Abstracts

Kees Storm (Leiden/Eindhoven)
Large-scale structure in supercooled glycerol
Liquids that are cooled well below their melting point typically show an extraordinary increase in viscosity as the glass transition is approached. The static structure of a glass, however, is identical to that of a liquid and it would therefore appear that there is no increasing static correlation length to accompany the glass transition. I will report on experiments studying heterogeneity in glycerol, a molecular glass-former. These experiments demonstrate the existence of an extended solid network dividing the fluid into separated, long-lived liquid pockets where the rotational diffusion of fluorescent probe molecules reveals markedly different local viscosities. The existence of such a network is further evidenced by bulk rheological measurements, that reveal the hallmarks of soft glassy rheology well above the glass transition: The network ages, has a yield stress, shear melts and shows history dependence.

Eduardo Sanz (Utrecht)
Liquid-to-solid nucleation and gel formation in suspensions of oppositely charged colloids
We present a numerical study of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study.We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for sub-critical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe should be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation. Besides, we study gel formation at strong particle interactions. We analyse the mechanism by which a gel is formed starting from a homogeneous dilute suspension. Both the experiments and the simulations show that the mechanism is related to an arrested phase separation. Furthermore, we use Brownian Dynamics simulations to study the relation between gel formation and the equilibrium phase diagram. We calculate real space correlation functions to analyse the dynamics of the system and we find qualitative differences between ergodic and non-ergodic states. Gels are found in regions of the phase diagram where the fluid-fluid coexistence is metastable. This does not necessarily imply that the critical point is metastable (short-range interactions); gel formation is also observed for long-range attractions below the triple temperature.

Thijs Vlugt (Utrecht)
Tail of the force distribution in granular materials.
We numerically study the distribution P(f) of contact forces in frictionless bead packs. We resort to umbrella sampling to resolve the asymptotic decay of P(f) for large f, and determine P(f) down to values of order 10^-45 for ordered and disordered systems in two and three dimensions. Our findings unambiguously show that, in the ensemble approach, the force distributions decay much faster than exponentially: P(f) ~ exp(-c f ^a), with a ~ 2.0 for 2D systems, and a ~ 1.7 for 3D systems. In MD simulations however, we always find an exponential decay of P(f) provided that the repulsive part of the interaction potential is steep enough.

Jasper van der Gucht (Wageningen)
Trajectories of actin-propelled biomimetic colloids
Eukaryotic cell movement is driven by polymerizing actin filaments that push against the cell membrane. The same actin polymerization machinery can be used to propel simpler objects, like bacteria, vesicles and plastic beads coated with an activator of actin nucleation. Here, we analyze the motion of colloids propelled by a comet-like tail of polymerizing actin filaments. We analyze the curvature of the particle trajectories, and find that it deviates strongly from a Gaussian distribution, with a relatively high propensity of high curvatures. This implies that the underlying microscopic processes are fluctuating in a non-independent manner. A stochastic simulation that includes nucleation, force-dependent dissociation, growth, and capping of filaments, shows that the non-Gaussian curvature distribution can be explained by a positive feedback mechanism in which attached chains under higher tension are more likely to snap.

Gijs Wuite (VU Amsterdam)
Visualizing the formation and collapse of DNA toroids
In eukaryotic and prokaryotic organisms as well as viruses, strongly positively charged polyamines play an important role in the organization of DNA. Interestingly, polyamines can cause DNA to undergo a sharp condensation phase transition in vitro. In this process, DNA molecules aggregate into highly ordered structures, generally with toroidal or rod-like shapes, consisting of many circumferentially wound DNA strands. Toroidal DNA condensates are of particular interest because of their striking similarity with the morphology of compacted DNA found in viruses and sperm cells. Despite previous (single-molecule) experiments addressing the forces and energies involved in the process of DNA condensation and de-condensation, little is known about the dynamics of toroid formation and the stability of such a condensate. Here we show that the reversible condensation of single DNA molecules into toroidal structures under the influence of polyamines can be visualized with high spatial- and temporal resolution using a combination of optical trapping and fluorescence microscopy. We demonstrate that the minimal stable condensate consists of two DNA loops and we follow the unravelling of this structure into a non-condensed form when the DNA overlap is further reduced. In addition, we observe the nucleation dynamics of toroidal structures as well as its stepwise growth. Finally, we show that toroidal condensates can incorporate DNA from both sides at equal rates, providing a basis for diffusion-driven mobility of condensates along DNA. Moreover, this result suggests a mechanism that explains the formation of closely spaced toroids, as observed in sperm cells. An extensive, quantitative understanding of DNA toroid formation dynamics will be pertinent to the development and optimization of applications based on DNA-condensation, such as gene therapy.

Hans Tromp (NIZO / Ede)
Water-water phase separation in sheared and confined conditions.
Due to its relatively large and tunable capillary length and the relatively low viscosity, phase separating aqueous solutions of dextran and non-gelling gelatine are in particular suited to be studied in confined and sheared conditions.
Phase separation in acquiescent conditions in the confinement of a capillary tube give rise to a quasi-stable plug sequence. In the work presented here, the formation and final state of this plug state is described and a tentative physical interpretation in terms of wetting, Rayleigh instabilities and an additional length scale (apart from the capillary radius) is given.
Microscopic observation in a zero velocity plane of continuously sheared phase separated dextran/gelatine systems shows a novel form of vorticity shear banding. Doughnut shaped rings or bands usually consisting of the phase of the lower viscosity form from tail-to-head and subsequent lateral coalescence of droplets stretched by the shear. The rings extend along the full circumference of the rotating cone-plate shear geometry. Stopping the shear renders the rings unstable. From microscopic monitoring of the subsequent Rayleigh instability the interface tension between the coexisting phases can be obtained. Preliminary results suggest that the formation of the rings is dependent on the volume fraction of the broken up phase, and on shearing history.   

Paul van der Schoot (TU Eindhoven)
Assembly kinetics of icosahedral virus capsids
A phenomenological model is presented to describe the assembly kinetics of icosahedral virus capsids in aqueous dispersions of the coat proteins. We find that the late-stage relaxation rate scales with the square of the concentration of the coat proteins, in agreement with measurements on some but not all of the viruses probed. Failure of the theory we attribute to nucleation events disregarded in the model.

Daniela Kraft (Utrecht University)
Binary thermodynamically stable pickering emulsions
It was recently found that a methacrylate oil phase can be stabilized in water by various types of charged colloids, amongs others by magnetite and Ludox AS 40 silica. The thermodynamically stable emulsions form spontaneously without requiring mechnical agitation. These meso-emulsions differ from the known micro emulsions, which are stabilized by a surfactant, as well as from the normal emulsions which contain unstable oil droplets. Two sorts of colloids which by themselves were found to stabilize the oil droplets were used to further investigate the mechanism of adsorption to the surface and the associated stabilization. Using colloidal particles of different sizes we also aim at the creation of 'patchy particles'.

Marteen Biesheuvel (TU Delft)
Electroactuation of Microcantilevers using Polyelectrolyte Brushes
When a microcantilever is coated on one side with a polyelectrolyte brush, it will deflect (bend) to a certain extent, depending on environmental parameters such as pH, ionic strength, and the presence of absorbing species (e.g., protein). The brush-coated microcantilever can therefore function as a sensor or actuator. When an AC or DC voltage is applied to the cantilever, actuation is strongly enhanced (electroactuation) while high switching rates (~1 Hz) can be achieved. I show briefly how brush theory can be applied to this system, how the brush conformations respond to the applied cantilever voltage, and how actuation is influenced. This project is a cooperation with Wilhelm Huck of the Melville Laboratory, Cambridge University, UK.

Marina Soares e Silva (Amolf)
Active motor-driven dynamics in actin-myosin networks probed by microtubule bending.
The cytoskeleton mediates various active cellular processes such as migration or division. Non-thermal random fluctuations in the actomyosin cytoskeleton have been suggested as a possible mechanism of modulation of the cell elasticity without changes in density or polymerization of actin filaments.To elucidate the role and mechanical action of myosin in these processes, we study a three-component in vitro model system. Actin filaments, myosin filaments and fluorescently labelled microtubules are assembled together in order to quantitatively determine the amplitude of microtubule bending fluctuations with time-resolved fluorescence microscopy. In order to examine these active fluctuations we vary parameters such as myosin processivity, ATP concentration and the presence of actin cross-linkers.

Izabela Piechocka (Amolf)
Nonlinear elasticity of collagen networks
Collagen is the most abundant protein constituent of the extracellular matrix in mammalian tissue. Network of collagen fibers support most tissues and contribute to the external shape of cells. As an example of a semiflexible protein network, collagen is very interesting: when sheared between two plates it stiffens strongly and gives a non-linear stiffening response to an imposed deformation and a negative normal stress. This negative net tension occurs in a network of randomly oriented filaments because filaments that are stretched exert more force than the compressed one. We aim to understand the structural origin of the non-linear response of type I collagen by combining shear rheometry with confocal microscopy. Here we report first measurements that show how the temperature, pH and protein concentration affect collagen polymerization and rheology.

Ben Erné (Utrecht)
How to Determine the Electrical Dipole Moment of Q-Dots
When the electrical dipole moment of Q-Dots is large, it cannot easily be determined from electrical impedance spectra because dipolar structures are present. The method of choice is then to analyze the cluster size distribution seen by cryogenic transmission electron microscopy.

Nienke Geerts (Amolf)
Clustering versus percolation in the assembly of colloids with long DNA
The highly specific nature of binding between base pairs in double stranded DNA offers many possibilities for the use of this biomolecule in technology for specific recognition on the molecular level. Here we present an experimental study in which we compare the self assembly of micron-sized PS beads bridged through hybridization of complemetary ssDNA strands (12bp) attached to variable-length dsDNA spacers that are grafted to the colloids. In all cases the same ssDNA pairs were used, however confocal microscopy revealed that the aggregation behavior is very different: ranging from finite-sized clusters to percolating systems.

Liesbeth Huisman (Leiden)
Characteristics of Semi-flexible Polymer networks
Networks of Semi-flexible Polymers are present both in cells as well in the intercellulair material. By computer simulations of these networks, we aim at characterising and understanding the behavior of these networks, e.g. the mechanical response under shear.

Peter van Oostrum (Utrecht)
Characterizing single colloidal particles with digital video holographic microscopy
We use in-line digital holographic microscopy [1] and Mie scattering theory to characterize and track individual colloidal particles. Each holographic snapshot provides information on the size and refractive index of hundreds of individual colloidal particles to within a few percent, while simultaneously allowing to measure the three dimensional position of the particles in the sample with nanometer accuracy. By studying the dynamics from multiple images we can analyze the distribution in size, refractive index, density, diffusion coefficient, sedimentation coefficient and correlations thereof.
[1] Sheng et al. Applied Optics 45, 3893 (2006)

Markus Bier (Utrecht)
Nonequilibrium steady states of platelet fluids
I will briefly describe an investigation of nonequilibrium steady states of fluids of platelike colloidal particles by means of dynamic density functional theory.

Andrei Pethukov (Utrecht)
Tiny-angle x-ray scattering from colloidal elephants
I would like to say a few words on the microradian x-ray scattering in application to large (3 to 4 orders of magnitude larger than the x-ray wavelength) colloids. My take-home message is simple: Dutch scientists have an easy access to a unique machine with a world-top angular resolution.


Vijayakumar Chikkadi (UvA Amsterdam)
Slow shear of colloidal glasses- real space investigations
We study the shear of a colloidal glass using a confocal microscope and shear cell arrangement. The motion of individual particles is tracked to investigate the effect of shear. In the context of deformation of amorphous materials, the idea of shear transformation zones has been used by many authors. Recent simulations of amorphous systems subject to shear in athermal, quasistatic limits reveal that the shear transformations during the individual plastic events organise into a cascade. We would like to test these ideas by performing experiments on colloidal glasses.

Katheryna Lyakhova (Eindhoven)
Impact of impurities on the network formation in liquid crystal colloids
The influence of impurities on the network formation in the mixture of liquid crystals and colloidal particles is studied in the framework of phenomenological Landau-de Gennes theory.  Phase diagrams of the mixture are obtained in three regimes: weak, intermediate and strong coupling between colloids and liquid crystal.  We have found that presence of colloids does not insure significant shift in isotropic-nematic transition temperature.  As an alternative model we have studied the mixture of liquid crystal and small amount of non-mesogenic impurity. Our general conclusion is that the transition temperature mainly shifts due to the presence of the third component (non-mesogenic impurity) while the soft solid formation itself is guaranteed by coupling between nematic liquid and colloids.

Michiel Hermes (Utrecht)
Locating the hard sphere glass transition
Using Molecular Dynamics simulations, we study suspensions of pure and polydisperse hard spheres at volume fractions along the metastable extension of the fluid branch. We calculate very accurately the equation of state and observe a sudden deviation from the fluid equation of state proposed by Speedy at a volume fraction of about 57.7+-0.3. A closer study of this deviation (only possible for polydisperse spheres) seems to indicate a kink in the compressibility and a discontinuity in the second derivative of the pressure as a function of volume fraction. Ageing of the system has a strong effect on the precise location of the discontinuity.

Timon Idema (Leiden)
A little motor, a big switcher!
Molecular motors can cooperatively pull membrane tubes. Some types of motors are very efficient can continuously pull for a long time. Others are seemingly useless as they stop pulling after a single step. We study the emergent behaviour of a minimal system containing these inefficient motors and find that they have rich dynamics of their own.

Marieke Bode (Utrecht)
Vesicle mysteries
Molybdenum-iron-clusters tend to form hollow superstructures in solution. The interactions that cause this structureforming are yet unknown. What is the role of the cluster-charge on the interactions between the clusters?

Sandra Veen (Utrecht)
Self-assembly of single molecular inorganic clusters
Self-assembly is a process seen in many different systems, ranging from surfactant micelles, lipid bilayers to virus capsids. It recently became clear that also single molecular inorganic clusters known as polyoxometallates (POMs) can organize themselves into superstructures in a globally comparable manner. They have been shown to spontaneously and reversible form large hollow spherical superstructures, much like surfactant micelles or virus capsids. Interestingly some of the POM's are spherically symmetric yet form the hollow vesicle resembling structures. In the soundbite running projects regarding the research on the vesicle superstructures will be presented.

Andriy Kyrylyuk (Eindhoven)
Percolation of Carbon Nanotubes in Solid Polymer Composites and in Fluid Colloidal Media
We apply continuum connectedness percolation theory to carbon nanotubes and predict hoe their bending flexibility, length polydispersity, and attractive interactions between them influence the percolation threshold. We argue that the host matrix in which the nanotubes are dispersed controls this percolation threshold through the interactions it induces between them during processing, and through the degree of connectedness that must be set by the tunneling distance of electrons, at least in the context of conductivity percolation. This provides to manipulate the percolation threshold and the level of conductivity in the final product.

Joshua Dijksman (Leiden)
Rheology of weakly vibrated granular materials
Weak vibrations induce liquid-like behavior in otherwise static granular packings. Here we report some preliminary results of a more quantitative probe into this phenomenon: we study the average torque necessary to sustain the rotation of a disk buried in a weakly vibrated granular packing. We find that the usual rate independence in these systems breaks down when weak vibrations are introduced. Furthermore the stress-strain rate curves for different vibration amplitudes can be collapsed onto a master curve.

Eelco Eggen (Utrecht)
Charged Colloidal Rods
In the context of Onsager's theory for the phase behaviour for hard rods, we formulate a theory for charged colloidal rods in the finite length limit. Qualitatively, we match the charge and interaction screening dependence by fitting effective hard rod dimensions. This is done in the description of a second order virial expansion.

Duc Nguyen (UvA Amsterdam)
Controlled growth of hard-sphere colloidal crystal
We use a colloidal system to study crystal growth on the atomic scale. By applying a temperature gradient we are able to control the growth of large single crystals. We can visualize nucleation and solidification at the interface in three dimensions by using confocal microscopy. Studying the crystal-fluid interface will give us insight into the mechanism of solidification

Hua Guo (UvA Amsterdam)
Experimental evidence for the long range attration in the colloidal system
We report observations of fluid-fluid and fluid-solid phase transitions in a density matched system of charge stabilized polystyrene spheres suspended in a quasi binary 3-methylpyridine(3MP) /H2O/D2O mixture.The phase transitions occur close to the temperature where the binary liquid mixture demixes. Using small angle  x-ray scattering , turbidity measurement and macroscopic observations, we determine the phase diagram for colloidal system. The similarity with the diagram proposed by Frenkel for attractive colloidal systems is striking. Using temperature as a control parameter we can induce phase transitions with external control.

Mounir Aytouna (UvA Amsterdam)
Transient surface tension sffects on the impact dynamics of surfactant laden droplets
We study the impact and retraction of surfactant laden droplets upon high sped impact on hydrophobic surface. We show that the spreading stage and some extend the retraction rate depend on intrinsically properties of the surfactants. We capture those properties by a single measurement : the dynamic surface tension.

Jos Zwanikken (Utrecht)
Spontaneous oil-in-water emulsification
Since the experimental evidence of thermodynamically stable oil-in-water emulsions, we are trying to understand the mechanism from the microscopic details of the system. We consider ion partitioning, strong ion correlations in the oil phase, and the addition of small colloidal particles, in a modified Poisson-Boltzmann model.

Klaas Besseling (Delft)
Reversible supramolecular polymers at surfaces
Recent advances in synthetic supramolecular chemistry yielded a fascinating new class of responsive materials: reversible supramolecular polymers (RSPs, also known as equilibrium polymers or living polymers). These are chains formed by reversible association of purposely designed molecules. Typically, such molecules possess two (self)complementary binding sites, and by dimerisation of these ‘stickers’ linear chains are formed. RSPs have similarities with ‘ordinary’ polymers, but there are also important fundamental differences: RSPs are dynamic: supramolecular bonds break and form continuously. As opposed to ‘ordinary’ polymers the degree of polymerisation of RSPs is not a fixed property but responds to conditions such as temperature and concentration. Furthermore, the chain-length distribution is intrinsically polydisperse. In recent years we have pioneered the collective behaviour of these objects. In particular their behaviour at surfaces. It is well known that dissolved polymer induces interactions between immersed surfaces (of e.g. colloidal particles). In the case that the polymer has no affinity for the surfaces a depletion layer develops near each surface for entropic reasons. A decreased polymer concentration in a slit between two surfaces yields an attractive surface force, usually called depletion interaction. Using colloidal-probe atomic force microscopy and the EHUT/DBUT system we succeeded recently to obtain the first experimental results on depletion interactions due to a reversible supramolecular polymer. Furthermore, it was demonstrated how these interactions are controlled by ‘stoppers’, monofunctional monomers that induce ‘dead chain ends’.

Esther Vermolen (Utrecht)
Creating templates for nucleation and growth of colloidal (photonic) crystals
We present easy-to-use and inexpensive techniques to make epitaxial colloidal templates, which can be used to selectively grow colloidal photonic crystals, e.g. the binary Laves structure MgCu2 [1]. This binary structure consists of large particles in the photonically advantageous diamond structure and small particles in the pyrochlore structure, which also recently has been identified as a good candidate for a photonic crystal with a large complete band gap in the visible. Using templates made by time-shared optical tweezers or by lithography, and by controlling the volume fraction of the dispersion by dielectrophoresis, we can study the nucleation and growth of metastable photonic crystals from colloidal dispersions as a function of the volume fraction and the symmetry and spacing of the template.
[1] A.-P. Hynninen, J. H. J. Thijssen, E. C. M. Vermolen, M. Dijkstra, and A. van Blaaderen, Nature Materials 6, 202-205 (2007).

Chase Broedersz (VU Amsterdam)
Effective medium theory for flexibly cross-linked stiff polymer networks
Motivated by recent experiments showing novel rheological properties of cytoskeletal networks, we develop an effective medium theory for rigid filament networks with flexible cross-linkers. Specifically, we treat such a network as a collection of stiff polymers mechanically connected by highly compliant cross-linkers to an elastic continuum, which self-consistently resembles the non-linear elasticity of the surrounding network. The model yields a linear elastic regime dominated by cross-link elasticity for low strains.  However, the finite compliance of the cross-linkers causes the network to stiffen dramatically at higher strains. This non-linear regime is consistent with recent experimental studies of networks made of the cytoskeletal polymer F-actin cross-linked by filamin.

Enrico Conti (VU Amsterdam)
Simulations of semiflexible networks: nonaffine deformations and nonlinear elastic response
The cytoskeleton of eukaryotic cells has several important functions such as mechanical stability, force sensing, intracellular transport and locomotion. The elasticity of the cytoskeleton is mainly due to a filamentous network of semiflexible polymers that includes both actin and microtubules. Such filaments are qualitatively different from flexible polymers because of their bending rigidity. This results in, among other things, strongly nonlinear elasticity of cytoskeletal networks. Such systems have recently been shown to exhibit negative normal stresses. We simulate model 2D networks of semiflexible filaments using energy minimization techniques, in order to study the origins of such anomalous normal stress behavior. In particular, we examine the connection between normal stresses, nonaffine or heterogeneous strain and geometry of these networks.

Maurice Mourad (Utrecht)
Gelation and liquid crystal phase separation for model clays
Attraction and repulsion induces gelation of charged colloids in water depending on particle concentration and ionic strength. In between these gel states liquid crystalline phase separation is observed for aqueous suspensions of Gibbsite platelets. The isotropic-nematic and the sol-gel transitions have been studied in detail for this system by complementary static and dynamic techniques, ranging from direct visual observations to polarization microscopy, small angle X-ray scattering and rheometry. The results of these experiments facilitate the understanding of the gel and phase transitions that occur in this system.

Laura Filion (Utrecht)
Binary Hard Sphere Crystal Structures Predicted By Genetic Algorithms
We study the close packed structures of binary hard spheres for various stoichiometries and small-to-large sphere diameter ratios using a genetic algorithm. Instead of relying on a set of preselected structures, the genetic algorithm can be used to search the entire phase space of possible structures with only the maximum number of elements in the unit cell restricted. In addition to predicting many of the known structures, such as NaCl and AlB2, we find additional structures which may play a role in the phase diagrams of hard spheres with small-to-large sphere diameter ratios near 0.8.