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Workshop Programme

for period 19 - 21 August 2013

Complex Fluids in Evolving Domains

19 - 21 August 2013

Timetable

Monday 19 August
08:00-08:50 Registration
08:50-09:00 Welcome - Uwe Thiele/Peter Olmsted
09:00-09:30 Squires, T (University of California, Santa Barbara)
  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.

 
09:30-10:00 Buzza, M (University of Hull)
  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)

 
10:00-10:30
  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

 
10:30-11:00 Morning Coffee
11:00-11:30 Hawkins, R (University of Sheffield)
  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.

 
11:30-12:00 Lushi, E (Imperial College London)
  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.

 
12:00-12:30 Keaveny, E (Imperial College London)
  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.

 
12:30-15:00 Lunch
15:00-15:30 Karpitschka, SA (Max-Planck-Institut für Kolloid- und Grenzflächenforschung)
  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).

 
15:30-16:00 Chakrabarti, B (Durham University)
  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.

 
16:00-16:30 Afternoon Tea
16:30-17:00 Koepf, MH (Technion - Israel Institute of Technology)
  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

 
17:00-17:30 Plapp, M (CNRS/École Polytechnique)
  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.

 
18:00-18:30 Wine reception
19:00-20:00 Dinner
Tuesday 20 August
09:30-10:00 Juel, A (University of Manchester)
  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

 
10:00-10:30 Qian, T (Hong Kong University of Science and Technology)
  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.

 
10:30-11:00 Morning Coffee
11:00-11:30 Knobloch, E (University of California, Berkeley)
  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).

 
11:30-12:00 Fontelos, M (ICMAT)
  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.

 
12:30-14:30 Lunch
14:30-15:00 Vermant, J (Katholieke Universiteit Leuven)
  Swirling and swarming in bacterial colonies: Interfacial driven flows and rheological complexity
 

Co-authors: raf.dedier@cit.kuleuven.be (chemical engineering, KU Leuven), Jan.michiels@biw.kuleuven.be (CPMG KU Leuven), wouter.sempels@chem.kuleuven.be (Chemistry, KU Leuven), johan.hofkens@chem.kuleuven.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.

 
15:00-15:30 Voigt, A (Technische Universität Dresden)
  Vesicle flickering, surface viscosity and exterior differential calculus - a mathematical approach to coarsening dynamics in lipid membranes
15:30-16:00 Afternoon Tea
16:00-16:30 Rucklidge, AM (University of Leeds)
  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.

 
16:30-17:00 Fielding, SM (Durham University)
  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.

 
19:30-22:00 Conference Dinner at Hinsley Hall
Wednesday 21 August
09:30-10:00 Archer, A (Loughborough University)
  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.

 
10:00-10:30 Sens, P (ESPCI ParisTech)
  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.

 
10:30-11:00 Morning Coffee
11:00-11:30 Wang, Q (University of South Carolina)
  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.

 
11:30-12:00 Watson, S (University of Glasgow)
  The Annealing-to-Driven Transition of Coarsening Nano-Faceted Crystals
12:30-14:00 Lunch
14:00-14:30 Depart by coach

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