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  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.
 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)
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 . 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.
 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.