Abstracts

Wilson Poon (Edinburgh)
The Physics of Active Colloids'
 

Ran Ni (Utrecht)
Nucleation of colloidal short rods
The interest in positionally and orientationally ordered assemblies of anisotropic particles is driven by their great technological potential as they exhibit anisotropic optical properties, but arises from a more fundamental point of view as well. However, the kinetic pathways of the self-assembly of anisotropic particles are not well understood. For instance,the phase diagram of hard rods has been known for around fifteen years, and shows that there are stable isotropic, nematic, smectic and crystal phases depending on the aspect ratio. Only very recently, the kinetic pathway of isotropic-nematic(IN) phase transition for long rods was reported, but the isotropic-smectic(ISm) and isotropic-crystal(IX) phase transitions of short rods still remain unknown. In this work, we study the nucleation of colloidal short rods from isotropic fluid to the crystal and smectic phases by using computer simulations. We identify three dynamic regimes in supersaturated isotropic fluid of short hard rods: (i) for moderate supersaturations, we observe nucleation of multilayered crystalline clusters which is in marked contrast to an earlier study[1]; (ii) at higher supersaturations, we find nucleation of small crystallites which arrange into long-lived locally favored structures; and (iii) at even higher supersaturations, the dynamic arrest is due to the conventional cage-trapping glass transition. For longer rods we find that the nucleation of the (stable) smectic phase out of a supersaturated isotropic state is strongly suppressed by an isotropic-nematic spinodal instability that causes huge spinodal-like orientation fluctuations [2].
[1]T. Schilling and D. Frenkel, Phys. Rev. Lett. 92, 085505 (2004).
[2] R. Ni, S. Belli, R. van Roij, and M. Dijkstra, Phys. Rev. Lett. 105, 088302 (2010).

Daniel Bonn (UvA Amsterdam)
Why is granular rheology so complicated?
The transport and handling of granular materials is responsible for roughly 8% of the world energy consumption. It is consequently important to understand the way these materials flow. I will discuss some experiments that aim at determining the flow resistance of granular materials and show that this is not at all easy due to many perturbative effects. If these are removed, the surprising result is that the rheology is close to that of a simple liquid.

Moumita Das (VU Amsterdam)
Mechanics and force transmission in the cell cytoskeleton
The mechanical response of most living cells is largely governed by their cytoskeleton, a composite polymeric scaffold made of a variety of stiff biopolymers and crosslinking proteins. Two major filament systems in the cytoskeleton are actin filaments (F-actin) and microtubules. Actin filaments are semiflexible, while the much stiffer microtubules behave as rigid rods. Recent experiments have shown that novel, synergistic interactions between F-actin and microtubules can lead to very rich mechanics, including a dramatically improved load bearing capacity for the microtubules and enhanced compressibility for the actin-microtubule composite. In this talk I will discuss theories that can explain the cooperative mechanical interplay between F-actin and microtubules. First, I will discuss how the direct coupling to the surrounding cytoskeleton allows intracellular microtubules to bear large compressive forces ~ 100 pN, and controls the range of force transmission along the microtubules which can be as large as tens of microns. Next, I will describe the collective mechanical properties of actin-microtubule composites. We find that stiff filaments such as microtubules and stress fibers can not only enhance the stiffness of the cell cytoskeleton, but can also dramatically endow an initially nearly incompressible F-actin matrix with enhanced compressibility relative to its shear compliance with very important consequences for cell mechanics.

Vijayakumar Chikkadi (UvA Amsterdam)
Anisotropic strain correlations in sheared colloidal glasses
Soft glasses show remarkable mechanical properties: they can arrest at high density, but can also flow easily when small stresses are applied. This transition from rigidity to flow is central to a wide range of amorphous materials in geology, biology, and material science. We investigate the role of microscopic correlations in this transition using colloidal glasses: In three dimensions and real time, we track the motion of the individual particles in a flowing colloidal glass, and we visualize correlations in the deformation directly in real space. At the transition from rigidity to flow, shear is neither macroscopically uniform nor strongly localized: We observe system-spanning strain correlations that reveal a novel form of mechanical criticality at the transition from rigidity to flow. These correlations are isotropic when thermal rearrangements dominate at small applied stress. However, when the applied stress increases, the scaling of correlations becomes anisotropic. These exciting new, anisotropic correlations can cause flow instabilities, and can ultimately lead to failure of the material, when correlations span the entire material. While our model system allows direct imaging of these critical strain fluctuations, we expect very similar mechanisms to hold for a wide range of amorphous materials from soft glasses to granular and molecular glasses.

Detlef Lohse (Twente)
Are there different states of fully developed turbulence?
Fully developed turbulence has been seen as the holy-grail problem of physics of fluids or even classical physics. Since the work of Kolmogorov it has been a paradigm that there is only one state of fully developed turbulence. In recent years several indications suggest that this paradigm may have to be revised and that in closed-flow configurations there are different states of fully developed turbulence. We will give evidence for this view from detailed experiments, numerical simulations, and theory. The two closed-flow configurations we address are two classical fluid dynamics flow configurations, namely Rayleigh-Bénard flow and Taylor-Couette flow.

Wouter Ellenbroek (TU Eindhoven)
How to glue charge with charge
When exposed to a solution of multivalent salt (e.g. calcium chloride), charged large molecules such as polymers and lipids can experience a net attractive force despite the fact that the "bare" Coulomb interaction between them is repulsive. In aggregates of such large molecules this leads to nontrivialities such as rigidification, domain formation, and theorists' headache. I will briefly discuss how computer simulations can help us to understand this phenomenon.

Ana Vila Verde (UvA Amsterdam)
How biomolecules influence water structure and dynamics
Interactions between solutes and water impact both water structure and structural dynamics as well as solute properties (e.g. conformational fluctuations of proteins). To understand these interactions we investigate water near disaccharides using classical atomistic molecular dynamics simulations. Disaccharides show topological and chemical complexity characteristic of larger biomolecules but are sufficiently small to permit detailed study.  We observe that increases in hydrophobicity precisely map slow down in water translation and rotation of local water populations. In line with recent studies of proteins, we find that chemically similar functional groups may interact differently with water depending on neighboring functional groups.
To explain these observations we examine the mechanism of hydrogen bond exchange for waters hydrogen bonded to other waters but within the sugar first solvation shell, as well as waters hydrogen bonded to the sugar.  Recent work showed that water in bulk rotates through large angular jumps that pass through bifurcated hydrogen bond intermediates and that rotation rates can be rationalized through transition state theory. Previous reports found that the rotational slow down of water near small solutes can be predicted from changes in the accessible transition state volume or the enthalpy of the hydrogen bonds. For our larger solutes we find that accounting for the transition state volume alone overestimates water rotational slow down.  Differences in hydrogen bond enthalpy are also insufficient to predict rotational slowdown. Water slowdown can only be understood by additionally accounting for subtle changes in the free energy landscape associated with water rotation - reduction in the number of available reactant states and broadening of the transition state barriers.  The presence of solutes of even moderate size thus affects water dynamics in ways difficult to predict using simple scaling considerations from bulk, making water response system dependent.
 

Juriaan Luiken (UvA Amsterdam)
Anisotropic superstructures from isotropic nanoparticles
Polymer nanocomposites (PNCs) are an exciting class of materials, with physical properties strongly depending on the size and shape of the nanoscale filler and the identity of the polymer matrix. Recent experimental efforts to control the self-assembly of colloidal nanoparticles, isotropically coated with polymer chains identical to the matrix, unexpectedly led to the assembly of anisotropic superstructures. Using molecular simulation techniques I investigated the formation of such superstructures with a simple coarse-grained model based on the Asakura-Oosawa model. The superstructure morphology is determined by a combination the polymer-colloid size-ratio and chain functionality, and can be summarized in a morphology phase diagram.

Dominik Michler (UvA Amsterdam)
Migration of surfactant vesicles towards an oil/brine interface
Within a brine AOT-solution (cationic surfactant) at concentrations above the cmc micrometer sized bi-lamelar structures (vesicles) can be observed by white light microscopy. Creating a common interface with oil  leads to a spontaneous migration of these vesicles towards the oil phase, where the vesicles form a layer of micro-emulsion. This behaviour is considered to be applied in oil recovery, where the vesicles serve as carriers for sensors, which are supposed to detect an change of PH value when the vesicles hit the interface. The micro-emulsion that forms at the interface eventually leads to a rheological character, which may facilitate the actual recovery process. However it is necessary to control such a system and hence to understand the governing mechanisms, to gain the mentioned benefits. Our first approach is to uncover the potential mechanisms by tracking the vesicles, as well as tracer particles in the surrounding medium and to investigate the influence of parameters like the interfacial area, oil chain length, as well as surfactant concentration on the velocity fields. Though, the basic condition which is necessary to carry out these experiments is a stable and controllable interface and in the same time the exclusion of buoyancy and evaporation.  This can be provided by a closed micro-fluidic device.

Burak Eral (Twente)
Suppressing the coffee stain effect: how to control colloidal self-assembly in evaporating drops using electrowetting
We study the influence of electrowetting on the formation of undesired solute residues, so-called coffee stains, during the evaporation of a drop containing non-volatile solvents. Electrowetting is found to suppress coffee stains of both colloidal particles of various sizes aand DNA solutions for alternating (AC) frequencies ranging from a few Hertz to a few tens of kHz. Two main effects are shown to contribute to the suppression: (i) the time-dependent electrostatic force prevents pinning of the three phase contact line. (ii) internal flow fields generated by AC electrowetting counteract the evaporation driven flux and thereby prevent the accumulation of solutes along the contact line. 

Stefan Frijters (TU Eindhoven)
Deformation of particle-stabilized droplets in multicomponent fluids
Traditionally, enhanced oil recovery processes often employ surfactants to improve oil yield. In this project, nanoparticles are considered as an alternative to these surfactants. To determine in which ways these particles might be useful we need to understand how they modify the properties of a droplet of fluid suspended in another fluid. To investigate this we use a simulation algorithm based on a multicomponent lattice Boltzmann model to describe the solvents combined with a molecular dynamics solver for the description of the solved particles. We have the ability to tune numerous relevant parameters of the system, such as particle sizes and wettability and surface tensions between the fluids and we use this ability to study the behaviour of fluid droplets stabilized by particles when subjected to shear. We then consider how these effects compare to those of surfactants.

Florian Günther (TU Eindhoven)
Colloidal particles in multiphase flow
Emulsions stabilized by particle are ubiquitous in the food and cosmetics industry, but our understanding of the influence of microscopic fluid-particle and particle-particle interactions on the macroscopic rheology is still limited. In this contribution we present a simulation algorithm based on a multicomponent lattice Boltzmann model to describe the solvents combined with a molecular dynamics solver for the description of the solved particles. In nature colloids are generally not spherical, such as clay particles, which have a plateletlike shape. As an example of anisotropic particles we study ellipsoids. Our model allows a wide variation of fluid properties, the aspect ratio m of the ellipsoid and arbitrary contact angles on the particle surfaces. We investigate some features of a single particle at a flat interface between two fluids such as the contact angle depending on particle attributes and the adsorption trajectories.
Furthermore, we study the parameter dependence of the model and demonstrate its applicability by studying, at least for the special case of m=1, the formation and rheology of a ``bicontinuous interfacially jammed emulsion gel'' (bijel) and of a ``Pickering emulsion''.

Junyou Wang (Wageningen)
Complex Coacervate Core Micelles from Iron-Based Coordination Polymers
We have studied the complex coacervate core micelles (C3Ms) from cationic poly (N-methyl-2-vinyl-pyridinium iodide)-b-poly (ethylene oxide) (P2MVP41-b-PEO205) and anionic iron coordination polymers. Micelle formation is studied by light scattering for both Fe(II) and Fe(III) containing C3Ms. At the stoichiometric charge ratio, both Fe(II)-C3Ms and Fe(III)-C3Ms are stable for at least one week at room temperature. Excess of iron coordination polymers has almost no effect on the formed Fe (II)-C3Ms and Fe(III)-C3Ms while excess of P2MVP41-b-PEO205 copolymers in the solution can dissociate the formed micelles. Upon increasing salt concentration, the scattering intensity decreases. This decrease is due to both a decrease in the number of micelles (or an increase in CMC) and a decrease in aggregation number. The salt dependence of the CMC and the aggregation number is explained using a scaling argument for C3M formation.

Sebastian Schmieschek (TU Eindhoven)
Lattice Boltzmann simulations of multiphase flows in microfluidics
A brief overview is given of the scope of the PhD project, its methods and the physics of interest.

José Alvarado (Amolf)
Claustrophobic networks: actin filaments self-organize under cell-sized confinement
In living cells actin filaments integrate many structures necessary for cells to exert and withstand forces. Different actin crosslinker proteins organize actin filaments into task-specific structures. Next to this biochemical regulation actin filaments are also prone to organize due to physical interactions with each other or the limiting boundary. We polymerize actin filaments in the absence of cross-linkers inside cell-sized micro-chambers and systematically vary chamber dimensions, actin concentration, and filament length. We show that confining actin filaments to a length scale similar to their own average length leads to the formation of bundle-like structures. Surprisingly, shorter filaments give stronger bundles. We propose a model  where in a polydisperse solution of actin long filaments align together as a result of quasi-2D confinement combined with depletion interactions caused by short filaments.

Anika Embrechts (TU Delft)
Micro-rheology of Electro-responsive Polyelectrolyte Gels for Cardiovascular Applications
This project concerns the development and investigation of electro-responsive hydrogels, which will be used as part of a micro-system for less-invasive cardiovascular surgery (EU FP7 Heart-e-Gel).  Elastic and mechanical properties of these electro-responsive hydrogels will be investigated in situ on the micro- and millimeter length scale using Atomic Force Microscopy-based micro-rheology.

Jos van Rijssel (Utrecht)
Direct observations of nanoparticle interactions
Using cryogenic transmission electron microscopy we study the equilibrium clusters of PbSe nanoparticles. The analysis of the structure of these clusters provides important information about the interaction between the nanoparticles. The visualization of these particles in 2D and 3D allows us to observe what happens on these small scales.

Frans Leermakers (Wageningen)
MC-SCF hybrid for polymer depletion
We combine MC simulations and Self-Consistent Field calculations to model polymers at interfaces. The intra-molecular excluded-volume effects are accounted for in a Flory-way. Inter-molecular excluded-volume effects are naturally accounted for as we solve the problem in a 3d computation box. The method improves on the classical Scheutjens-Fleer results: For non-adsorbing polymers with N = 1000, the scaling of the depletion layer thickness with chain length (in the dilute) and with polymer concentration (in the semi-dilute regime) follow the proper excluded-volume scaling.