Axel Arnold, AMOLF
Segregation of flexible polymers in confinement
During the cell replication cycle of a bacterium, it is necessary for it to replicate the DNA and separate the two resulting DNA rings such that exactly one ring goes to each of the two daughter cells. It is commonly believed that a not yet detected active process has to be involved in this separation. However, scaling arguments show that the confinement of a polymer in a pore alone leads to an entropy-driven segregation. Recent analytical results obtained in the framework of the de Gennes blob model give estimates for the scaling of the segregation time for two DNA strands in a pore. We present some preliminary results from MD simulations to check these predictions.
Daniel Bonn, UvA Amsterdam
Behnaz Bozorgui AMOLF
Lattice-Based Monte Carlo for Telechelic Molecules
We present a novel Monte-Carlo scheme that makes it possible to perform efficient simulations of dense systems of self-avoiding polymers on a lattice. We show that the method is particularly useful to simulate dense systems of polymers with functionalized end groups. We compare the efficiency of the new scheme with Configurational Bias MC and indicate the regime where the present approach is the method of choice.
Patrick Charbonneau, AMOLF
Simulation and theory of equilibrium and out-of-equilibrium behavior in colloidal gels
We compute the phase behavior of model colloidal systems with short-ranged attractions and an additional long-ranged repulsion, using Gibbs ensemble Monte Carlo. A preliminary investigation of out-of-equilibrium behavior is also performed. The results suggest that the recent observations of stable cluster phases in systems without long-ranged repulsions are intimately related to gas-crystal and metastable gas-liquid phase separation. With the inclusion of an additional long-ranged repulsion, a quantitative determination of the phase boundaries and microphase ground states is made. We demonstrate that gelation may occur in such systems as the result of arrested microphase separation.
Edwin Devid, Utrecht University
Mixed metal hydroxides (MMH)
For my masterthesis I synthezise hydrotalcites composed of Mg2+, Al3+ and Mg2+, Fe3+, and further investigate the structure (size), point of zero charge and phasebehavior.
Joshua Dijksman, Leiden University
Packing fraction and rigidity in granular matter
My experimental research focuses on how the rigidity of granular media changes when the packing fraction varies between 'random loose' and 'random close'-packing, e.g. in the window of mechanically stable packings for frictional grains. We are currently investigating how vibrations (or rather 'taps') can be used to create packings of different.
Marjolein Dijkstra, Utrecht University
Colloidal self-assembly: Predicting and designing new structures
Recently, many new experimental colloidal model systems became available, e.g., colloidal molecules, oppositely charged colloids, and patchy particles, showing a overwhelming number of structures. My aim is to obtain a fundamental understanding of self-assembly (phase behaviour, kinetics, nucleation, glasses) of these new classes of building blocks using computer simulations and to explore the possibilities for new structures and advanced materials. In addition, I plan to reverse-engineer colloidal building blocks that can self-assemble into target structures with a specific function.
Eelco Eggen, Utrecht University
The second virial coefficient for charged colloidal rods
In the context of the second order virial expanion for colloidal particles, charged rods are usually described only in the infinite length limit. Like the Onsgager limit does for hard rods, this gives simple results. The theory for hard rods, however, can be easliy extended to include finite-length rods. My aim is to devise an analogous treatment for the theory of charged rods.
Wouter Ellenbroek, Leiden University
A diverging length scale in jammed granular media
The rigidity of an amorphous granular packing is directly related to the average number of contacts per particle. The difference between this number and the minimum value it can have for a jammed system sets the minimum length scale on which the system can be described like an elastic continuum.
Annalisa Fasolino, UvA /RU
Magnetic deformation of self-assembled molecular nanocapsules
The deformation in high magnetic fields of spherical nanocapsules, self-assembled from bola-amphiphilic sexithiophene molecules, has been measured allowing the determination of their bending rigidity. At high fields, an enhanced rigidity is found which cannot be explained within the model proposed by Helfrich. We propose a complete form of the free energy functional that accounts for this behaviour, and allows the discussion of the formation and stability of nanocapsules in solution.
Nienke Geerts, AMOLF
DNA-Driven assembly of colloidal clusters
Novel types of nano-structured materials can be made using DNA-coated colloids. The specificity of interaction between two complementary DNA strands makes it possible to “tune” the interactions between colloids with different DNA coatings. Most systems studied thus far consisted of (nano) colloids functionalized with short DNA oligomers. Here we consider the behavior of micron-sized colloids functionalized with long DNA strands.
Bastiaan Huisman UvA Amsterdam
Simulations of self-assembling supramolecular structures using patchy molecules
We present a coarse grained model to describe molecules that self-assemble into complex supramolecular structures due to their directionally dependent, selective interactions. Examples of these molecules include (globular) proteins, complementary molecules such as DNA and aromatic molecules that favor stacking through p bonds. We simulate rigid molecules made up of spherical, coarse grained beads on which several types of patches can be placed to model selective and directional intermolecular interactions. As a first test of our model we study the linear aggregation of discoids and compare it to equilibrium polymerization theory.
Blandine Jérôme, UvA Amsterdam
Polymer thin films
We study the behaviour of amorphous polymer thin films and how this behaviour changes as the film thickness decreases. We focus more particularly on visco-elastic properties (probed with an atomic force microscope), transport properties (diffusion and absorption of solvent), and the influence of the supporting substrate.
Gijs Katgert, Leiden University
Linear shear of quasi-2D foams
We have investigated the linear shear of quasi 2D foams. In the presence of a glass top plate, this system exhibits shearbanding. We have looked at what parameters govern this shearbanding, and compared data to a simple model
Asmae Khaldoun, UvA Amsterdam
Quicksand is the generic name for unstable soils reputed to trap anyone who treads on it. Popular wisdom has it that one should not move when trapped in quicksand, as motion makes one sink in even deeper and that once trapped, it is difficult to escape1. Here we provide an explanation for these observations by studying the most commonly encountered form of natural quicksand. We show that a spectacular liquefaction of the material occurs when a stress is applied to the material: the liquefaction is the reason why one sinks away, and it is more pronounced for larger stresses. By constructing ’laboratory quicksand’, we demonstrate that the liquefaction is due to the structure: quicksand is a loose granular packing of sand particles stabilized by a clay matrix that forms a particulate gel. The stress liquefies the clay matrix, and the granular assembly collapses, expulsing water. This results in a densely packed system that practically impossible to dilate: it is for this reason that once trapped it is difficult. A sinking test demonstrates that, due to buoyancy, it is impossible to drown in the quicksand.
Willem Kegel, Utrecht University
Internally frustrated matter
I will give an overview of our current work on micro-phase separating systems. Examples are virus capsids, inorganic macromolecules, anisometric colloids and emulsions stabilized by nanoparticles.
Mark Klokkenburg, Utrecht University
Experimental evidence for hexagonal band formation in magnetic dipolar fluids
Field-induced structures in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment are analyzed on a single-particle level by in situ cryogenic transmission electron microscopy (2D). The field-induced columnar phase locally exhibits hexagonal symmetry and confirms the structures observed in simulations for ferromagnetic dipolar fluids. The columns have a liquid-like internal structure, distorted by lens-shaped defects, due to the fluctuation-induced inter-chain attraction relative to field-directed dipolar attraction. Both dipolar coupling and the dipole concentration determine the dimensions and the arrangement of the columns. Their regular spacing manifests long-range end-pole repulsions.
Daniela Kraft, Utrecht University
Fabrication of anisometric particles
My PhD-project will focus on the self-organization of anisometric colloids on finite as observed in biological systems, e.g. the formation of micelles, vesicles or virus capsids. At the moment my research focuses on the development of a model system with cone-shaped geometry. Different techniques for fabricating these particles such as phase separating spherical colloids are under development.
Andriy Kyrylyuk, TU Eindhoven
Continuum Percolation of Carbon Nanotubes/Polymer Composites
The clustering and percolation of continuum systems consisting of a dispersion of single-wall carbon nanotubes (SWNT), which are modeled as hard rods, and polymeric nanoparticles is considered. Integral equation theory at the level of the second virial approximation is employed to study depletion-induced continuum percolation of hard rods. The percolation threshold is calculated as a function of the sticky potential and the aspect ratio of the rods. The percolation threshold is predicted to be strongly dependent on the size of the polymeric nanoparticles.
Mirjam Leunissen, Utrecht University
Manipulating colloids with charges and electric fields
I will show how one can obtain novel colloidal phase behavior by creating opposite charges, by applying one or more external electric fields, or by adding water to a low-polar suspension. I will also demonstrate the power of dielectrophoretic forces for manipulating the particle concentration in small, sealed samples.
Katya Lyakhova, TU Eindhoven
Formation of soft solids in liquid crystal colloid
Mixtures of thermotropic liquid crystals with spherical particles are studied. In such mixtures formation of soft solid with a high storage modulus was reported by experimentalists. The existence of 3 different regimes during soft solid formation is proposed for consistent description of temperature dependency of storage modulus. The combination of phenomenological Landau-deGennes and Carnaham-Staling theories is used. We found that interfacial tension between nematic and isotropic liquid crystal is an important contribution to elastic modulus of the system on the early stage of phase separation while late stages of the process are dominated by nematic pressure and depletion interaction of colloids
Alexey Lyulin, TU Eindhoven
Computer modelling of dendrimers
Perspectives of the computational studies of the hyperbranched structures are discussed. Brownian dynamics simulations of dilute solutions of dendrimers and hyperbranched polymers with different degree of branching are presented. Non-monotonic molecular-weight dependence of the zero-shear-rate intrinsic viscosity for non-draining structures is simulated and explained in detail. The overcharging phenomenon is observed for complexes of linear polyelectrolytes and a dendrimer.
Fred MacKintosh, VU Amsterdam
Nonergodicity in cells and cytoskeletal networksThe mechanics of cells is largely governed by a skeleton-like structure of stiff protein polymers. These cytoskeletal networks are, however, far out of equilibrium. We show how specifically non-equilibrium contractile activity dramatically changes the mechanics of these networks. We also show how these networks behave as glassy materials, both in vitro and in vivo.
Tommi Markkula, AMOLF
Ordering of complexed PEO-P2VP thin films
PEO-P2VP copolymers, complexed with an AZO-sulfonate group, were found to form lamellae corresponding to the complexing molecule dimensions, and cylinders, arising from the copolymer micro phase separation. X-ray reflectivity and GISAXS were used to clarify a model based on AFM observations.
Matthias Mobius, Leiden University
Non-affine bubble motion in sheared 2D foams
We use a 2D foam as a model system to study the behavior of disordered, jammed media far from equilibrium. In particular, we investigate the bubble motion while applying a linear shear to the foam. The bubbles rearrangements are non-affine and depend on the liquid fraction of the foam. In order to characterize the collective bubble motion we measure the probability distribution of the relative displacement angle of neighboring bubbles, P(alpha) (Ellenbroek et al.). As the liquid fraction is increased, the distribution becomes more peaked around 90 degrees. This is a signature for approaching the point at which the system unjams. It is the first experimental measurement of P(alpha), which has only been measured in simulations so far.
Jakub Otwinowski, UvA Amsterdam
Reverible aggregation in a polystyrene sphere suspension in a quasi binary liquid mixture
We study the aggregation of polystyrene in the 3-methylpyridine/H2O/D2O density matched system with temperature as a control parameter. Theory predicts adsorption-induced aggregation of colloidal particles in the binary mixture. Using dynamic light scattering, we study the adsorption layer as function of temperature when one approaches the phase separation line of the binary mixture. The aggregation phase could be characterized by the measurement of the structure factor with small angle x-ray scattering as a dense liquid, glass and fcc crystal.
Salima Rafai, UvA Amsterdam
Live Rekvig, AMOLF
Molecular simulations of coalescence in oil/water/surfactant systems
We use Dissipative Particle Dynamics simulations to study coalescence in oil/water emulsions with surfactant. Coalescence is a rare event in the sense that the waiting time between events exceeds the duration time of the event by many orders of magnitude. We therefore use Forward Flux Sampling to enhance sampling of the transition state. We use simple bead-spring models to determine how the structure of the surfactant affects the film rupture rate. We also study the nature of the transition state and the molecular mechanism of merging the two surfactant monolayers.
Peter Schall, UvA Amsterdam
Strain distribution in sheared colloidal crystals and glasses
Nazly Sedghinejad, UvA Amsterdam
Controlled Growth of Single Crystals in Colloidal Suspensions
The goal of this project is to investigate the growth of a crystal from a nucleus on the particle scale in colloidal systems. We apply a temperature gradient to control the growth of colloidal single crystals. We use fast confocal microscopy to visualize nucleation and crystal growth on the particle scale in three dimensions. By tracking the individual particles at the surface of the growing crystal, we aim to obtain insight into the particle assemble at the interface
Evaporation of salt solutions: Crystallisation and deposition pattern.
Dannis 't Hart, Utrecht University
Incorporating Lanthanides into SiO2 nanoparticles
Lanthanide doped silica colloids have interesting applications due to their luminescent properties, and can be used as optical probes inside photonic crystals, functionalized optical markers inside biological cells, or as luminescent particles in the study of colloidal systems. We describe a general method which allows for the incorporation of lanthanides into silica colloids. The method is shown to work for Eu, Er, Sm, Tb and Nd, and is believed to be easily extendable to other lanthanides. The method relies on the complexation of lanthanide ions with compatible ligands, as well as a silica synthesis method based on micro-emulsions.
Brian Tighe, Leiden University
Nonlinear elastic stress response in granular materials
We study the response of two-dimensional granular materials to a boundary force, for which classical elasticity predicts identical stress states in the cases of isotropic and hexagonally anisotropic materials. We probe the differences in these two cases by including corrections from the full nonlinear elasticity theory. Additionally, we model the effect of discrete microstructure by taking the magnitude of multipole stress response terms, which are induced in the nonlinear system, as material parameters. By so incorporating both anisotropy and microstructure, we are able to fit experimental stress response profiles in hexagonal packings of photoelastic grains, while either correction alone is insufficient.
Chantal Valeriani, AMOLF
Homogeneous bubble nucleation in a Lennard-Jones fluid: pathway analysis and rate calculation
The formation of a vapor phase from a superheated liquid is a significant not yet completely understood phenomenon, although it has fundamental importance in many engineering applications. By means of Molecular Dynamics simulations and Forward Flux Sampling technique, we analysed the nucleation pathways in the Gibbs free energy landscape in order to understand the bubble formation mechanism, and we calculated the nucleation rate to form a critical bubble.
Martin van Hecke, Leiden University
Future experimental directions for granular flows
I'll briefly describe some of the main open questions in granular flows, and discuss two future lines of experiments I have in mind to address these questions. (incidentally, I have two openings for PhD students to work on this ;-)
Michel Versluis, University of Twente
Leaping shampoo and the stable Kaye effect
Shear-thinning fluids exhibit surprisingly rich behaviour. One example is the Kaye effect which occurs when a thin stream of a solution of polyisobutylene in decalin is poured into a dish of the fluid. As pouring proceeds, a small stream of liquid occasionally leaps upward from the heap. This surprising effect, which lasts only a second or so, is named after its first observer, Kaye, who could offer no explanation for this behaviour. Later, Collyer and Fischer suggested from 250 frames/s cine recordings that the fluid must be highly shear thinning as well as elastic and ‘pituitous’ (slimy or sticky). In addition, their results suggested that a rigid surface is required to back the reflected liquid stream. While the words bouncing and reflection are associated with elastic effects, we will show here that the Kaye effect is in fact a continuous flow phenomenon. We show that the Kaye effect works for many common fluids, including shampoos and liquid soaps. We reveal its physical mechanism (formation, stability and disruption) through high-speed imaging. The measurements are interpreted with a simple theoretical model including only the shear thinning behaviour of the liquid; elastic properties of the liquid play no role. We show that the Kaye effect can be stable and that it can be directed. We even demonstrate a stable Kaye effect on a thin soap film excluding the necessity of a rigid backing surface.
Jos Zwanikken, Utrecht University
Colloids near Interfaces: Narcissistic Interactions
Using DFT we explore the diverse behaviour of colloids near interfaces of different dielectric media. We find the colloids to be affected by their image charge resulting in non-trivial screening effects, and high electric fields at the interface.