Abstracts November 8, 2013

Ulrich Wiesner (Cornell) (10.30-11.15)
Soft Matter: From Self-Assembly Concepts to Advanced Materials
Solutions to global problems including energy conversion and storage, clean water and human health require increasingly complex, multi-component hybrid materials with unprecedented control at the nanoscale. This talk will give examples for the synthesis and characterization of nanomaterials based on the self-assembly of organic macromolecules with inorganic, solid-state type materials. The aim of the described work is to understand the underlying fundamental chemical, thermodynamic and kinetic formation principles enabling generalization of results over a wide class of materials systems. Targeted applications of the prepared systems will include the development of nanostructured electrodes for energy conversion and storage devices, the formation of asymmetric membranes for water filtration and separation, as well as advanced optical materials referred to as metamaterials.
Daniela Kraft (Leiden) (11.15-11.45)
Brownian motion of anisotropic particles
Marjolein van der Linden (Utrecht) (13.30-14.00)
Formation and crystallisation of long-range repulsive glasses
We studied long-range repulsive glasses formed in suspensions of sterically stabilised charged colloidal poly(methyl methacrylate) particles (diameter = 2.23 um) with low polydispersity (4%) in the low-polar solvent cyclohexyl bromide (dielectric constant = 7.92). Particle interactions were described by a long-range repulsive Yukawa potential. Glasses were obtained upon compression of the suspensions by centrifugation from a body-centred-cubic crystalline structure at low initial volume fraction (phi =~ 0.02) to a close-packed amorphous structure (phi =~ 0.64). Subsequent expansion of the sediment in gravity resulted in long-range repulsive glassy structures with volume fractions phi = 0.16-0.64. The presence of small clusters (mostly dumbbells; clustered fraction >= 0.12) formed by centrifugation prevented the glasses from crystallising for several weeks, while the sediment was still expanding. We used confocal microscopy to obtain three-dimensional data sets of the system and quantitatively analysed the structure of the glasses. The structure of the glasses was found to be remarkably similar to that of hard-sphere glasses, for which experimental data were obtained by centrifugal compression of silica spheres with a hard potential, despite the much longer-range interaction potential. After more than ten weeks the clustered fraction decreased due to spontaneous dissociation of the clusters, and finally bulk crystallisation of the glasses was observed into face-centred-cubic crystals with a volume fraction around 0.22.
Dmitry Denisov (Amsterdam) (14.00-14.30)
Universality of slip avalanches in flowing granular matter
We study the evolution of slowly sheared granular systems deforming via discrete strain bursts (slips). The granular sample consisting of 105 hard spheres is subjected to applied shear and studied with the combination of two techniques - precise stress-strain measurements and laser sheet imaging. Fluctuations in the stress-strain profile correspond to internal slip avalanches leading abrupt reconstructions in the system due to the shear. The magnitude of the fluctuations is taken as the size of the avalanche events. The power-law distribution of the slip sizes signifies the existence of the dynamically critical state in granular samples under the shear. Laser sheet imaging allows us to visualize each individual slip event, estimate its spatial distribution together with local strain change and connect it to the global fluctuation in the stress-strain curve. Such unique combination of the techniques and analysis lead us to comprehensive understanding of slip avalanches.
Oscar Enriquez (Twente) (15.30-16.00)
Dynamics of bubble growth under small supersaturation.
Beer, champagne and soft-drinks are water-based solutions which owe their 'bubbliness' to a moderate degree of carbon dioxide supersaturation. Bubbles grow sequentially from nucleation sites due to solute concentration gradients and detach due to buoyancy. The leading mass transfer mechanism is diffusion, but the advection caused by the moving surface also plays an important role. Now, what happens at the limit of very weak supersaturation? We take an experimental look at CO2 bubbles growing in water under such a condition. Nucleation sites are provided by hydrophobic micro-cavities on a silicon chip, therefore controlling the number and position of bubbles. Although advection is negligible, measured growth rates for an isolated bubble differ noticeably from a purely diffusive theoretical solution. We can explain the differences as effects of the concentration boundary layer around the bubble. Initially, its interaction with the surface on which the bubble grows slows the process down. Later on, the growth rate is enhanced by buoyancy effects caused by the depletion of the solute in the surroundings of the bubble. When neighbouring bubbles are brought into play they interact through their boundary layers, further slowing down their growth rates.
Ran Ni (UvA/Utrecht) (16.00-16.30)
Active hard spheres: Glass transition & Crystallisation
Active matter consists of a collection of objects that are able to continuously convert stored biological or chemical energy to drive their motion. The interest in studying the dynamics of active matter originates from the aim to understand the intriguing self-organization phenomena in nature, including bird flocks, bacteria colonies, tissue repair, and cell cytoskeleton. Very recently, breakthroughs in particle synthesis have enabled fabrications of artificial colloidal microswimmers, which not only show strikingly new phenomena and physics but also offer new possibilities and insights in touching some classic yet unsolved problems in condensed matter physics. In this talk, I will present our recent work on systems of active hard spheres, and focus on the non-equilibrium glass transition [1] and crystallisation [2].

[1] R. Ni et al., Nature Communications, 2013, in press, [arXiv:1306.3605].
[2] R. Ni et al., 2013, submitted.

Soundbites November 8, 2013

Frank Aangenendt (Technische Universiteit Eindhoven)
Mechanics of materials built from sponge-like particles
Materials built from sponge-like particles are quite common in every day products. Until now their behaviour is predicted using hard sphere models although the behaviour of hard spheres is fundamental different from soft spheres. By experimentally investigate soft porous spheres with tunable size, stiffness, porosity and interactions predictions can be made about the macroscopic properties of soft systems as a function of the microscopic properties of the sponge-like particle.
Bryan Chen (Instituut-Lorentz, Leiden)
Topological solitons in isostatic spring networks
I will show an example of a soliton arising as a domain wall between two different topological phases of a 1D chain of rotors studied by Kane and Lubensky. A two dimensional example might also be presented.
Corentin Coulais (Leiden University)
Buckling of meta-beams
Buckling is an instability that occurs in slender structures under axial load. This instability is well understood since the early works of Euler in the inextensible limit. We show here that when we confer non-standard effective elastic properties to the material, buckling strongly deviates from the inextensible limit and display a new phenomenology.
Gary Davies (University College London, Eindhoven Technical University)
Lattice Boltzmann simulations of Soft Matter Systems
Lattice Boltzmann simulations predicted the existence of the bicontinuous interfacially jammed emulsion gel. Since then, researchers have used lattice Boltzmann simulations as a qualitative tool to gain insight into the dynamics and behaviour of systems with multiple immiscible fluids and immersed particles. In lattice Boltzmann simulations, hydrodynamic forces are incorporated intrinsically and the forces relevant in soft matter scales emerge from them. We show that the correct forces do indeed emerge from our lattice Boltzmann simulations and investigate how much energy it takes to detach a single particle from a liquid-liquid interface.
Simone Dussi (Soft Condensed Matter Group, Utrecht University)
Investigating the entropy-driven isotropic-cholesteric transition in colloidal liquid-crystals.
Whereas the nematic director of a common nematic liquid-crystal phase is homogeneous throughout the system, the cholesteric phase displays an helical arrangement of the director field. In spite of the huge relevance for optoelectronic applications, the relation between the macroscopic chirality of the liquid crystalline phase and the microscopic chiral details (shape & interactions) of the constituent molecules is still not well understood. Suspensions of sterically stabilized fd virus particles exhibit a cholesteric phase, that might be driven by entropy alone (Grelet 2003). Such stabilization mechanism was already proposed by Straley in 1976 within the framework of the the elastic theory of liquid crystals. Extending Straley's approach we develop a second-virial theory to determine the equilibrium pitch of selected hard chiral particles. Performing ad-hoc computer simulations we are able to investigate the entropy-driven cholesteric phase and to speculate over the validity of the theoretical predictions.
Saida El Asjadi (TuDelft)
Viscoelastic behaviour of bio-polymer stabilised emulsions
Jeffrey Everts (Utrecht University)
Colloidal charge regulation in oily solvents
The effect of charge regulation in colloidal dispersions is often ignored: the colloidal charge is assumed to be fixed regardless the ionic strength, pH, and colloid packing fraction. In aqueous electrolytes this fixed charge is often quite realistic, but in apolar or low-polar solvents with a relative dielectric constant in the regime 2-10 the net colloidal charge may be strongly dependent on the thermodynamic state. We are interested in incorporating the chemical equilibrium between dissolved charge carriers (screening ions) and surface- adsorbed ones (colloidal surface charges) in a Poisson-Boltzmann (PB) type theory.
Zahra Fahimi (TU/e)
Anjan Gantapara (Soft Condensed Matter Group, Utrecht University)
Phase Behavior of Experimentally Realizable Polyhedral Colloidal Particles
Motivated by recent breakthroughs in the synthesis of colloidal polyhedral particles and their experimental application, we study the phase behavior of these faceted particles using simulations. We apply Monte Carlo simulations and free-energy calculations to determine the phase behavior of of a family of bi-frustums adhering to a two-dimensional air-toluene interface. We employ theoretical free-energy calculations to determine the equilibrium configuration of these particles at the interface and use this as input for simulations to determine the phase behavior. The phase behavior of these bi-frustums adhered to the air-toluene interface matches the experimental observations. Our results illustrate the intricate relation between phase behavior and building-block shape and can guide future experimental studies on polyhedral-shaped nanoparticles.
Piotr Glazer (TU Delft)
Multi-stimuli responsive hydrogel cilia
jan Groenewold (UU)
statistical mechanics of antifreeze proteins
Florian Günther (TU Eindhoven)
Timescales of emulsion formation induced by anisotropic particles
Particle stabilized emulsions have received an enormous interest in the recent past, but our understanding of their formation dynamics is still limited. Ellipsoids are known to be more efficient stabilizers of fluid interfaces than spheres, but their anisotropy and the related additional rotational degrees of freedom influence emulsion formation.
Dennis Hessling (TU Eindhoven)
Towards Green Ink with lattice Boltzmann and molecular dynamics
To improve waterborne inks, a multicomponent suspension, we use simulations to determine the influence of various parameters on the resulting agglomeration depth of pigments. Currently thermal fluid fluctuations and the resulting Brownian motion of MD-particles, coupled to LBM, as well as coalescence of microdroplets are under investigation.
Katie Humphry (Shell)
Microemulsion Flow in Porous Media: Implications for Alkaline-Polymer-Surfactant Flooding
Work flows to assess the technical and economic suitability of an enhanced oil recovery (EOR) technique for a particular field generally involve laboratory testing, such as core flooding experiments, and field-scale reservoir modelling. It is important to understand the flow properties of all phases present when building field scale models or interpreting laboratory experiments that involve an EOR technology. In alkaline-surfactant-polymer (ASP) flooding, surface-active molecules decrease the interfacial tension between water and crude oil, increasing the capillary number. The relative decrease in interfacial tension compared with viscous forces leads to the recovery of oil trapped in the reservoir pore space. The ultra-low interfacial tensions needed for ASP flooding occur when the interfacial surfactant film has zero average curvature, and the surfactant solubilizes equal amounts of oil and brine. Under these conditions, in addition to the brine and oil phases, a third thermodynamically stable phase is formed. This third phase is known as a microemulsion. While the flow properties of crude oil and polymer-enriched brine are well understood in porous media, relatively little is known about the flow properties of the microemulsion phase, particularly with respect to porous media. Because the shear and extensional forces experienced by a fluid flowing through porous media are different to those exerted by a shear rheometer, knowing of the viscous response of microemulsion flowing through porous media is an important parameter in interpreting ASP core flooding experiments, and in modeling ASP flooding on the field scale. In this study, litres of microemulsion, with and without polymer, are generated using a model AS and ASP systems. Preliminary rheological measurments for these microemulsions are reported. The microemulsions are studied using conventional shear rheology. Additionally, an \textit{in situ}, or apparent, viscosity is recovered from a core flooding experiment in Berea sandstone, where pressure drop across the core is recorded as a function of field-relevant flow rates of the microemulsion through the core. The implication of these results for ASP flooding is discussed.
Andreas Härtel (ITF, UU)
Energy storage in supercapacitors - Density Functional Theory for ionic liquids
The safe and secure storage of energy is an important scientific challenge with outcome that is applicable immediately in technics. Low-cost nanoporous supercapacitors that are filled with an ionic liquid and have an internal surface area of the order of a square km per kg are promising candidates for storing large amounts of electric energy with a high (dis)charging rate. However, the degree of understanding to fully optimize these devices require a detailed knowledge of the underlying physics, in particular of the molecular structure of the densely packed ionic liquid confined in the nanopores of the charged supercapacitor [1]. Since crystal-fluid interfaces in colloidal hard-sphere systems are well described by fundamental measure density functional theory [2], we extend this framework by incorporating Coulomb interactions to achieve a microscopic theory of ionic liquids. This modern approach allows us to study the packing of ions near highly charged electrodes by analyzing the structure of the electric double-layer in comparison with the capacitance of the system. [1] C. Merlet et al., "On the molecular origin of supercapacitance in nanoporous carbon electrodes", Nature Mat. 11, 306 (2012). [2] A. Härtel et al., "Tension and Stiffness of the Hard Sphere Crystal-Fluid Interface", Phys. Rev. Lett. 108, 226101 (2012).
Mathijs Janssen (Institute for Theoretical Physics, Universiteit Utrecht)
Blue engines with temperature gradients
Where rivers flow into the sea, an enormous amount of energy (about 2kJ/L, equivalent to a 200m waterfall) is dissipated, due to irreversible mixing of fresh and salty water. A so-called ?blue engine? was proposed [1], extracting electrical energy from this process by cyclic (dis)charging of (super)capacitors immersed in water of varying salinity. Previous work [2] found an optimal work cycle for such engines. We propose a modified cycle which, next to the salinity gradient, contains a temperature gradient as well. Depending on the temperature difference, this modification can significantly improve the power output. [1]D.Brogioli, Phys. Rev. Lett. 103, 058501 (2009) [2]N.Boon and R. van Roij, Mol. Phys. 109, 1229 (2011)
Janne-Mieke Meijer (Utrecht University)
Directed self-assembly of colloidal hematite cubes
The self-assembly of colloidal particles can be directed in various ways, of which one is changing the building block shape. We have stdied micron-sized cubic hematite colloids and their spontaneous self-organization. Colloidal cubes are synthesized with different sizes and the interactions are controlled by solvent composition, surface coating and external fields. Using microscopic techniques and microradian X-ray diffraction we are able to investigate the structure of self-organised densely packed cube sediments in detail. We show that the flat faces of the cubes cause alignment and form short to long-range ordered layered structures. Additional alignment in an external magnetic field causes the formation of a single BCM crystal.
Pepijn Moerman (UU)
Virus capsid polymorphism
The equilibrium between HBV virus capsids and dimers was successfully described using a combination of hydrophobic attraction and electrostatic repulsion, showing that complicated biophysical systems can be described using coarse thermodynamic models. The observed ratio between T=3 and T=4 capsids however remained unaccounted for. Here we address the fundamental question whether this structural dimorphism is a consequence of kinetics or thermodynamics. To this end we provide two independent models and compare them with experimental data on HBV. We show that contrary to previous reports kinetics cannot cause the observed dimorphism for HBV.
Alexandre Georges Louis Olive Olive (TUD - ChemE - ASM)
Catalyst-assisted micropattern formation
A bio-mimetic approach based on catalyst-assisted self-assembly is used to create micro-patterns of supramolecular nanofibers and subsequently hydrogels. Gelator precursors react on catalytic sites producing building blocks necessary for the self-assembly of hydrogel fibers. Growth of fibers occurs in specific location and lead to directed structures. This versatile system offers spatiotemporal control over formation of supramolecular structures. A unique mechanism of fiber formation for a synthetic system has been observed.
Jayson Paulose (Leiden University)
Fluctuations make spherical shells softer
Thermal fluctuations strongly affect the mechanical response of thin solid membranes, giving rise to anomalous, temperature-dependent elastic constants. Shells, or membranes with curvature in the ground state, have a qualitatively different relationship between deformations and strains compared to flat membranes. We evaluate the effects of temperature on the response of spherical shells to point forces and external pressures, showing that fluctuations soften the shell.
Francesca Tesser (Tu/e)
Population dynamics and flow structures
The dynamics of a population composed by distinct types of individuals can be modeled in terms of binary reactions implementing birth and coagulation processes and progressing at specific rates. By means of numerical simulations in 1d or 2d domains we study the time evolution, including diffusion and interaction amongst individuals and we perform quantitative analysis by measuring statistical observables such as the heterozygosity and the fixation time. The same model can be applied in situations where, on top of the processes mentioned above, a flow transports the individuals allowing the study of the influence of typical flow structures on the population size as well as on its genetic composition. Finally we perform experiments with bacteria colonies growing in controlled microfluidic devices.
Ruben van Drongelen (TU Delft)
Moving without a leader; the advantages of collective migration
Many biological systems consist of a collection of identical individuals. Such systems range from macroscopic flocks of birds to microscopic unicellular organisms like amoebae. We investigate the benefits of collectively migrating by simulating these systems as self-propelled soft particles with Vicsek-type neighbour alignment interactions. We find that by simply packing together, individuals can boost their diffusion constant up to two orders of magnitude without the need for additional mechanisms.
Thijs van Westen (TU Delft)
One plus one equals three? - Binary mixtures of Liquid Crystals
Using a recently developed statistical-mechanical-based equation of state for liquid crystalline fluids, it is shown that the mixing of liquid crystals of different molecular length can results in surprising results for the physical properties of these fluids.
Scott Waitukaitis (Leiden)