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SoftMatterMeeting2

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Abstracts Talks 

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Vincenzo Vitelli (Leiden)
Sound propagation in jammed solids
We study three dimensional jammed packings of soft spheres as a function of their packing fraction and obtain the energy diffusivity, a spectral measure of transport that controls sound propagation and thermal conductivity. The crossover frequency between weak and strong phonon scattering gradually shifts to zero as the system is decompressed towards the critical packing fraction at which rigidity is lost. We present a scaling analysis backed by numerics that relates the packing fraction dependence of the crossover frequency to the scaling of the shear modulus with compression. Below the crossover, the diffusivity displays a power-law divergence with inverse frequency consistent with Rayleigh law, which suggests that the vibrational modes are primarily transverse waves, weakly scattered by disorder. Above it, a large number of modes appear whose diffusivity plateaus at a nearly constant value before dropping to zero above the localization frequency.

Rut Besseling (Edinburgh)
Shear localization and flow instability in colloidal glasses
We show, via simultaneous imaging and rheology, that hard sphere colloidal glasses in steady shear exhibit rate dependent and continuous shear localization. Near yielding and deep in the glass phase, shear is typically strongly localized near the walls and vanishing in the bulk, while at larger stress the nonlinearity smoothly decreases. While the data show apparent similarity with predictions for cooperative glassy flow, we argue instead that the phenomenology is due to small concentration gradients. We show theoretically that the system is unstable to growth of concentration fluctuations for vey moderate shear rates. These become 'frozen' in near yielding, which accounts for the variety of experimental observations. The instability is generic for systems with (i) a sufficient shear-induced contribution to the pressure and (ii) a strong concentration dependence of the characteristic (yield/flow) stress

Simeon Stoyanov (Unilever)
Learning from nature to design food microstructures
As long as we have existed on the planet, natural foods have been derived form locally available plant and animal sources. Over the last few millennia man has become adept in developing ways of breeding, growing, harvesting, processing and preserving this natural material to enable widespread use across the globe. Many of these raw materials have become commodities and are used globally. In parallel, our knowledge in medicine and nutrition has grown rapidly and we now have good understanding of what our raw materials deliver in terms of nutrition and health benefits and where and how they are adsorbed during digestion. Since Nature evolved plants and animals to breed more plants and animals and not necessarily to provide convenient food for mankind, there is a technical challenge for the physical-chemists to understand and manipulate the natural raw materials used in consumer products to provide nutritious, healthy and good tasting offerings with functional health benefits.
In this talk, I will give two examples, where a Soft Condensed Matter approach is critical for being able to learn from Nature and to apply this knowledge to design novel food products. The first example is based on a study of how plants structure water internally and how we can use these natural structures and knowledge to develop low fat, healthy products such as mayonnaise.  In the second example we will look at another nature structure: plant pollens. The outer layer (exine) of spores and pollen grain of mosses and ferns is composed largely of a substance known as sporopollenin. It has been described as “one of the most resistant organic materials of direct biological origin found in nature and in geological samples” as intact spores are found in ancient sedimentary rocks, dated at least 500 million years old. It is also known that certain plant pollens or other spores are capable of crossing the gut wall, largely intact, but is then destroyed within the blood stream, thereby releasing its contents. It has been recently recognised that sporopollenin can be used to develop novel drug and nutraceutical delivery systems, providing an injections alternative, based on oral (food) delivery, where drugs, foods supplements and actives can be delivered exactly targeting the lower gut.

Laura Filion (Utrecht)
Efficient Method for Predicting Crystal Structures: Variable Box Shape Simulations
The ability of atomic, colloidal, and nanoparticle systems to self organize into crystal structures makes the prediction of crystal structures in these systems an important challenge for science. In a recent Nature Review Article, Scott Woodley and Richard Catlow claim "The prediction of structure at the atomic level is one of the most fundamental challenges in condensed matter science." Thus it is unsurprising that the subject has received much attention from the scientific community over the last several decades. The question itself is deceivingly simple: assuming that the underlying interaction between constituent particles is known, which crystal structures are stable?
In this talk we describe an efficient and robust method for predicting crystal structures based on variable box shape NPT Monte Carlo simulations. We apply the method to various hard, soft, attractive, and anisotropic interactions, as well as to binary mixtures demonstrating the effectiveness of the method for a wide range of systems. For binary hard-sphere mixtures, star polymers,and binary Lennard-Jones mixtures, the crystal structures predicted by this algorithm are consistent with literature, thereby providing confidence in the robustness of the method.

James R. T. Seddon (Twente)
Surface Nanobubbles
Surface nanobubbles are nanoscopic, spherical cap bubbles that form at the solid-liquid interface. They differ from normal bubbles in that they appear stable to diffusion, lasting at least 10-11 orders of magnitude longer than expected. Moreover, nanobubbles grow during this time, and show no obvious evidence of imminent collapse. Results are presented of an experimental investigation into the effects of liquid temperature and gas concentration on the formation of surface nanobubbles. Previously, both effects have always been investigated together, whilst here we independently investigate the role of each.

Esther van den Pol (Utrecht)
Biaxial phases in colloidal goethite dispersions
Colloidal dispersions of goethite (a-FeOOH) contain boardlike crystallites (of volume LxWxT) which form nematic, smectic and columnar liquid crystals. These goethite particles also display peculiar magnetic properties combining a considerable permanent magnetic moment along their length (L) with an induced magnetic moment along the shortest particle dimension (T). Therefore, particles align parallel to a small external magnetic field but perpendicular to a large magnetic field. We here present a goethite dispersion, for which L/W — W/T, so that a biaxial nematic phase might be expected. Small angle X-ray scattering (SAXS) measurements revealed that both a biaxial nematic and biaxial smectic phase are observed in this system. The biaxial phases could be aligned in different directions by applying a very small magnetic field which allowed us to construct a complete picture of the biaxial phases.

 

Abstracts Soundbites

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Andriy Kyrylyuk (Utrecht)
Random Packing of Spheres in Confined Geometries
The conventional Bernal random packing of spherical particles, which occupy around 64% of the volume, is related to a large collection of particles. In this case all the finite-size effects due to a finite number of particles and a finite system size can be neglected. However, in real systems the packing always consists of a certain amount of particles that are confined in a box with some walls. The question then is what is the effect of confinement on the Bernal random close packing density as well as on the microstructure of the packing. Here, we study random packing of spheres in a cubic box with two (quasi 1D), four (quasi 2D) and six confining walls (quasi 3D confinement) opposite to each other. We find that confinement lowers the packing density but at the same time hardly changes the microstructure of the random packing in the bulk of the box. The crystallization is found to be limited to one layer of particles near the walls.

Paulina Skrzeszewska (Wageningen)
Collagen-inspired self aggregating materials

Duc Nguyen (UvA Amsterdam)
Critical Casimir Effect provides direct Control of Colloidal Interactions
The Casimir Effect is a celebrated phenomenon in quantum physics. It manifests itself as the effective attraction between two dielectrics brought close to each other to confine fluctuations of the electromagnetic field. A similar force arises between two surfaces in a liquid mixture close to its critical point: Confinement of critical fluctuations of the liquid results in an attractive force between the walls, when the wall separation is of the order of the correlation length of the liquid. We use this effect for a fine control of colloidal interactions. The temperature dependence of the correlation length allows us to `freeze' a colloidal gas into a colloidal liquid, and a liquid into a solid. This offers novel opportunities for the assembly of micro- and nanomaterials. I will present recently developed optically transparent systems that allow use of conventional light scattering and confocal microscopy to study Critical Casimir-mediated particle assembly.

Marc Lemmers (Wageningen)
Multi-responsive reversible gels based on charge-driven assembly
In my soundbite I will show that it is possible to produce a new class of multi-responsive reversible gels based on charge-driven co-assembly of two water soluble components; an ABA triblock copolymer with hydrophilic middle-block and polyelectrolyte end-blocks, and an oppositely charged homopolymer.
I will show that these reversible gels are truly multi-responsive. The viscosity of these gels increases strongly as function of the total polymer concentration: six orders of magnitude with a four-fold increase in concentration. Contrary to this, the viscosity can be drastically decreased again by the addition of a little salt. Besides concentration and ionic strength, the gel properties can also be drastically altered by changing temperature, charge composition or pH.

Peter Prinsen (Leiden)
Multi-shell structures of virus coat proteins
Under conditions of low ionic strength and a pH ranging between about 3.7 and 5.0, solutions of purified coat proteins of cowpea chlorotic mottle virus (CCMV) form spherical multi-shell structures in the absence of viral RNA. The outer surfaces of the shells in these structures are negatively charged whereas the inner surfaces are positively charged due to a disordered cationic N-terminal domain of the capsid protein, the arginine-rich RNA-binding motif that protrudes into the interior.  We argue that the main forces stabilizing these multi-shells are counterion release combined with a lower charge density in the RNA-binding motif region of the outer shells due to their larger radii of curvature, which compensate for the outer shells not being able to adopt the smaller, optimal, radius of curvature of the inner shell. This explains why the structures are only stable at low ionic strengths at pHs for which the outer surface is negatively charged and why the larger outer shells are not observed separately in solution. The spacing between shells is determined mainly by the entropic elasticity of the RNA-binding motifs.

Burak Eral (Twente)
Asymetric diffusion in micron size closed cylinders
In this work, we study dynamics/diffusion of colloidal particles (PT) confined in 3D micron scale cylinders embedded in a multilayer microfluidic device measured by Particle tracking. We show that in the vicinity of the walls total diffusion is hindered furthermore this decrease is asymmetric for diffusion coefficient in radial direction (D?) i.e. perpendicular to wall and tangential direction (D?) parallel to wall. Behavior observed in cylindrical cavity is significantly different from a single particle in the vicinity of flat solid wall.

Joost de Graaf (Utrecht)
Towards Adsorption Dynamics of Colloids at the Liquid-Liquid Interface
Using a new numerical technique [J. de Graaf et al., Phys. Rev. E 80, 051405 (2009)], we determine the adsorption free-energy landscape of anisotropic colloids at a flat liquid-liquid interface. The free energy is based on surface and line tension contributions, building upon Pieranski's original adsorption model. We apply simplified Langevin dynamics to the free-energy landscape to study the colloidal adsorption process. Our results show that the adsorption mechanism for anisotropic particles can be extremely complex, and give confidence in our technique as an initial step towards accurately modeling experimental systems.

Kiri Nichol (Leiden)
Melting a granular solid with oscillatory shear
In the absence of mechanical excitations a container of glass beads behaves as a solid. However, agitating the grains with a disk spinning in the bottom of the container causes the particles to behave collectively as a liquid. Liquid-like behaviour is also apparent when the disk oscillates at large amplitudes, but at smaller oscillation amplitudes the system is more solid-like: heavy objects sink irregularly while low-density objects remain stuck.

Niels Boon (Utrecht)
Charge regulation, ionic screening, and effective interaction of patchy surfaces
The properties of flat surfaces with chargeable patches were studied using non-linear Poisson-Boltzmann theory in combination with charge regulation. We calculated the force between surfaces and observed that a repulsive force at large separations can become attractive at smaller separations, due to charge regulation at the patches. The size of the potential barrier between attraction and repulsion can be tuned by varying the patch size.

Marieke Schor (UvA Amsterdam)
Silk: stacking, sliding or templating?
Silk-based proteins have been used succesfully  as building blocks for self-assembling nanomaterials. However, the mechanisms by which the silk-based proteins assemble to form fibrils remains unclear. Here we compare three hypothetical mechanisms for assembly of two proteins via molecular dynamics simulations.

Daniel Florea (Eindhoven)
Colloidal particles as a model for supramolecular polymers
I
n the last 20 years the design of polymers changed from classical macromolecules to a class of reversible supramolecular polymers. Supramolecular polymers are assembled using non-covalent interactions and this feature assures adjustable properties that can react to external stimuli.  The remarkable properties of these materials are already applied in the following fields: adhesives, printing, cosmetics and coatings. We study a colloidal model that mimics the behavior of a supramolecular polymer. The advantage of our approach is given by the longer length scale which makes these systems easier to study.

José Alvarado (Amolf)
Confined Active Networks
Just as skyscrapers depend on materials like steel and concrete to resist external forces, cells depend on proteins like actin to define their mechanical properties. But unlike buildings, cells have motor proteins like myosin which allow them to move, exert forces, and stiffen. Existing studies show that actin-myosin networks exhibit a rich phase space in-vitro. But equilibrium actin filaments have been shown to organize spontaneously when confined to cellular dimensions. How does this spontaneous organization affect active networks? To answer this question, we encapsulate actin-myosin networks in cell-sized cavities. We aim to quantify their spatial organization with microscopy while measuring their mechanical properties with microrheology. Preliminary results suggest confinement induces a greater degree of bundling and richer behavior compared to the unconfined case.

Ethayaraja Mani (Utrecht)
Self-assembly of patchy particles: Implications for protein crystallization

Ronald Otten (Eindhoven)
Tactoids of plate-like particles in a magnetic field
We investigate how a magnetic field deforms nematic droplets, or tactoids, that spontaneously form in suspensions of colloidal gibbsite platelets, for the cases of sterically stabilised gibbsite in an apolar solvent and charged gibbsite in a polar solvent (water). Our findings provide the first experimental evidence for the existence of a so-called split-core defect structure that was predicted 10 years ago. To rationalise our observations, we also calculated state diagrams of the shape and director-field structure as a function of tactoid size, magnetic field strength, and material parameters, which (qualitatively) reproduce the observed director-field and shape transformations.

Triet Dang (UvA Amsterdam)
Colloidal crystal-liquid Interface: Free energy and Surface tension
I
nvestigation of the crystal--liquid interface is of central importance for understanding crystal growth, and nucleation. This interface is the most difficult to study experimentally because it is buried between two condensed phases. We use micron-size colloidal particles as models to visualize atomic processes of crystal growth. We determine all thermodynamic properties of the system: pressure, chemical potential, and free energy density. We use interface fluctuations to determine the interfacial tension. Remarkably, the interfacial tension value that we find shows a very good agreement with that determined in simulations.

Liesbeth Huisman (Leiden)
Fast deformation of biopolymer networks
We develop a new method to model the viscoelastic response of biopolymer networks on oscillatory shear. We observe an intermediate regime of frequency dependent strain stiffening. At these intermediate frequencies, the single-filament modes have enough time to fully relaxed but the network as whole has not.

Wouter den Otter (Twente)
Self-assembly of clathrin into polyhedral cages
Clathrins are protein complexes with three long legs wich enable them to self-assemble into closed polyhedral cages. These intricate cages contain pentagonal and hexagonal faces, with one triskelion hub residing at every vertex and four legs running along every edge. Cage formation, and the concommittant bending of a membrane into a cargo-loaden vesicle play an important role in endocytosis, the uptake of large molecules by living cells. We briefly present some simulation results on the self-assembly of clathrin cages.

Igor Saulo Santos de Oliveira (Twente)
Simulating viscoelastic fluids and embedded particles
We are investigating the structure formation of colloids in sheared viscoelastic solvents using Brownian dynamics
simulations. We started by simulating the viscoelastic solvent, represented by a shear-thinning elastic polymer solution. While Newtonian liquids react to shear deformations by developing stresses proportional to the applied shear rates, complex fluids display a variety of more complex stress versus rate of strain relationships. In many cases, the apparent viscosity, i.e. the ratio of stress and rate of strain decreases with an increase in shear rate, this phenomenon is called shear thinning. During the simulations of the polymer solution, entanglements between chains appear and disappear continuously. This effect is included in the Brownian dynamics by a recent method dubbed Responsive Particle Dynamics (RaPiD), in which the overlap of two different polymers are described by the number of entanglements between both polymers. The results obtained from the simulations agree with experimental shear-thinning curves. From this point forward, we want to include different particles to the system, starting with spherical ones, in order to observe and analyze the structure formation of these particles in the viscoelastic fluid.

Bas Kwaadgras (Utrecht)
The Coupled Dipole Method For Calculating Effective Interactions
We present the Coupled Dipole Method (CDM), by which we calculate the effective interactions between nanoclusters of dipoles. This method takes into account all many-body dipole interactions and can therefore be expected to be more accurate than, e.g., pairwise summation of dipole-dipole interactions. We also apply CDM to determine the polarizability of a given dipole cluster.

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