# Seminars (DNMW03)

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Event When Speaker Title Presentation Material
DNMW03 13th May 2019
09:40 to 10:20
Randall Kamien Packing Liquid Crystal Domains
Focal conic domains are complex, geometric configurations found in cholesteric and smectic liquid crystals: they are not topologically protected but are very low energy states. How do they pack on finite geometries? Come and listen!
DNMW03 13th May 2019
10:20 to 10:40
Thomas Machon Contact Topology and the Cholesteric Landscape
Cholesterics, chiral liquid crystals, typically exhibit a large number of metastable states for a given geometry. This is both a blessing and a curse, it affords great potential for the creation of new devices but can also mean that tight control of a structure can be difficult to achieve. In this talk we will discuss why it is that the tendency of cholesterics to twist means that they have a complex energy landscape. Our principle tools will be drawn from the field of contact topology. By describing cholesterics as contact structures we will show that non-vanishing twist implies conservation of the layer structure in cholesteric liquid crystals. This characterises the morphological richness of these systems, leads to a number of additional topological invariants for cholesteric textures that are not captured by traditional descriptions, and gives a geometric characterisation of cholesteric dynamics in any context, including active systems, those in confined geometries or under the influence of an external field.
DNMW03 13th May 2019
11:10 to 11:50
Margarida Telo da Gama Self-organization of patchy colloidal particles: 2 & 3D

We investigate the self-organization of patchy colloidal particles deposited on flat substrates in three (2+1) and two (1+1) spatial dimensions. We propose and use a simple stochastic model for the interaction between the particles, which allows the simulation of very large systems, to probe the long time and large-scale structure of the deposited films. The latter exhibit well defined surface, liquid and interfacial regions except when the growth is dominated by the formation of chains, which occurs for systems with an effective valence close to two. We also investigate the interfacial roughening in (1+1) systems and compare our results with those obtained experimentally for evaporating droplets. We find, in line with the experiments, that when the film growth is dominated by chains the generic Kardar-Parisi-Zhang (KPZ) interfacial roughening is replaced by quenched KPZ. We discuss this somewhat surprising result.

DNMW03 13th May 2019
11:50 to 12:30
Anja Schlömerkemper Evolution of magneto-viscoelastic materials
In this talk I will survey our recent approach to the modeling of magneto-viscoelastic materials. Our system of partial differential equations consists of the Navier-Stokes equations, the Landau-Lifshitz-Gilbert equation and an evolution equation for the deformation gradient. I will address modeling aspects, analytical results and potential applications.
DNMW03 13th May 2019
13:30 to 14:10
Hillel Aharoni Making Faces: Universal Inverse Design of Thin Nematic Elastomer Surfaces
Thin nematic elastomer sheets can be programmed, via the nematic director field embedded into them, to take different shapes in different environments. Recent experiments from various groups demonstrate excellent control over the director field, thus opening a door for achieving accurate and versatile designs of shape-shifting surfaces. At the crux of any effort to implement this design mechanism lies the inverse design problem -- given an arbitrary surface geometry, constructing the director field that would induce it. In this talk I describe several aspects of this inverse problem. I present a numerical algorithm for finding global approximate solutions for any 2D geometry. I also show that many exact solutions always exist locally and can be readily integrated, and classify the set of all director fields that deform into an arbitrary given geometry. These results allow optimizing the resultant director fields for different purposes, e.g. maximizing the domain of a global solution, increasing its robustness, reducing residual stresses, or controlling the entire shape-shifting path.
DNMW03 13th May 2019
14:10 to 14:50
Ard Louis Simplicity bias in random design
The design of a soft-matter system can be recast as an input-output map, where the inputs are the parameters that fix the components and their interactions, and the outputs describe the outcome of a self-assembly process. By extending the coding theory from algorithmic information theory, we have recently shown [K Dingle, C. Camargo and AAL, Nat Comm. 9, 761 (2018)] that for many computable maps, the a priori probability P(x) that randomly sampled inputs generate a particular output x decays exponentially with the approximate Kolmogorov complexity $\tilde{K}(x)$ of that output. While Kolmogorov complexity is technically uncomputable, we show how to make approximations that work in practice, allowing for a tight upper bound on P(x). For soft matter systems, simplicity bias implies that randomly sampling design inputs will naturally lead to outputs that have low descriptional complexity. Since high symmetry structures typically have low descriptional complexity, simplicity bias implies that randomly picking design patterns can lead to the spontaneous emergence of highly symmetric self-assembled structure. We provide evidence for these trends for self-assembled RNA and protein structures.
DNMW03 13th May 2019
15:20 to 16:00
Gareth Alexander Geometric Topology of Liquid Crystal Textures: Chirality and Bend
The textures and phases of liquid crystals are replete with geometric motifs, and the geometric approach to elasticity underpins a large portion of nonlinear theories. Despite this, the basic characterisation of topology comes from the homotopy theory without particular attention to geometric features. I will describe our recent work developing geometric approaches to liquid crystal topology, describing cholesteric point defects and topological chirality, and the geometric features of bend distortions, illustrated by applications to the twist-bend nematic phase.
DNMW03 13th May 2019
16:00 to 16:40
Alex Travesset Soft Skyrmions and Programmable Self-Assembly of Superlattices
Materials whose fundamental units are nanocrystals (NC)s, instead of atoms or molecules, are gradually emerging as major candidates to solve many of the technological challenges of our century. Those materials display unique structural, dynamical and thermodynamical properties, often reflecting deep underlying geometric, packing and topological constraints. In this talk, I will discuss the rational design of NC materials by programmable self-assembly. I will present the Orbifold Topological Model (OTM), which successfully describes the structure of crystal or quasicrystal arrangements of NCs (superlattices) by considering capping ligands as Skyrmion textures, which determine the bonding very much like atomic orbitals in lattices of simple atoms. I will show that the OTM describes “atomic orbitals” as consisting of vortices, which enable the generation of a spontaneous valence and reveal the universal tendency of these systems towards icosahedral order, allowing to describe them as quasi-Frank-Kasper phases.  These results will be confirmed by numerical simulations. I will elaborate on the success of the OTM in describing all existing experimental structural data on single component and binary superlattices obtained by solvent evaporation and present new candidate phases.
DNMW03 13th May 2019
16:40 to 17:00
Shayandev Sinha Thermally actuated portable microvalves using elastomeric focusing
Thermally actuated controlled shape changes in soft materials is a challenge as the material shows non-linear expansion characteristics. CTE of many materials is not properly available. In order to focus the expansion of the soft solid into large displacements a confined geometry is created to amplify the shape changes. Here we use an elastomer (PDMS sheet) confined between two rigid layers, which when locally heated using resistive heating expands into the micromolded channels, resulting into a massive relative displacement compared to the case of an unconfined geometry. This principle is used to make microfluidic valves which are electrically controlled (using a 3.3V-5V cellphone battery) and close in less than 100 ms. They operate within a power range of 140-160 mW generated by the specifically designed resistive heating element (in-house made ink) screen printed on the chip. We investigate the parameters of the heating element design, height dimensions and flow conditions through the valves. This technique helps us to make multiple valves along the fluidic pathway with arbitrary positioning. The size of these really help to make the devices portable as one does not need a separate controller for the actuating the valves.
DNMW03 14th May 2019
09:00 to 09:40
Daniel Joseph Needleman Structure, Mechanics, and Thermodynamics of Mixtures of Microtubules and Molecular Motors
The self-organization of the microtubule cytoskeleton underlies diverse cell biological processes, ranging from chromosome segregation to neuronal morphogenesis. In order to gain insight into these biological processes, and the properties of active matter more generally, we are studying the large-scale structure, mechanics, and thermodynamics of collections of microtubules and molecular motors in cell extracts and reconstituted systems of purified components. I will present our recent work characterizing spontaneous contractions, ordering, instabilities, and heat production in these systems.
DNMW03 14th May 2019
09:40 to 10:20
Jorn Dunkel Towards rationally designed active metamaterials
Recent advances in 3D printing and lithography have spurred rapid progress in the development of passive metamaterials. By interweaving simple subunits in intricate geometric arrangements, metamaterials can be custom designed to have many remarkable response features, from acoustic and photonic band gaps to auxetic behavior and topological robustness. In parallel, the last few years have seen the introduction of new classes of artificial and bio-inspired active materials based on colloidal and microbial suspensions or internally actuated gels. These non-equilibrium systems show great promise as components in autonomous soft robotic and microfluidic devices, and have reached a level of understanding where these applications can now be fruitfully developed. In this talk, I will discuss our recent work that aims to implement a computational framework for the inverse design of discrete active metamaterials. Building a network-based description, we will illustrate how optimized material structures can be used to harvest energy from correlated fluctuations [1,2], and outline basic design principles for active topolectrical circuits [3]. [1] Woodhouse et al, Phys Rev Lett 121: 178001, 2018 [2] Ronellenfitsch et al, Phys Rev Lett 121: 208301, 2018 [3] Kotwal et al, arXiv:1903.10130
DNMW03 14th May 2019
10:20 to 10:40
Nicholas Tito Exploiting entropy to enhance toughness in polymer gels with reversible crosslinks
Co-Authors: Costantino Creton, Cornelis Storm, Wouter Ellenbroek

Entropy is the daunting "second half" of thermodynamics, universally encountered yet often overlooked when designing molecular recipes for new soft materials and structures. This talk seeks to inspire a line of thought on how entropy can be harnessed as a central design element in soft polymeric materials, for imbuing adaptability, robustness, and functional uniqueness.

Highly elastic yet failure-resistant polymer gels with reversible crosslinks [1] will be showcased as a recent example where entropy provides unexpected functionality. Using a combination of theory, molecular simulation, and polymer self-consistent field theory for networks [2], I will discuss how entropy counter-intuitively leads to spatial clustering of reversible crosslinks around permanent crosslinks in the polymer gel. This entropy-induced order leads the gel to be less prone to failure, while maintaining its high degree of extensibility [3]. Practical guidelines will be outlined to optimise this design in experiment, along with a discussion of key kinetic and timescale considerations.

[1] Kean, Z. S.; et al. Adv. Mat. 2014, 26, 6013.
[2] Tito, N. B.; Storm, C.; Ellenbroek, W. G. Macromolecules 2017, 50, 9788.
[3] Tito, N. B.; Creton, C.; Storm, C; Ellenbroek, W. G. Soft Matter 2019, 15, 2190.
DNMW03 14th May 2019
11:10 to 11:50
Bianca Stroffolini Function spaces meet material science: Orlicz-Sobolev nematic elastomers
In the last decade, models for nematic elastomers and magnetoelasticity has been extensively studied. These models consider both an elastic term where a polyconvex energy density is composed with an unknown state variable defined in the deformed configuration, and a functional corresponding to the nematic energy (or the exchange and magnetostatic energies in magnetoelasticity) where the energy density is integrated over the deformed configuration. In order to obtain the desired compactness and lower semicontinuity, one has to face the regularity requirement that maps create no new surface. I'll discuss that this in fact the case for maps whose gradients are in an Orlicz class with an integrability just above the space dimension minus one. The results presented in this talk have been obtained in collaboration with Duvan Henao (Pontificia Universidad Cat\'olica de Chile).
DNMW03 14th May 2019
11:50 to 12:30
Carme Calderer Modeling and analysis of chromonic liquid crystal condensates
The discovery of the liquid crystal phases of DNA and their study has attracted the attention of many scientists, for several decades. These include the contributions by soft matter physicists such as Professor F. Livolant, that began in the mid-1970’s. On the other hand, the observation of clustering phenomena in lyotropic liquid crystals, analogous to that formed by DNA condensates and bacteriophage viral genome in a capsid domain, led John Lyndon to coin the chromonic denomination of the liquid crystals formed by plank-like molecules (2013). All these liquid crystals are found to form hexagonal columnar chromonic phases, although they differ in order of magnitude by a factor of 106. This presentation addresses modeling and analysis of bacteriophage viruses, the toroidal structures formed by condensed DNA in free solutions, and the analogous phenomena observed in lyotropic chromonic liquid crystals phases of materials with plank-like molecular shapes. Part of the presentation will focus on the experiments performed by ProfessorLavrentovich’s group on materials such as food dyes--sunset yellow--and anti-asthmatic drugs. A special feature determining the arrangement of DNA in a capsid is the dominant contribution of the elastic energy penalizing distortion of the cross sections perpendicular to the column axis. The central mathematical problem is formulated as a free boundary problem for the Oseen-Frank and Ericksen’s energies, where the domain and the vector (or tensor) field are unknown. The admissible set includes volume constraints as well as those expressing the high resistance of the chromonic structures to splay and twist deformation. The first part of the presentation will involve general geometries of the domain, resorting to earlier analyses of liquid crystal droplets. We will subsequently show that minimizers of the bending dominated constrained energy have toroidal shapes. Moreover, we will show that axisymmetric configurations lead to families of polyconvex energies for which minimization can be established by standard methods of calculus of variations. Moreover, in the case of bacteriophage viruses, we will identify the absolute minimizer as the coiling DNA configuration. We will conclude the presentation with the discussion of a numerical algorithm aimed at the design of viruses for applications to drug delivery and nanotransport.
DNMW03 14th May 2019
13:30 to 14:10
Julia Yeomans Bacteria: self-motile liquid crystals?
We discuss recent work showing that the concepts of liquid crystal physics can give insight into the behaviour of colonies of bacteria. Dense bacterial layers show local nematic order and the appearance of associated topological defects can act as preferential sites for biofilm formation. Moreover less dense swimming bacterial suspensions can be focused by an underlying passive nematic, a step towards exploiting their energy for microfluidic transport.

Women in Materials Science

DNMW03 14th May 2019
14:10 to 14:50
Elisabetta Matsumoto Twisted topological tangles: or the knot theory of knitting
Shashank Markande, Michael Dimitriyev, Krishman Singal and Elisabetta Matsumoto Imagine a 1D curve, then use it to fill a 2D manifold that covers an arbitrary 3D object – this computationally intensive materials challenge has been realized in the ancient technology known as knitting. This process for making functional materials 2D materials from 1D portable cloth dates back to prehistory, with the oldest known examples dating from the 11th century BCE. Knitted textiles are ubiquitous as they are easy and cheap to create, lightweight, portable, flexible and stretchy. As with many functional materials, the key to knitting’s extraordinary properties lies in its microstructure. At the 1D level, knits are composed of an interlocking series of slip knots. At the most basic level there is only one manipulation that creates a knitted stitch – pulling a loop of yarn through another loop. However, there exist hundreds of books with thousands of patterns of stitches with seemingly unbounded complexity. The topology of knitted stitches has a profound impact on the geometry and elasticity of the resulting fabric. This puts a new spin on additive manufacturing – not only can stitch pattern control the local and global geometry of a textile, but the creation process encodes mechanical properties within the material itself. Unlike standard additive manufacturing techniques, the innate properties of the yarn and the stitch microstructure has a direct effect on the global geometric and mechanical outcome of knitted fabrics.
DNMW03 14th May 2019
15:20 to 16:00
Anton Souslov Odd elasticity in soft active solids
An active material is either a solid or a fluid in which microscopic constituents convert energy into motion. These microscopic engines can be organised to output collective macroscopic work. For active solids, we show that the theory of elasticity can be modified to describe this work-extraction process. This talk focuses on the specific example of how an antisymmetric (or odd) component of the elastic tensor leads to the extraction (or injection) of work during quasi-static cycles of elastic deformations. Such materials can be designed based on active mechanical components that include sensors and actuators. Inside the material, work-extraction cycles manifest themselves in signal propagation: in an overdamped active solid, elastic waves propagate via a balance between energy injection and dissipation. In addition, activity can be measured via static deformations, including activity-induced auxetic behaviour. This theory of odd elasticity suggests design principles for emergent autonomous materials in which work is locally injected, transported, and then extracted.
DNMW03 14th May 2019
16:00 to 16:40
Martin Copic Q-tensor model of twist-bend and splay nematic phases
The twist-bend nematic phase is characterized by a conically twisting director and by a dramatic softening of the bend elastic constant. The instability towards bend can theoretically also induce a splay -bend phase with a bend-splay modulation along the director. Recently we found another modulated nematic phase  where the splay elastic constant tends to zero, resulting in a splay modulation perpendicular to the director. These phases can be modeled by a single Q-tensor free energy with a  term that breaks the degeneracy between the splay and bend elastic constant and with a flexoelectric coupling of the divergence of the Q-tensor with polarization.

Martin Čopič and Alenka Mertelj - Insitute J. Stephan, Ljubljana, Slovenia
DNMW03 14th May 2019
16:40 to 17:00
Mikhail Osipov Orientational ordering and self-organisation of nanoparticles in liquid crystal and polymer nanocomposites
Nematic liquid crystals (LCs) and block copolymers doped with nanoparticles possess a number of interesting properties. In particular, anisotropic nanoparticles are orientationally ordered in the boundary region between the blocks [1-3] and a small concentration of nanoparticles can shift the transition temperatures between different phases, orientational ordering of nanoparticles is responsible for the enhanced dielectric anisotropy of the composite lamellae and hexagonal phases which opens a possibility to align block copolymers by external fields. This may enable one to solve various application problems. We first summarise the results of a molecular theory of nematic LCs doped with anisotropic nanoparticles and describe the effect of nanoparticles on the N-I phase transition, the nematic order parameter and consider the formation of chains of polar nanoparticle [4-7]. We then present the results of a molecular theory of the induced orientational order of anisotropic nanoparticles in the lamellae and in the hexagonal phase of a diblock copolymer taking into anisotropic interaction between nanoparticles and the polymer chains. Numerical concentration and orientational order parameter profiles are presented for different values of the model parameters including the strength of the anisotropic interaction. We also present the results of the general mean-field theory which enables one to describe both the effect of segregation of monomers between different blocks on the orientational order of nanoparticles and the effect of nanoparticles on the stability of different phases..
Finally we present the results of the computer simulations of the lamellae and hexagonal copolymer nanocomposites doped with nanoparticles of different length and affinity, and the simulated concentration and order parameter profiles are compared with theoretical results [1,3]. We also discuss the corresponding phase diagrams which illustrate how the nanoparticles may effect the phase behaviour of block copolymers.
References
[1] Osipov, M. A., Gorkunov, M. V., Berezkin, A. V., Kudryavtsev, Y. V., Phys. Rev. E, 97, 042706 (2018)
[2] M.A. Osipov and M.V. Gorkunov, Eur.Phys.J., 39, 126 (2016)
[3] A.V. Berezkin, Y.V. Kudryavtsev, M.V. Gorkunov, and M.A. Osipov, J. Chem. Phys., 146, 144902 (2017) [4] M.V. Gorkunov and M.A. Osipov, Soft Matter, 7, 4348 (2011)
[5] M.A. Osipov and M.V. Gorkunov, ChemPhys.Chem. 15, 1496 (2014)
[6] M.A. Osipov and M.V. Gorkunov, Phys. Rev. E , 92, 032501 (2015)
[7] Osipov, M. A. and Gorkounov, M. V. in Liquid Crystals with Nano and Microparticles. Lagerwall, J. P. F. and Scalia, G. (eds.). Singapore: World Scientific Publishing Company, 2016.

DNMW03 15th May 2019
09:00 to 09:40
Monica Olvera de la Cruz Control of Magnetoelastic Matter
Magnetic materials hold tremendous potential for precision control of matter due to their tunable interactions in dynamic magnetic fields. Flexible superparamagnetic filaments and membranes under the influence of precessing magnetic fields, for example, can exert controllable forces to generate microscopic actuation. We characterize the resulting changes of shapes in terms of their material parameters, as well as of the strength of the magnetic field. In particular, we show how by controlling the magnetic field, open membranes may form either rippled or helicoidal surfaces, whereas closed membranes can buckle into convex and concave shapes with specific symmetries. Shape control via magnetic fields is also discussed in three-dimensional gels reinforced with ferromagnetic matter. These systems might be suitable for constructing devices with controllable conformational changes such as artificial muscles.
DNMW03 15th May 2019
09:40 to 10:20
Daphne Klotsa A touch of non-linearity: mesoscale swimmers and active matter in fluids
Living matter, such as biological tissue, can be seen as a nonequilibrium hierarchical assembly of assemblies of smaller and smaller active components, where energy is consumed at many scales. The functionality and versatility of such living or “active-matter” systems render it a promising candidate in a discussion on the optimal design of soft matter. While many active-matter systems reside in fluids (solution, blood, ocean, air), so far, studies that include hydrodynamic interactions have focussed on microscopic scales in Stokes flows, where the active particles are <100μm and the Reynolds number, Re <<1. At those microscopic scales viscosity dominates and inertia can be neglected. However, what happens as swimmers slightly increase in size (say ~0.1mm-100cm) or as they form larger aggregates and swarms? The system then enters the intermediate Reynolds regime where both inertia and viscosity play a role, and where nonlinearities in the fluid are introduced. In this talk, I will present a simple model swimmer used to understand the transition from Stokes to intermediate Reynolds numbers, first for a single swimmer, then for pairwise interactions and finally for collective behavior. We show that, even for a simple model, inertia can induce hydrodynamic interactions that generate novel phase behavior, steady states and transitions.
DNMW03 15th May 2019
10:20 to 10:40
Anja Pusovnik Liquid crystal metamaterials from nematic colloidal platelets
Metamaterials are artificial materials with properties otherwise not existing in nature. This is achieved through the design of its constituent building blocks, which are generally several times smaller than the operating wavelength. An interesting route for the fabrication of photonic metamaterials is their self-assembly in liquid crystals. Here, we firstly determine the optimal geometrical parameters of a single split ring resonator (SRR) colloidal particle in order to achieve the stability of the 2D and 3D SRR structures in liquid crystals using free energy calculations. Then we focus on the optical response of such a composed material, notably the resonances in the transmissivity spectra, and tunability of optical properties of the SRR colloidal crystal with external fields.
DNMW03 15th May 2019
11:10 to 11:50
Miha Ravnik Design of passive and active passive nematic defects
Complex –passive or active- nematic fluids are characterised by internal orientational order, which upon tuning or frustration, can exhibit topological defects. The type of defects and their role naturally depend on dimensionality of the system, but importantly also on the geometry, confinement, flow, driving or even activity. Here, we present design of topological defects in passive and active nematic complex fluids – forming umbilic defects, singular loops, point defects and disclinations. Specifically, we show in passive nematics how confinement in the form of complex geometry and fractal surfaces can lead to formation of various defect-based nematic profiles, including exhibiting high-elastic multipoles. In active nematics, we show defect profiles in three-dimensional active nematic droplet, also highlighting the role of different surface coupling regimes.
DNMW03 15th May 2019
11:50 to 12:30
Davide Marenduzzo Self-assembly of liquid crystal mixtures: cubic fluid cylinders, elastic emulsions and colloid-active gels composites
In this talk we will show results from lattice Boltzmann simulations probing the behaviour of soft matter mixtures based on a liquid crystalline host (which can be either passive or active).   In the first part of the talk we will investigate the behaviour of a phase-separating mixture of a blue phase I liquid crystal with an isotropic fluid. The resulting morphology is primarily controlled by an inverse capillary number, setting the balance between interfacial and elastic forces. When this dimensionless number and the concentration of the isotropic component are both low, the blue phase disclination lattice templates a novel cubic array of fluid cylinders. In different regions of parameter space, we find elastic emulsions which coarsen very slowly, rewiring the blue phase disclination lines as they do so.   In the second part of the talk, we will study the dynamics of a dispersion of passive colloidal particles in an active nematic host. We find that activity induces a dynamic clustering of colloids even in the absence of any preferential anchoring of the active nematic director at the particle surface. When such an anchoring is present, active stresses instead compete with elastic forces and re-disperse the aggregates observed in passive colloid-liquid crystal composites.
DNMW03 15th May 2019
13:30 to 14:30
Tom Lubensky Colloquium: Review of the 2019 NAS Decadal Survey on Materials Research
At the request of the US National Science Foundation (NSF) and the Department of Energy (DOE), the National Academies of Sciences, Engineering and Medicine undertook a broad study of the current status and promising future directions of materials research in the United States.  This talk will present an overview of this report.
DNMW03 16th May 2019
09:00 to 09:40
Igor Musevic Topological defect formation in a nematic undergoing an extreme temperature quench
The Kibble-Zurek mechanism (KZM) describes the formation of topological defects during the rapid crossing of a second-order phase transition. Several experiments have been performed using nematic liquid crystals, with the aim being to observe the KZM. Most of the experiments report on the late-stage coarsening dynamics of the defect tangle, whereas the mechanism of defect formation in the early stage is still not convincingly demonstrated and lacks solid evidence. We have designed an experiment that can generate an extremely rapid crossing of the isotropic-nematic phase transition which currently achieves cooling rates in excess of 40,000 K/s, with cooling rates as fast as 1,000,000 K/s being achievable in principle. We have developed a novel illumination technique that can take instantaneous images of the quenched sample area with an exposure time of 20 nanoseconds. We have also developed a technique to measure the time dependence of the temperature during the quench. This makes it possible to study defect formation with very accurate timing and an accurate measurement of the local temperature during the quench. The robustness of the experiment allows for several thousand repetitions, which can greatly improve the statistics of the measurements. The current status of the experiments is reported. Coauthored by Uros Jagodic and Anna V. Ryzhkova.
DNMW03 16th May 2019
09:40 to 10:20
Sriram Ramaswamy Fluid flocks with inertia
I will show that inertia can stabilise flocks in bulk fluid provided their order is vectorial, not nematic, and their active stresses are extensile. Among our results is a flocking transition driven by inertia, and two kinds of turbulent states, one of which is ordered "phase turbulence". This work was done with Rayan Chatterjee, Aditi Simha and Prasad Perlekar.
DNMW03 16th May 2019
10:20 to 10:40
Ziga Kos Design of micro-confinement for controlled structure formation in non-equilibrium nematic fluids
Nematic fluids can be designed for a specific purpose by changing their chemical structure, or also by changing the properties of the confinement. I will discuss how the interface between orientational structures in non-equilibrium nematic fluids in microfluidic confinement is affected by the viscoelastic properties of the nematic, flow rate, and shape of the channels [1]. Furthermore, by combining multiple channels into junctions, we were able to create an advanced platform for generation of various topological states, where the strength of the topological singularity in the nematic orientational field is related to the strength of the stagnation point in the junction [2]. The position and strength of the nematic defect can be tuned by the number of channels meeting in a junction and the flow rates through the channels. Flow of confined nematics is of further interest as the nematic structure can allow for the control of the transport properties in porous materials, or the external field-induced modulation of the nematic structure can be designed as a local flow pump, which is a contribution towards using the internal structure of fluids for advanced microfluidic techniques. [1] T. Emeršič, R. Zhang, Ž. Kos, S. Čopar, N. Osterman, J. J. de Pablo, and U. Tkalec, Sculpting stable structures in pure liquids, Sci. Adv. 5, eaav4283 (2019). [2] L. Giomi, Ž. Kos, M. Ravnik, and A. Sengupta, Cross-talk between topological defects in different fields revealed by nematic microfluidics, Proc. Natl. Acad. Sci. 114, E5771 (2017).
DNMW03 16th May 2019
11:10 to 11:50
Tom Lubensky Elasticity and Response in Mechanical Topological Lattices
Ball-and-Spring lattices that have a perfect balance between the number of degrees of freedom and the number of constraining springs under periodic boundary conditions have topologically protected zero-energy surface modes and nonlinear elastic Guest-Hutchinson modes. This talk will provide an overview of these modes in various model systems, including one whose excitation spectrum matches that of a quantum model on a honeycomb lattice introduced by Kitaev. It will also discuss bulk and surface excitation in systems in which the number constraining springs exceeds the number of degrees of freedom.
DNMW03 16th May 2019
11:50 to 12:30
John Ball Some remarks on mathematical theories of liquid crystals
The talk will concern two different topics. First a quick new proof will be given of a result of Fatkullin & Slastikov (2005), Liu, Zhang & Zhang (2005) (see also Zhou et al (2005)), to the effect that stationary solutions to the Onsager equation with the Maier-Saupe interaction are radially symmetric. Second, a description will be given of joint work with Lu Liu on exterior problems in the 2D one-constant Oseen-Frank theory.
DNMW03 16th May 2019
13:30 to 14:10
Claudio Zannoni Bottom Up Modelling of Liquid Crystals and Device Applications
Liquid crystals (LC), with their unique combination of physical properties, offer an increasing number of novel fascinating applications ranging from optical and haptic displays to organic electronics devices, waveguides etc... The variety of observables of interest, and the complexity of LC mesogens require their bottom up modelling and computer simulations to be performed at different (micro-, nano- and Angstrom) length scales, that can be tackled respectively with lattice, molecular and atomistic approaches. In the talk we plan to present some recent examples of application of these different simulations. In particular, we show that Monte Carlo simulations of lattice models [1] can help investigating the structure of defects in photopatterned hybrid nematic films with different in plane surface order [2]. A much more detailed, atomistic level description is instead required to try and understand the role of liquid crystal ordering, if present, in rationalizing charge mobility in organic semiconductors and, possibly in designing better organic electronic materials. We shall discuss, in particular, the proposed hypothesis [3] that a smectic E organization is key to the unusually high performance of certain organic seminconductors, e.g. Ph-BTBT-C10 [4].
[1] C. Chiccoli, L. R. Evangelista, P. Pasini, G. Skačej, R. Teixeira de Souza and C. Zannoni, Scientific Reports, 2018, 8, 2130.
[2] C. Chiccoli, P. Pasini, , C. Zannoni, G. Skačej, H. Yoshida,T. Hiroshima, K. Sunami, T. Ouchi, and M. Ozaki, submitted (2018)
[3] H. Iino, T. Usui and J.-i. Hanna, Nature Comm. 6, 6828 (2015).
[4] A. Baggioli, M. Casalegno, G. Raos, L. Muccioli, S. Orlandi, and C. Zannoni submitted (2019).
DNMW03 16th May 2019
14:10 to 14:50
Simon Copar Flow-induced states in channel-confined nematics
Anisotropy of liquid crystals couples their orientational order to velocity shear, and consequently, induces different regimes in flows with different strengths. A flow-aligning nematic liquid crystal, confined to a channel with homeotropic surface alignment, is known to undergo a transition from a homeotropic to a flow-aligned state. I will present a more detailed view of this behaviour, including a hidden pre-transitional state with broken chiral symmetry and dynamics of flow-aligned domain under uniform or alternating flow. Additionally, I will present a Landau model that captures the stability of different states with respect to material parameters.
DNMW03 16th May 2019
15:20 to 16:00
Apala Majumdar Nematic Pattern Formation on 2D Polygons - a Landau de Gennes study
DNMW03 16th May 2019
16:00 to 16:40
Lidia Mrad Constrained Energy Minimization for Bent-Core Liquid Crystals
One of the important applications of liquid crystal materials is their use in optical and display devices. There are several phases of liquid crystals, some of which promise more efficient and less expensive optical devices than others. A recently discovered phase is made up of bow-shaped molecules, a characteristic that endows them with spontaneous ferroelectricity. Under the effect of an applied electric field, two competing mechanisms of switching can be detected in the tilted structure of these materials. An important question in this setup is how the dominant mechanism - switching here - is affected by specific system parameters. We formulate the model as an energy minimization problem allowing us to use several variational tools in its analysis. We emphasize how we can deal with challenges that arise from constraints and nonlinearities peculiar to this problem. Our results address existence and uniqueness of solutions to the ensuing partial differential equations, which in turn shed light on the physical mechanisms observed.
DNMW03 16th May 2019
16:40 to 17:00
Katherine Macmillan Materials from Colloidal Particles using Optical Fields
Katherine A. Macmillan, Erick Sarmiento and Stefan U. Egelhaaf The interaction of light with colloidal particles has been widely exploited in optical tweezers [1]. In addition, multiple traps or extended potential energy landscapes (optical fields) have been applied using periodic interference patterns, speckle patterns created using ground glass and freely configurable patterns created using spatial light modulators [1, 2]. The capability of these optical potential energy landscapes to trap multiple colloidal particles in a designed structure has yet to be fully explored. In order to pursue this goal, here we study a two dimensional colloidal glass in a periodic potential. We find that a periodic potential with a periodicity commensurate with the lattice spacing for a hexagonally close packed array can induce the particles to crystallise. We have investigated the influence of parameters describing the potential on the formation of crystals from disordered structures. Upon the removal of the periodic potential, the colloidal particles can return to a more disordered state rendering the crystal structures only transient. The possibility of fixing this transient state by attaching the particles together has begun to be investigated. In the future, we aim to use optically-created potential energy landscapes to imprint a structure on a dispersion of colloidal particles that can be fixed by covalently bonding the particles together. [1] Richard D. L. Hanes, Matthew C. Jenkins and Stefan U. Egelhaaf, Review of Scientific Instruments, 2009, 80, 083703 [2] F. Evers, R.D.L. Hanes, C. Zunke, R.F. Capellmann, J. Bewerunge, C. Dalle-Ferrier, M.C. Jenkins1, I. Ladadwa, A. Heuer, R. Castañeda-Priego and S.U. Egelhaaf, European Physical Journal Special Topics, 2012, 222, 2995–3009
DNMW03 17th May 2019
09:00 to 09:40
Man-made nano- and micro-motors are key to many future applications. I will describe highly reconfigurable self-assembly of colloidal micro-motors that exhibit a repetitive rotation when immersed in a liquid crystal and powered by a continuous exposure to unstructured ~1nW light. A monolayer of self-assembled azobenzene molecules defines how the liquid crystal’s optical axis mechanically couples to the colloidal particle’s surface, as well as how they jointly rotate as the light’s polarization changes. The rotating particle twists the liquid crystal, which, in turn changes polarization of the light traversing it. The resulting feedback mechanism spontaneously yields a continuous opto-mechanical cycle and drives the unidirectional particle spinning, with handedness and frequency robustly controlled by polarization and intensity of light. I will discuss how this may enable new forms of active matter and self-assembled machines.
DNMW03 17th May 2019
09:40 to 10:20
Antonio DeSimone Reconfigurable surfaces with controlled stretching and shearing: from biological templates to engineering devices
In recent years, we have studied locomotion and shape control in Euglena gracilis using a broad range of tools ranging from theoretical and computational mechanics, to experiment and observations at the microscope, to manufacturing of prototypes.

As a concrete example, the behavior of Euglena gracilis is particularly interesting.This unicellular protist is particularly intriguing because it can adopt different motility strategies: swimming by flagellar propulsion, or crawling thanks to large amplitude shape changes of the whole body (a behavior known as metaboly).

We will survey our most recent findings [1-4] within this stream of research.

This is joint work with M. Arroyo, G. Cicconofri, A. Lucantonio, and G. Noselli, and is supported by ERC Advanced Grant 340685-MicroMotility.

References
[1] Rossi, M., Cicconofri, G., Beran, A., Noselli, G., DeSimone, A.: “Kinematics of flagellar swimming in Euglena gracilis: Helical trajectories and flagellar shapes”, Proceedings of the National Academy of Sciences USA 114(50), 13085-13090 (2017).
[2] Noselli, G., Beran, A., Arroyo, M., DeSimone, A.: “Swimming Euglena respond to confinement with a behavioral change enabling effective crawling”, Nature Physics, 2019.
[3] Noselli, G., Arroyo, M., DeSimone, A.: “Smart helical structures inspired by the pellicles of euglenids”, J. Mech Phys Solids 123, 234-246 (2019).
[4] Caruso, N., Cvetkovic, A., Lucantonio, A., Noselli, G., DeSimone, A.: “Spontaneous morphing of equibiaxially pre-stretched elastic bilayers: The role of sample geometry”, Int J Mech Sci 149, 481-486 (2018).
DNMW03 17th May 2019
10:20 to 10:40
Henrik Ronellenfitsch Inverse design of discrete mechanical metamaterials
Mechanical and phononic metamaterials exhibiting negative elastic moduli, gapped vibrational spectra or topologically protected modes enable precise control of structural and acoustic functionalities. While much progress has been made in their experimental and theoretical characterization, the inverse design of mechanical metamaterials with arbitrarily programmable spectral properties and mode localization still poses an unsolved problem. Here, we present a flexible computational inverse-design framework that allows the efficient tuning of one or more gaps at nearly arbitrary positions in the spectrum of discrete phononic metamaterial structures. The underlying algorithm optimizes the linear response of elastic networks directly, is applicable to ordered and disordered structures, scales efficiently in 2D and 3D, and can be combined with a wide range of numerical optimization schemes. We illustrate the broad practical potential of this approach by designing mechanical bandgap switches that open and close pre-programmed spectral gaps in response to an externally applied stimulus such as shear or compression. We further show that the designed structures can host topologically protected edge modes, and validate the numerical predictions through explicit 3D finite element simulations of continuum elastica with experimentally relevant material parameters. Generally, this network-based inverse design paradigm offers a direct pathway towards manufacturing phononic metamaterials, DNA origami structures and topolectric circuits that can realize a wide range of static and dynamic target functionalities. Joint work with Norbert Stoop, Josephine Yu, Aden Forrow, Joern Dunkel
DNMW03 17th May 2019
11:10 to 11:50
Douwe Jan Bonthuis Charging of neutral solutes in water
Owing to the small length scales involved, aqueous interfaces dominate the properties of colloidal materials suspended in water. Surface charges, in particular, control the stability of colloidal suspensions and the self-assembly and organization of nanoparticles. Apart from charging by surface groups, ions and protons adsorb at the surfaces of colloids, lipid membranes and biological molecules, affecting their electrostatic and hydrodynamic interactions.

We study the interfacial structure of the aqueous interfaces of oil droplets, air bubbles and solid surfaces. A combination of analytical work, molecular dynamics simulations and continuum theory allows for direct comparison to experimental results for surface tensions, conductivities and electrokinetic mobilities.
DNMW03 17th May 2019
11:50 to 12:10
In this talk, we address the building of finite-time adiabatic processes at the mesoscale, i.e. processes in which the average heat exchange between the system and its surroundings vanishes. Specifically, we consider a Brownian particle trapped by a harmonic potential and immersed in a fluid. Therein, we analyse some general properties and, in particular, we show that there emerges a minimum time for connecting two equilibrium states with such a finite-time adiabatic process. Also, we look into a different optimisation problem, namely that of the final temperature for a given connection time. Interestingly, we find out that this second problem is closely related to the first one: both of them are controlled by the same function. Finally, we discuss some perspectives for future work.

(In collaboration with Carlos A. Plata, David Guéry-Odelin and Emmanuel Trizac)
DNMW03 17th May 2019
12:10 to 12:30
Dwaipayan Chakrabarti Colloids Get Creative: Key to Open Crystals
Open crystals are sparsely populated periodic structures, which, when composed of colloidal particles, are appealing for their variety of applications, for example, as photonic materials, phononic and mechanical metamaterials, as well as porous media [1-4]. Programming self-assembly of colloidal particles into open crystals has proved a long-standing challenge due both to the mechanical instability and lack of kinetic accessibility that colloidal open crystals typically suffer from. Building on our recent work [5-7], I will here introduce a hierarchical self-assembly scheme for triblock patchy particles to address the challenges met with programming self-assembly into colloidal open crystals [8].  The presentation will demonstrate in silico the hierarchical self-assembly of colloidal open crystals via what we call closed clusters, which stop to grow beyond a certain size in the first stage and are thus self-limiting [8].  Our designer patchy particles are spherical in shape, having two attractive patches at the poles across a charged middle band – a close variant of those synthesised recently [9]. By employing a variety of computer simulation techniques, I will show that the design space supports different closed clusters (e.g. tetrahedra or octahedra with variable valences) en route to distinct open crystals. Our design rules thus open up the prospects of realising a number of colloidal open crystals from designer triblock patchy particles, including, most remarkably, a diamond crystal [8], much sough-after for is attractive photonic applications. The relevant photonic band structure will be presented.

References
[1] J. D. Joannopoulos, P. R. Villeneuve and S. Fan, Nature 1997, 386, 143.
[2] K. Aryana and M. B. Zanjani, J. Appl. Phys. 2018, 123, 185103.
[3] X. Mao and T. C. Lubensky, Annu. Rev. Condens. Matter Phys. 2018, 9, 413.
[4] X. Mao, Q. Chen and S. Granick, Nature Mater. 2013, 12, 217.
[5] D. Morphew and D. Chakrabarti, Nanoscale 2015, 7, 8343.
[6] D. Morphew and D. Chakrabarti, Soft Matter 2016, 12, 9633.
[7] D. Morphew and D. Chakrabarti, Nanoscale 2018, 10, 13875.
[8] D. Morphew, J. Shaw, C. Avins and D. Chakrabarti, ACS Nano 2018, 12, 2355.
[9] Q. Chen, S. C. Bae and S. Granick, J. Am. Chem. Soc. 2012, 134, 11080.