of such self-propelled colloids, and examine their collective behaviour when they interact via the gradient of the fields that they also use for self-propulsion. I will present some results on interesting collective effects such as instabilities and emergent dynamical behaviours.
CFMW01 28th June 2013
14:45 to 15:30
Vascularization patterns and fluid flow in growing tumours
Growing tumours remodel the vascular network by generating new blood vessels (angiogensis), by co-opting already existing ones and by vessel regression. We want to understand the physical determinants of the emerging tumour vascularization patterns and the characteristics of the resuting blood and interstitial fluid flow. For this purpose we develop a theoretical model combining a dynamically evolving and blood flow carrying pipe network with a non-liear growth process, intercommunicating via nutrient and growth factor fields. With it we discuss mechanisms leading to tumor compartmentalization, hot spot formation, and interstitial fluid flow patterns impeding drug delivery.
CFMW01 28th June 2013
16:00 to 16:25
Cell motility due to active gel flows
The cell cytoskeleton, consisting of filaments and molecular motors (such as actomyosin), can be modelled as a polar or nematic active gel. Here a cell embedded in a surrounding medium is modelled as a drop of active gel. Motility of the model cell depends on flows of the contractile active gel. I will present analytical calculations of velocity fields for different polarisation fields and boundary conditions in two and three dimensions. These solutions will be compared to numerical simulations in which the approximations required for analytical tractability are lifted and steady state polarisations and velocities can be found. Generic features of persistent motion will be discussed and comparisons to available experimental data made.

Co-author: Carl Whitfield (University of Sheffield)

CFMW01 28th June 2013
16:25 to 16:50
A New Theory of Micro-Robots: multiple scales & distinguished limits
This paper is devoted to the theory of three types of micro-robots consisting of rigid spheres connected by rods or springs of oscillating lengths:

1. N-sphere linear micro-robot;

2. Dumbbell micro-robot driven by flow oscillations;

3. Triangular micro-robot;

In all three cases the velocities of self-propulsion and the angular velocity of self-rotations have been calculated analytically with the use of the two-timing asymptotic procedure. The results are discussed and compared with the known experiments and with the results of other authors.

CFMW01 28th June 2013
16:50 to 17:15
Conditions of hydrodynamic synchronization in models of beating cilia
Motile cilia are highly conserved structures in the evolution of organisms, generating the transport of fluid by periodic beating, through remarkably organized behavior in space and time. It is not known how these spatiotemporal patterns emerge and what sets their properties.

Individual cilia are nonequilibrium systems with many degrees of freedom. However, their description can be represented by simpler effective force laws that drive oscillations, and paralleled with nonlinear phase oscillators studied in physics.

Here I will describe synthetic model phase oscillators, where colloidal particles are driven by optical traps. The complex structural details of the cilia are coarse-grained into the details of how the colloidal particles are driven. We explore experimentally two types of colloidal model, finding in each case the conditions for optimal coupling. The applicability of this approach to biological data is illustrated by successfully mapping the behavior of cilia in the alga Chlamydomonas onto one of the coarse-grained models.

CFMW01 28th June 2013
17:15 to 17:40
J Dunkel Fluid dynamics of bacterial turbulence
Self-sustained turbulent structures have been observed in a wide range of living fluids, yet no quantitative continuum theory exists to explain their properties. We report experiments on active turbulence in highly concentrated 3D suspensions of Bacillus subtilis and compare them with a minimal fourth-order vector-field theory for incompressible bacterial dynamics. Velocimetry of bacteria and surrounding fluid, determined by imaging cells and tracking colloidal tracers, yields consistent results for velocity statistics and correlations over two orders of magnitude in kinetic energy, revealing a decrease of fluid memory with increasing swimming activity and linear scaling between energy and enstrophy. The best-fit model parameters allow for quantitative agreement with experimental data.

Co-authors: Sebastian Heidenreich (PTB Berlin), Knut Drescher (Princeton University), Rik Wensink (CNRS Orsay), Markus Baer (PTB Berlin), Ray Goldstein (University of Cambridge)

CFM 2nd July 2013
11:00 to 12:00
Modeling and simulations for two-phase flows at solid surfaces
CFM 2nd July 2013
12:00 to 13:00
F Peruani From bacteria to collective motion in heterogeneous media
CFM 4th July 2013
14:00 to 15:00
E Lushi Chemotactic dynamics in suspensions of micro-swimmers
CFM 4th July 2013
15:00 to 16:00
O Jensen Modelling the growth of plant cells and tissues
CFM 9th July 2013
11:00 to 12:00
Emergent Coarsening Dynamics and Scaling Laws via the Principle of Maximum Dissipation
CFM 9th July 2013
12:00 to 13:00
Coarsening Rates for the Dynamics of Interacting Slipping Droplets
CFM 11th July 2013
14:00 to 15:00
P Olmsted Some effects of boundaries and interfaces on bulk non-Newtonian fluids
CFM 11th July 2013
15:00 to 16:00
L.S Luo Lattice Boltzmann model for visco-elastic fluids in 2D
CFM 16th July 2013
11:00 to 12:00
Surface anchoring mediated hierarchical self-assembly in orientationally ordered plasmonic complex fluids
CFM 16th July 2013
12:00 to 13:00
A Hazel Geometry-induced complexity in internal two-phase flows
CFM 18th July 2013
14:00 to 15:00
M Muller Studying slow collective phenomena by concurrently coupling particle-based and continuum descriptions.
CFM 18th July 2013
15:00 to 16:00
Synchronous versus asynchronous transport of a paramagnetic particle in a modulated ratchet potential
CFMW02 22nd July 2013
09:30 to 11:00
E Knobloch Localized structures in fluid flows
In these two lectures I will show examples of spatially localized structures arising in different types of fluid flows and will describe physical and mathematical approaches that have proved useful to understanding the origin of localization and the properties of the localized structures that result. I will illustrate these ideas using model equations such as the Swift-Hohenberg equation and the conserved Swift-Hohenberg equation and apply the results to systems modeled by the Navier-Stokes equation on the one hand and dynamical density function theory on the other. I will also remark on the motion of fronts separating different but structured phases and relate the growth of localized structures to the problem of structure formation on growing domains.
CFMW02 22nd July 2013
11:15 to 12:00
Surfactants and Thin Liquid Layers 1
CFMW02 22nd July 2013
14:00 to 14:45
Surfactants and Thin Liquid Layers 1
CFMW02 22nd July 2013
15:00 to 16:30
R Kamien Topological defects in crystals and liquid crystals 1
CFMW02 23rd July 2013
09:30 to 10:15
Bridging the scales near the contact line 1
Interface is where the macroscopic meets the microscopic; even a simple fluid becomes there a complex fluid. The origins of surface tension and disjoining pressure lie in nanoscale density gradients governed by molecular interactions. We shall see how the paradox of the moving contact line is resolved on the molecular scale when it is viewed as a physico-chemical problem dependent on fluid–substrate interactions.

There is enormous scale separation between molecular and hydrodynamic scales, which makes computation difficult but facilitates analytical theory. We ascend from molecular to macroscopic scales - from density functional theory to lubrication equations - by the approximation ladder. Multiscale perturbation theory elucidates dynamics of the contact line and provides tools for the study of various instabilities, as demonstrated taking as an example the motion of droplets driven by surface forces.

CFMW02 23rd July 2013
10:15 to 11:00
Gradient dynamics formulations of thin film equations
The course starts with a brief review of a number of experiments on dewetting and evaporating thin films/drops of simple and complex liquids. Then the concept of a gradient dynamics description of the evolution of interface-dominated films and drops on solid substrates is introduced starting with the case of a single layer of simple non-volatile liquid, and advancing towards the formulation for films of mixtures.

The second part of the course uses the obtained models to investigate depinning transitions and deposition patterns in a number of different settings that can all be described by the introduced evolution equations.

An extended abstract and a reference list may be found in the attached .txt file.

CFMW02 23rd July 2013
11:15 to 12:00
D Holm Geometric approach to modelling complex fluids 1
CFMW02 23rd July 2013
14:00 to 14:45
D Holm Geometric approach to modelling complex fluids 1
CFMW02 23rd July 2013
15:00 to 16:30
I Stewart Introduction to liquid crystal continuum theory: statics and dynamics
An introduction to the mathematical modelling of liquid crystals will be presented. A brief review of the static theory will lead in to a presentation of the Ericksen-Leslie theory for the dynamics of nematic liquid crystals. Recent developments related to smectic and other liquid crystals phases will also be discussed. Applications to model the influence of flow in 'switching phenomena' (e.g., the time taken to switch a pixel 'on' or 'off') in flat panel liquid crystal displays (LCDs and LEDs) will also be discussed.
CFMW02 24th July 2013
09:30 to 11:00
E Knobloch Localized structures in fluid flows
In these two lectures I will show examples of spatially localized structures arising in different types of fluid flows and will describe physical and mathematical approaches that have proved useful to understanding the origin of localization and the properties of the localized structures that result. I will illustrate these ideas using model equations such as the Swift-Hohenberg equation and the conserved Swift-Hohenberg equation and apply the results to systems modeled by the Navier-Stokes equation on the one hand and dynamical density function theory on the other. I will also remark on the motion of fronts separating different but structured phases and relate the growth of localized structures to the problem of structure formation on growing domains.
CFMW02 24th July 2013
11:30 to 13:00
Lattice Boltzmann Methods for multi-phase turbulent flows
I will review recent results obtained by numerical simulations of multi-phase and/or multi-component flows using Lattice Boltzmann Methods. In particular, I will discuss limitations and potentialities of the numerical method to study boiling systems and droplet dispersion under strong turbulent conditions.
CFMW02 25th July 2013
09:30 to 11:00
I Stewart Introduction to liquid crystal continuum theory: statics and dynamics
An introduction to the mathematical modelling of liquid crystals will be presented. A brief review of the static theory will lead in to a presentation of the Ericksen-Leslie theory for the dynamics of nematic liquid crystals. Recent developments related to smectic and other liquid crystals phases will also be discussed. Applications to model the influence of flow in 'switching phenomena' (e.g., the time taken to switch a pixel 'on' or 'off') in flat panel liquid crystal displays (LCDs and LEDs) will also be discussed.
CFMW02 25th July 2013
11:15 to 12:00
D Holm Geometric approach to modelling complex fluids 2
CFMW02 25th July 2013
14:00 to 14:45
D Holm Geometric approach to modelling complex fluids 2
CFMW02 25th July 2013
15:00 to 16:30
Fluid dynamic approaches to modelling bacterial biofilms growth
Biofilms are slimy colonies of bacteria that have settled on a fluid-solid interface. They are ubiquitous and often undesirable and present numerous challenges in medicine and industry. They consist of bacteria, polymeric substances and water to form a porous structure that changes as the biofilm grows and matures. In this talk, mathematical models will be presented to describe biofilm growth as an expanding viscous fluid. The talk will broadly be in two parts. Firstly, a model for the early stages of biofilm development using thin-film approaches, where Depending on the strength of interaction between bacteria and the substratum two limits naturally arise. Secondly, a model that uses ideas from mixture theory to describe mature biofilm development, enabling prediction of fluid flow regimes within the biofilm structure.
CFMW02 26th July 2013
09:30 to 11:00
R Kamien Topological defects in crystals and liquid crystals 2
CFMW02 26th July 2013
11:15 to 12:00
Bridging the scales near the contact line 2
Interface is where the macroscopic meets the microscopic; even a simple fluid becomes there a complex fluid. The origins of surface tension and disjoining pressure lie in nanoscale density gradients governed by molecular interactions. We shall see how the paradox of the moving contact line is resolved on the molecular scale when it is viewed as a physico-chemical problem dependent on fluid–substrate interactions.

There is enormous scale separation between molecular and hydrodynamic scales, which makes computation difficult but facilitates analytical theory. We ascend from molecular to macroscopic scales - from density functional theory to lubrication equations - by the approximation ladder. Multiscale perturbation theory elucidates dynamics of the contact line and provides tools for the study of various instabilities, as demonstrated taking as an example the motion of droplets driven by surface forces.

CFMW02 26th July 2013
14:00 to 14:45
Bridging the scales near the contact line 3
Interface is where the macroscopic meets the microscopic; even a simple fluid becomes there a complex fluid. The origins of surface tension and disjoining pressure lie in nanoscale density gradients governed by molecular interactions. We shall see how the paradox of the moving contact line is resolved on the molecular scale when it is viewed as a physico-chemical problem dependent on fluid–substrate interactions.

There is enormous scale separation between molecular and hydrodynamic scales, which makes computation difficult but facilitates analytical theory. We ascend from molecular to macroscopic scales - from density functional theory to lubrication equations - by the approximation ladder. Multiscale perturbation theory elucidates dynamics of the contact line and provides tools for the study of various instabilities, as demonstrated taking as an example the motion of droplets driven by surface forces.

CFMW02 26th July 2013
15:00 to 16:30
Depinning transitions and deposition patterns
The course starts with a brief review of a number of experiments on dewetting and evaporating thin films/drops of simple and complex liquids. Then the concept of a gradient dynamics description of the evolution of interface-dominated films and drops on solid substrates is introduced starting with the case of a single layer of simple non-volatile liquid, and advancing towards the formulation for films of mixtures.

The second part of the course uses the obtained models to investigate depinning transitions and deposition patterns in a number of different settings that can all be described by the introduced evolution equations.

An extended abstract and a reference list may be found in the attached .txt file.

CFMW02 29th July 2013
09:30 to 11:00
Mechanics and Thermodynamics of Nematic Shape Equations
CFMW02 29th July 2013
11:15 to 12:00
F Gay-Balmaz An introduction to the variational principles and Poisson brackets for complex fluids
CFMW02 29th July 2013
14:00 to 14:45
F Gay-Balmaz Equivalent theories of liquid crystals
CFMW02 29th July 2013
15:00 to 16:30
Recent progress on the moving contact line problem
The moving contact line problem is a long-standing and fundamental challenge in the field of fluid dynamics, occurring when one fluid replaces another as it moves along a solid surface. Moving contact lines occur in a vast range of applications, where an apparent paradox of motion of a fluid-fluid interface, yet static fluid velocity at the solid satisfying the no-slip boundary condition arises. In this talk we will review recent progress on the problem made by our group.

The motion of a contact line is examined, and comparisons drawn, for a variety of proposed models in the literature. We first scrutinise a number of models in the classic test-bed system of spreading of a thin two-dimensional droplet on a planar substrate, showing that slip, precursor film and interface formation models effectively reduce to the same spreading behaviour. This latter model, developed by Shikhmurzaev a few years ago, is a complex and somewhat controversial one, differentiating itself by accounting for a variation in surface layer quantities and having finite-time surface tension relaxation. Extensions to consider substrate heterogeneities in this prototype system for slip models are also considered, such as for surface roughness and fluctuations in wetting properties through chemical variability.

Analysis of a solid-liquid-gas diffuse-interface model is then presented, with no-slip at the solid and where the fluid phase is specified by a continuous density field. We first obtain a wetting boundary condition on the solid that allows us to consider the motion without any additional physics, i.e. without density gradients at the wall away from the contact line associated with precursor films. Careful examination of the asymptotic behaviour as the contact line is approached is then shown to resolve the singularities associated with the moving contact line problem. Various features of the model are scrutinised alongside extensions to incorporate slip, finite-time relaxation of the chemical potential, or a precursor film at the wall. But these are not necessary to resolve the moving contact line problem. Ongoing work to rigorously include non-local terms into models for contact line motion based on density functional theory will be discussed, with work analysing the contact line in equilibrium presented.

**Joint work with David Sibley, Andreas Nold & Nikos Savva

CFMW02 30th July 2013
09:30 to 11:00
T Ratiu Geometric approach to the Hamiltonian and Lagrangian formulation of complex fluids
CFMW02 30th July 2013
11:15 to 12:00
J Toner Fish gotta swim, birds gotta fly, I gotta do Feynmann graphs 'till I die: A hydrodynamic theory of flocking 1 (Part 1)
CFMW02 30th July 2013
14:00 to 14:45
J Toner Fish gotta swim, birds gotta fly, I gotta do Feynmann graphs 'till I die: A hydrodynamic theory of flocking 1 (Part 2)
CFMW02 30th July 2013
15:00 to 16:30
A Bertozzi Particle laden thin films: theory and experiment
CFMW02 31st July 2013
09:30 to 11:00
Computation of complex fluid flows
CFMW02 31st July 2013
11:30 to 13:00
Hydrodynamic Coordination at Low Reynolds Number
Microorganisms and the mechanical components of the cell motility machinery such as cilia and flagella operate in low Reynolds number conditions where hydrodynamics is dominated by viscous forces. The medium thus induces a long-ranged hydrodynamic interaction between these active objects, which could lead to synchronization, coordination and other emergent many-body behaviors. In my talk, I will examine these effects using minimal models that are simple enough to allow extensive analysis that sheds light on the underlying mechanisms for the emergent phenomena.
CFMW02 1st August 2013
09:30 to 11:00
Equation-free approach to deriving effective macroscopic equations for complex interacting systems
CFMW02 1st August 2013
11:15 to 12:00
Capillary Models for Liquid Crystal Fibers, Membranes, Films, and Drops
CFMW02 1st August 2013
14:00 to 14:45
Capillary Models for Liquid Crystal Fibers, Membranes, Films, and Drops (Part 2)
CFMW02 1st August 2013
15:00 to 16:30
J Yeomans Swimming at low Reynolds number
I shall introduce the hydrodynamics that underlies the way in which microorganisms, such as bacteria and algae, and fabricated microswimmers, swim. For such tiny entities the governing equations are the Stokes equations, the zero Reynolds number limit of the Navier-Stokes equations. This implies the well-known Scallop Theorem, that swimming strokes must be non-invariant under time reversal to allow a net motion. Moreover, biological swimmers move autonomously, free from any net external force or torque. As a result the leading order term in the multipole expansion of the Stokes equations vanishes and microswimmers generically have dipolar far flow fields. I shall introduce the multipole expansion and describe physical examples where the dipolar nature of the bacterial flow field has significant consequences, the velocity statistics of a dilute bacterial suspension and tracer diffusion in a swimmer suspension.
CFMW02 2nd August 2013
09:30 to 11:00
T Ratiu Geometric approach to the Hamiltonian and Lagrangian formulation of complex fluids
CFMW02 2nd August 2013
11:30 to 13:00
J Toner Fish gotta swim, birds gotta fly, I gotta do Feynmann graphs 'till I die: A hydrodynamic theory of flocking 2 ¯
CFM 6th August 2013
11:00 to 12:00
Coarse bifurcation studies of complex problems
CFM 8th August 2013
14:00 to 15:00
S Fielding Flow instabilities in complex fluids
CFM 8th August 2013
15:00 to 16:00
E Del Gado Soft matter in construction
CFM 13th August 2013
11:00 to 12:00
A Voigt A phase field approach to cell motility
CFM 13th August 2013
12:00 to 13:00
Phase separation kinetics of active particles
CFM 15th August 2013
15:00 to 16:00
Structure and Dynamics of Biological Liquid Crystals
CFMW03 19th August 2013
09:00 to 09:30
Dynamics of grainy liquid crystalline monolayers: visible and hidden grain boundaries and chiral rheology
Co-authors: Kyuhan Kim (UCSB ChE), SiYoung Choi (UCSB ChE), Joe Zasadzinski (Minnesota CEMS)

While the equilibrium properties of fluid interfaces have been manipulated and studied for centuries, their dynamic, rheological properties (e.g. viscosity and elasticity) have proven more elusive. Despite the dominant role that even molecularly-thin interfaces can play in multiphase flows, the viscosity of the bulk fluids on either side of the interface can easily overwhelm any attempt at measuring surface rheology. I will describe a technique we have developed to measure the interfacial rheology -- the viscous and elastic properties -- of fluid-fluid interfaces, typically laden with some surface-active species (molecular surfactants, copolymers, colloids, etc.). A novel feature is our ability to visualize the interface during the measurement, enabling us to directly relate the measured response to the microstructure of the interface.

In particular, we study model lung surfactant monolayers that consist of liquid-condensed phases of the phospholipid DPPC and, in some cases, cholesterol. We directly visualize the deformation of liquid-crystalline domains under both linear and nonlinear deformations. Despite the simplicity of the system -- a single-component, 2 nm-thick molecular monolayer -- we find an extraordinarily rich rheological response, including a soft, glassy response, elastic strain energy that is stored over a shockingly long time, two-dimensional yielding behavior, aging, rejuvenation, and anisotropically chiral rheology, exhibiting either ductile plasticity or brittle fracture, depending on the sense of the shear. We relate these rheological responses to observed boundaries between individual DPPC crystals, as well as hidden boundaries where tail group tilt orientations change rapidly.

CFMW03 19th August 2013
09:30 to 10:00
M Buzza Two-Dimensional Colloidal Alloys
Co-authors: Adam Law (Max-Planck-Institut fuer Intelligente Systeme), Melodie Auriol (Ecole Nationale Superieure de Chimie de Rennes), Dean Smith (University of Hull), Tommy Horozov (University of Hull)

We study the self-assembly of mixed monolayers of hydrophobic and hydrophilic colloidal particles adsorbed at oil/water interfaces both experimentally and theoretically. Experimentally, we find that by tuning the interactions, composition and packing geometry of the mixed monolayer, a rich variety of two-dimensional super-lattice [1] and cluster [2] structures are formed which are stabilised by strong electrostatic interactions mediated through the oil phase. The 2D structures obtained are in excellent agreement with zero temperature lattice sum calculations [1-3], indicating that the self-assembly process can be effectively controlled for the creation of novel 2D structures.

[1] A.D. Law, D.M.A. Buzza, T.S. Horozov, Phys. Rev. Lett., 106, 128302 (2011) [2] A.D. Law, M. Auriol, D. Smith, T.S. Horozov, D.M.A. Buzza, Phys. Rev. Lett., 110, 138301 (2013) [3] A.D. Law, T.S. Horozov, D.M.A. Buzza, Soft Matter, 7, 8923 (2011)

CFMW03 19th August 2013
10:00 to 10:30
Soundbites from Attendees
Emanuela Del Gado: Crowding and ordering in the adsorption of nanoparticles at air-water interfaces

Lorenzo Botto: Rod-like particles at fluid interfaces: adsorption, in-plane interactions, and Êmicromechanics of particle chains

Daniel Rings: SPH for complex fluids: A path to hydrodynamics with moving boundaries and inhomogeneities

Wieland Marth: Signaling networks and cell motility - a computational approach using a phase field description

Alice Thompson: Can consecutive droplet deposition yield liquid films of uniform depth?

Adriano Tiribocchi: A minimal model for a crawling cell

John Joseph Williamson: Domain registration transition in lipid bilayer phase separation

Wieland Marth: Signaling networks and cell motility

Lailai Zhu: Deformability-induced cell sorting in micro-fluidic devices

CFMW03 19th August 2013
11:00 to 11:30
Active gel flow in finite domains with applications to cell motility in confinement
Co-authors: Carl Whitfield (University of Sheffield), Raphael Voituriez (UPMC/CNRS, Paris), Davide Marenduzzo (University of Edinburgh)

Motility of cells in confinement is relevant to cell migration in tissues. Motility is powered by the cell cytoskeleton, which consists of biopolymer filaments and active cross linkers (molecular motors), fueled by biochemical energy. Modelling the cell cytoskeleton as a finite domain of active polar gel, we calculate internal flow fields. These velocity fields are dependent on the boundary conditions. In addition, coupling these internal flows to external media gives rise to mechanisms for motion of the active droplet. The internal dynamics also affect the shape of the active domain. I will present results of analytical calculations and numerical simulations of velocity fields with different boundary conditions. As well as showing results I will discuss some future challenges that are currently unsolved.

CFMW03 19th August 2013
11:30 to 12:00
E Lushi Active suspensions in domains with static or moving boundaries
I will briefly describe a novel fast computational method that enables us to trace the coupled dynamics of thousands on micro-swimmers that interact directly as well as via the collectively generated fluid flows. I will illustrate with results involving such an active'' micro-swimmer suspension inside a drop where the spontaneous organization that emerges depends not only on confinement and steric effects, but also on the presence of hydrodynamics. Lastly, I will discuss the case when the active suspension is inside a domain with moving boundaries, such as a peristaltic pump, and where the transport of passive tracers gets effected by the swimmers' collective motion.
CFMW03 19th August 2013
12:00 to 12:30
E Keaveny Undulatory locomotion in structured media
Many swimming microorganisms inhabit heterogeneous environments consisting of solid particles immersed in viscous fluid. Such environments require the organisms attempting to move through them to negotiate both hydrodynamic forces and geometric constraints. Here, we study this kind of locomotion by first observing the kinematics of the small nematode and model organism Caenorhabditis elegans in fluid-filled, micro-pillar arrays. We then compare its dynamics with those given by numerical simulations of a purely mechanical worm model that accounts only for the hydrodynamic and contact interactions with the obstacles. We demonstrate that these interactions allow simple undulators to achieve speeds as much as an order of magnitude greater than their free-swimming values. More generally, what appears as behaviour and sensing can sometimes be explained through simple mechanics.
CFMW03 19th August 2013
15:00 to 15:30
SA Karpitschka Sharp Border between Coalescence and Noncoalescence of Sessile Drops from Miscible Liquids
Co-author: Hans Riegler (MPIKG)

Recently it has been shown that sessile drops from different but completely miscible liquids do not always coalesce instantaneously upon contact. Quite un­ex­pec­ted it is observed that after contact, the drop bodies remain separated in a temporary state of non­coale­scence, connected only through a thin liquid bridge [1,2]. The connected drops move as a twin drop con­figuration over the surface. The surface energy difference between the liquids causes a Marangoni flow. This stabilizes the bridge and drives the drop motion [3]. Up to now studies regarding the (non)coalescence behavior of sessile drops from different liquids were performed only without a systematic variation of the con­tact angles. Therefore it is unknown: (I) at which con­tact angles the transition between temporary non­coalescence and immediate coalescence occurs, (II) whether this transition is sharp or gradual, and (III) whether the behavior is different f or static and dynamic contact angles, respectively. We present quan­titative experimental data on the contact angle de­pen­dence of the coalescence behavior of sessile drops from completely miscible liquids. We find quantitatively the same coalescence behavior for both static and dynamic contact angles. The border between the coalescence and the non­coalescence regime is sharp and given by a power law relation between contact angle and surface tension contrast. The power laws are explained within a fluid dynamic thin film approach by scaling arguments. The sharp transition is quantitatively reproduced by numerical simulations.

[1] H. Riegler, P. Lazar, Langmuir 24, 6395 (2008). [2] S. Karpitschka, H. Riegler, Langmuir 26, 11823 (2010). [3] S. Karpitschka, H. Riegler, Phys. Rev. Lett. 109, 066103 (2012).

CFMW03 19th August 2013
15:30 to 16:00
B Chakrabarti Shaping and sculpting of liquid drops using laser beams
Co-authors: David Tapp (Durham University), Jonathan Taylor (University of Glasgow), Colin Bain (Durham University)

Motivated by recent experiments on optical sculpting of liquid drops with ultralow interfacial tension I discuss modeling approaches that predict droplet shapes in single and multiple optical traps using simulations and theory.

CFMW03 19th August 2013
16:30 to 17:00
MH Koepf A continuum model of epithelial spreading
Co-author: Leonid M. Pismen (Department of Chemical Engineering, Technion - Israel Institute of Technology, 32000 Haifa, Israel)

We present a continuum model of unconstrained epithelial spreading. The tissue is described as a polarizable and chemo-mechanically interacting layer with neo-Hookean elasticity. Our model reproduces the spontaneous formation of finger-like protrusions commonly observed in experiment. Statistics of velocity orientation obtained from numerical simulation show strong alignment in the fingers opposed to an isotropic distribution in the bulk, as has been measured by Reffay et al. (Reffay et al., Biophysical Journal, 2011). The results faithfully reproduce faster relative advance of cells close to the leading edge of the tissue, as well as spatial velocity correlations and stress accumulation within the tissue, which proceeds in form of a "mechanical wave", traveling from the wound edge inwards (cf. Serra-Picamal et al., Nature Physics, 2012).

M. H. Koepf, L. M. Pismen: Non-equilibrium patterns in polarizable active layers, Physica D 259 (2013) 48-54

M. H. Koepf, L. M. Pismen: A continuum model of epithelial spreading, Soft Matter 9 (2013) 3727-3734

CFMW03 19th August 2013
17:00 to 17:30
Equilibrium and growth shapes of fiber-covered surfaces
Co-authors: Thi-Hanh Nguyen (CNRS, Ecole Polytechnique), Vincent Fleury (CNRS, Ecole Polytechnique), Hervé Henry (CNRS, Ecole Polytechnique)

Branched growth patterns are generally formed by an interplay between instabilities that favor branching and stabilizing effects that result from the microscopic structure of matter. We consider nematic surfaces (that is, surfaces that are covered by fibers which remain tangential to the surface) and investigate the consequences of an anisotropic bending rigidity (surfaces are easier to bend in the direction normal to the fibers than along it) on equilibrium and growth shapes. We formulate a continuum model that allows us to determine the organization of the fibers and the geometric shape of a simply connected domain which correspond to a minimum of the total (free) energy. The coupling with a simple diffusive growth mechanisms leads to growth shapes that could not have been obtained with a simple crystalline material. Possible connections with the growth of biological structures will be discussed.

CFMW03 20th August 2013
09:30 to 10:00
Oscillatory bubbles induced by geometric constraint
Co-authors: Alice Thompson (University of Manchester), Andrew Hazel (University of Manchester)

We show that a simple change in pore geometry can radically alter the behaviour of a fluid-displacing air finger [1,2]. A rich array of propagation modes, including symmetric, asymmetric, localised fingers, is uncovered when air displaces oil from axially uniform tubes that have local variations in flow resistance within their cross-sections. The most surprising propagation mode exhibits spatial oscillations formed by periodic sideways motion of the interface at a fixed distance behind the moving finger tip [2]. This rich behaviour is in contrast to the single, symmetric mode observed in tubes of regular cross-section, e.g. circular, elliptical, rectangular and polygonal.

We derive a two-dimensional depth-averaged model for bubble propagation through wide channels with a smooth occlusion, which is similar to that describing Saffman-Taylor fingering, but with a spatially varying channel height. We solve the resulting system numerically, using the finite-element library oomph-lib (www.oomph-lib.org), and find that numerical solutions to the model exhibit most the qualitative features of the experimental propagation modes, including the oscillatory modes of propagation.

The existence of these novel propagation modes suggests that models based on over-simplification of the pore geometry will suppress fundamental physical behaviour present in practical applications, where pore geometry often contains many regions of local constriction, e.g. connecting or irregularly shaped pores in carbonate oil reservoirs, and airway collapse or mucus buildup in the lungs. Moreover, these modes offer further potential for geometry-induced manipulation of droplets for lab-on-the-chip applications, in which geometric variations have so far been restricted to the axial direction.

[1] A. de Lozar et al. (2009) Phys. Fluids 21, 101702. [2] A.L Hazel et al.(2013) Phys. Fluids 25, 062106. [3] Pailha et al. (2012) Phys. Fluids 24, 0217

CFMW03 20th August 2013
10:00 to 10:30
Droplet Motion with Evaporation and Condensation in One-Component Fluids
Recently, the dynamic van der Waals theory (DvdWT) has been presented for the study of hydrodynamics in one-component fluids with liquid-vapor transition in inhomogeneous temperature fields [Onuki A 2005 Phys. Rev. Lett. 94 054501]. We first derive the hydrodynamic boundary conditions at the fluid-solid interface for the DvdWT using conservation laws and the positive definiteness of entropy production together with the Onsager reciprocal relation. We then apply the DvdWT to the study of droplet motion driven by thermal gradients at solid surfaces. The effect of thermal singularity at the liquid-vapor-solid three phase contact line is investigated. The droplet motion predicted by the continuum hydrodynamic model is also observed and semi-quantitatively verified by performing molecular dynamics simulations for confined one-component two-phase fluids.
CFMW03 20th August 2013
11:00 to 11:30
E Knobloch Front motion and the growth of localized patterns
Stationary localized patterns in bistable dissipative systems are confined by fronts connecting the pattern to a background homogeneous state. Such patterns are stationary whenever the fronts are pinned to the pattern state. When parameters are changed the fronts may unpin leading to the growth of the pattern as the pattern invades the stable homogeneous state. I will describe analytical techniques for computing the front speed focusing on the location and frequency of the phase slips that are necessary to grow the pattern at a rate that is consistent with the front motion. The process will be illustrated using numerical simulations of the Swift-Hohenberg equation and the forced complex Ginzburg-Landau equation in both one and two spatial dimensions.

This is joint work with R Krechetnikov (University of California at Santa Barbara) and Yi-ping Ma (University of Chicago).

CFMW03 20th August 2013
11:30 to 12:00
M Fontelos The dynamics of jets of polymeric liquids
The presence of polymers in a jet of Newtonian liquid leads to an extraordinary resistance of the jet to breakup intro drops. Instead, it undergoes a transition towards the so-called beads-on-string configuration, where thin filaments connect an apparently random sequence of drops. Later in the evolution, the drops exhibit an interesting dynamics that we discuss and explain.
CFMW03 20th August 2013
14:30 to 15:00
Swirling and swarming in bacterial colonies: Interfacial driven flows and rheological complexity
Co-authors: raf.dedier[at]cit.kuleuven[dot]be (chemical engineering, KU Leuven), Jan.michiels[at]biw.kuleuven[dot]be (CPMG KU Leuven), wouter.sempels[at]chem.kuleuven[dot]be (Chemistry, KU Leuven), johan.hofkens[at]chem.kuleuven[dot]be (Chemistry, KU Leuven)

Bacterial colonies have interesting dynamics and pattern formation, when moving atop a solid surface. In the present work we discuss how autoproduced bio-surfactants play a dominant role in pattern formation during either drying or swarming.

First, bacterial swarming is one of the most efficient methods by which bacteria colonize nutrient-rich environments and host tissues. Several mechanisms have been proposed to explain the phenomenon and the associated intricate macroscopic pattern formation. Here, by using a series of complementary genetic and physicochemical experiments and a simple mathematical analysis, we show how the bacterial swarming can be caused by a surface tension driven flow. The opportunistic pathogen, Pseudomonas aeruginosa, is studied, as it is relevant for such bacteria to control and arrest swarming. Moreover, P. aeruginosa bacteria secrete strong surface active component.

Second, auto-production or exogenous addition of a soluble non-ionic surfactant during the drying stages of the colonies induces complex flow patterns in a region near the edge of an evaporating droplet, even at very high surfactant concentrations. This is due to the generation of a Marangoni flow, itself being created by a heterogeneous distribution of surfactant molecules along the interface by an outward capillary flow, creating oscillatory or vortex dominated flows.

In all these systems Marangoni stresses can generate sufficiently strong forces to drive both surface and bulk flows, either in swarming colonies or during drying of bacterial systems.

CFMW03 20th August 2013
15:00 to 15:30
A Voigt Vesicle flickering, surface viscosity and exterior differential calculus - a mathematical approach to coarsening dynamics in lipid membranes
CFMW03 20th August 2013
16:00 to 16:30
Quasipatterns in problems with two length scales: Faraday waves and soft-matter polymer micelles
Co-authors: Anne Skeldon (University of Surrey), Mary Silber (Northwestern University)

In problems with two comparable length scales, it is possible for two waves of the shorter wavelength to interact with one wave of the longer, as well as for two waves of the longer wavelength to interact with one wave of the shorter. Consideration of both types of three-wave interactions can generically explain the presence of complex patterns, such as quasipatterns, and spatiotemporal chaos. Two length scales arise naturally in some examples of polymer micelles and in the Faraday wave experiment, where a viscous fluid is subjected to vertical vibration. Our results enable some previously unexplained experimental observations of spatiotemporal chaos in the Faraday wave experiment to be interpreted in a new light; application to quasicrystals recently observed in self-assembled colloidal systems is more speculative.

CFMW03 20th August 2013
16:30 to 17:00
SM Fielding Hydrodynamics and phase behaviour of active suspensions
We simulate a suspension of active squirming disks over the full range of volume fractions from dilute to close packed, with full hydrodynamics in two spatial dimensions. By doing so we show that "motility induced phase separation" (MIPS), recently proposed to arise generically in active matter, is strongly suppressed by hydrodynamic interactions. We give an argument for why this should be the case, and support it with counterpart simulations of active Brownian disks in a parameter regime more appropriate to hydrodynamic suspensions than in previous studies.
CFMW03 21st August 2013
09:30 to 10:00
A Archer Solidification fronts: how rapid fronts can lead to disordered glassy solids
Co-authors: Mark Robbins (Loughborough University), Uwe Thiele (Loughborough University), Edgar Knobloch (University of California at Berkeley)

We determine the speed and form of a crystallization (or, more generally, a solidification) front as it advances into the uniform liquid phase after it has been quenched into the crystalline region of the phase diagram. The speed is obtained by assuming a dynamical density functional theory (DDFT) model for the system and applying a marginal stability criterion. Our results also apply to phase field crystal (PFC) models of solidification. As the solidification front advances into the unstable liquid phase, the density profile behind the advancing front develops density modulations and the wavelength of these modulations is a dynamically chosen quantity. For shallow quenches, the selected wavelength is that of the crystalline phase and so well-ordered crystalline states are formed. However, when the system is deeply quenched, we find that this wavelength can be quite different from that of the crystal, so the solidification front naturally generates disorder in the system. Sig nificant rearrangement and aging must subsequently occur for the system to form the regular well-ordered crystal that corresponds to the free energy minimum. Additional disorder is introduced whenever a front develops from random initial conditions. We illustrate these findings with simulation results from DDFT and the PFC model.

CFMW03 21st August 2013
10:00 to 10:30
Maturation and exchange in cellular organelles
Co-authors: Serge Dmitrieff (EMBL), Madan Rao (NCBS)

Most molecules secreted or internalized by Eukaryotic cells follow well defined routes, the secretory and endocytic pathways, along which they are exposed to a succession of biochemical environments by sequentially visiting different membrane-bound organelles. Molecules internalized by endocytosis move from early to late endosomes before being sorted and carried to their final destination. Molecules synthesized in the endoplasmic reticulum go through the Golgi apparatus, itself divided into cis, medial and trans compartments (called cisternae), where they undergo post-transcriptional maturation and sorting. One fundamental issue underlying the organization and regulation of intracellular transport is whether progression along the transport pathways occurs by exchange between organelles of fixed biochemical identities (via the budding and scission of carrier vesicles), or by the biochemical maturation of the organelles themselves. In this talk, I will present some aspects of the Physics of out-of-equilibrium membrane system, and discuss their relevance to intra-cellular transport. I will particularly focus on the dynamical coupling between biochemical maturation and phase separation of membrane components, and its possible relevance for the generation and maintenance of the Golgi apparatus.

CFMW03 21st August 2013
11:00 to 11:30
Q Wang Analysis and computation of a polar active liquid crystal model
Co-authors: Xiagang Yang (Nankai University), Xiaofeng Yang (Univesity of south Carolina), M. Greg Forest (University of North Carolina a Chapel Hill)

We will present a systematic analysis of a polar nematic liquid crystal modl developed for solutions of active liquid crystals. We will (i). identify the mode of instability for simple equilibrium states to study the near equilibrium dynamics, (ii). study spatially heterogeneous structure of the model prediction in 1-D and 2-D space, (iii). investigate the capillary instabilit associated with the free surface active liquid crystal jet. For the capillary instability, we identified not only a classical Rayleigh mode and how it is modifed by the model parameters, but also, a couple of new modes exclusively tied to the activity of the active material system. Nonlinear simulations are performed using an equivalent phase field model to confirm and linear stability result.

CFMW03 21st August 2013
11:30 to 12:00
The Annealing-to-Driven Transition of Coarsening Nano-Faceted Crystals