Mirjam Leunissen (Amolf)
Daniela Kraft (Utrecht)
Demitri Desinov (VU Amsterdam)
Casimir-like effect on a granular pile
Casimir derived in 1948 that an attractive force between two metal plates should exist due to the vacuum fluctuations of electromagnetic waves. Since the Casimir effect is due to electromagnetic fluctuations, it seems plausible that confinement of other fluctuating fields could give rise to a similar effect. Here we show that a Casimir-like effect can be observed in a avalanching granular pile, which is a self-organized critical system. We study the Casimir-like effect between two plexiglass sheets placed on the pile parallel to each other and parallel to the mean avalanche flow direction. We find that indeed the sheets are moving together under the influence of the avalanches confined by the sheets. During 40 experiments, 25 hours each, for initial inter-sheet distances ranging from 20.0 mm to 90.0 mm we observe a decreases ranging from 6.0 mm to less than 1.0 mm.
Burak Eral (Twente)
Anisotropic and hindered diffusion of colloids in 3D closed cylinders
Video microscopy and particle tracking were used to measure the spatial dependence of the diffusion coefficient (D) of colloidal particles in a closed cylindrical cavity. Both height and radius of the cylinder were equal to 9.0 particle diameters. The number of trapped particles was varied between 1 and 16, which produced similar results. In the center of the cavity, D? turned out to be 0.75 times D0 measured in bulk liquid. On approaching the cylindrical wall, a transition region of about 3 particle diameters wide was found, in which the radial and azimuthal components of D? decrease to respective values of 0.1 and 0.4 times D0, indicating asymmetric diffusion. Hydrodynamic simulations of local drag coefficients for hard spheres produced very good agreement with experimental results. These findings indicate that the hydrodynamic particle-wall interactions are dominant, and that the complete 3D geometry of the confinement needs to be taken into account to accurately predict the spatial dependence of diffusion.
Thomas Krebs (Wageningen)
Droplet Coalescence in Flowing Dispersions
The control of the stability of flowing emulsions and dispersion is of crucial importance in e.g. the separation of crude oil from water during oil production. Theoretical considerations suggest an optimal collision force between two droplets for which a maximal coalescence rate is obtained. The experimental investigation of the coalescence kinetics in dense flowing dispersions has been difficult to achieve so far, since the high droplet concentration poses a hindrance for many experimental techniques under dynamic conditions. In this work we present the results of a microfluidic study on the break up of oil-in-water emulsions in linear flow. Monodisperse droplets of hexadecane in water formed at a microfluidic T-junction were injected into a collision chamber. Droplet coalescence was followed with a microscope and a high-speed camera. Experimental variables were droplet size, flow rate and dispersed phase volume fraction in the collision chamber. The evaluation of the data yields the evolution of the average droplet size and the rate constants for coalescence between different combinations of droplet sizes. We find that the coalescence rate constants decrease with increasing flow rate and vary rather weakly with droplet size. The distribution of individual coalescence times between droplets is also obtained and compared with existing theories.
Marleen Kamperman (Wageningen)
Bioinspired Reversible Adhesives by Micro- and Nanopatterning Techniques
The subject area of adhesion not only poses many interdisciplinary fundamental questions but is also of great interest in microfabrication, biomedicine, construction industry, sports equipment etc. Several materials strategies over the last years have been inspired by biological systems: the adhesion performance of flies, spiders and geckos was investigated by nanomechanical techniques and traced back to a combination of van der Waals and capillary forces. A common feature is the miniaturisation of fibrillar contact elements, which in the case of the gecko reach nanoscopic dimensions. We develop micro- and nanostructured surfaces using patterning technologies to investigate the effect of material properties and geometry on adhesive forces and energies. These geometry-property relationships are used to guide the design of novel bioinspired artificial adhesives.
Aatish Kumar (UvA Amsterdam)
Multiscale modeling of hybrid networks of telechelic polypeptides
Marlous Kamp (Utrecht)
Thermally switchable patchy particles
Patchy particles are expected to form gel networks in samples with a high volume fraction. We attempt to make colloidal patchy particles with thermally switchable interactions. We want to make the patches thermally switchable by - locally - grafting specific alkanes and alcohols. Silica spheres grafted with these moieties namely undergo a sol-gel transition in well-chosen apolar solvents upon temperature decrease.
Nina Elbers (Utrecht)
Smart nanoparticles at the oil-water interface
Lowering the surface tension to such an extent that spontaneous emulsification becomes possible has many industrial applications, for example in enhanced oil recovery. We want to develop a system that is capable of doing this by using nanoparticles that adsorb to the water-oil interface and hereby act as a Pickering emulsifier and can simultaneously act as containers for surfactants as a result of their porous and hollow structure. By coating these porous shells with a microgel that collapses or swells as a function of polarity, pH or temperature, the content (i.e. surfactants) can be released upon the trigger that expands the microgel and opens up the pores.
Wolfgang Lechner (UvA Amsterdam)
Reaction coordinates in the crystal nucleation
Using transition path sampling methods combined with a likelihood maximization of the committor function we find optimal reaction coordinates for the crystal nucleation of soft colloidal particles. The use of novel structural order parameters sheds a light on the interpretation of surface and volume term in free energy.
Haining An (TU Delft)
Soft Magnetorheological gels
Self-assembled physical gels of triblock copolymer SEBS swollen in paraffin oil containing pre-aligned magnetic particle strings provide an easy way to assemble highly responsive magneto rheological material. Magnetorheological gels are an intermediate system between magnetorheological elastomers and magnetorheological fluids both with directional dependent response and partial re-arrangement of the particles network.
Igor Saulo Santos de Oliveira (Twente)
Alignment of particles in sheared viscoelastic fluids
We investigate structure formation of colloids dissolved in a polymer solution and in a worm-like micellar solution, using Responsive Particle Dynamics (RaPiD) simulations [1,2]. While Newtonian liquids react to shear deformation by developing stresses proportional to the applied shear rates, complex fluids display a variety of more complex stress versus rate-of-strain relationships. The two viscoelastic fluids studied here are shear thinning, i.e. their apparent viscosities decrease with increasing shear rates. Both fluids contain long flexible chains, whose entanglements appear and disappear continuously as a result of Brownian motion and the applied shear flow. In the RaPiD simulation method, each chain is modelled by a single smooth Brownian particle with slowly evolving interparticle degrees of freedom accounting for the entanglements. The simulated storage and loss moduli of the two fluids agree well with experimental results , and both fluids are shear thinning. Spherical colloids dispersed in the quiescent fluids mix homogeneously. Under shear flow, however, the colloids in the micellar solution align to form strings in the flow direction while the colloids in the polymer solution remain randomly distributed. These observations are in agreement with recent experiments .
 A. van den Noort, W.K. den Otter and W.J. Briels, Europhys. Lett. 80, 28003 (2007).
 A. van den Noort, W.J. Briels, J. Non-Newtonian Fluid Mech. 152, 148 (2008).
 F. Snijkers, G. D’Avino, P.L. Maffettone, F. Greco, M. Hulsen, J. Vermant, J. Rheol. 53, 459 (2009).
 R. Pasquino, F. Snijkers, N. Grizzutti and J. Vermant, Langmuir Lett. 26, 3016 (2010).
Ceyda Sanli (Twente)
Morphological analysis of the density-dependent clustering on a standing Faraday wave
Floating granular particles on a standing Faraday wave accumulate at the nodal points of the wave due to the wave drift. By the help of the increasing importance of the capillary attraction, in the dense limit the motion of the particles becomes complex. Understanding the resultant spatial distribution of the particles and the clustering is of fundamental importance to characterize such a complex system since the resultant clustering directly affects the flow properties of the particles, and brings a different perspective to the granular flow of particles on surface waves. In this study, we show that the Minkowski functionals can quantify the different types of clustering that we have observed in our experiment.
Thijs Besseling (Utrecht)
Drying & fracture in colloidal suspensions
Drying of a colloidal suspension is commonly observed in everyday life, for example while painting a fence. During evaporation of the solvent enormous pressures are exerted on the particles, which can lead to large stresses and undesirable fracture of the material. Indicative of the complicated nature of this problem is that there are several models that contradict each other on even the most basic assumptions. The situation gets even more complex when the goal is to end up with regular arrangements of particles. We want to significantly increase our understanding of drying and fracture in colloidal suspensions using quantitative, real-space confocal microscopy analysis together with model systems in which we can tune the size, polydispersity and interactions of the particles.
Li Liu (Twente)
Simulations on Coarse-grained polymer melts
The transport properties of polymer melts with different molecular weights have been studied in Responsive Particle Dynamics (RaPiD) model, by representing each polymer chain as a single particle in Brownian Dynamics simulations. We investigate the scaling behaviour of this model, which will allow us to map the coarse grained model with experimental data and develop an efficient approach to simulate different polymer melts.
Joost de Graaf (Utrecht)
Triangular Tessellation: Crystal Structures of Anisotropic Particles
Recent advances in the ability to synthesize highly anisotropic particles have created a demand for simulation techniques capable of handling such complex shapes. By combining two existing methods, we have obtained a way to predict the crystal structure of anisotropic particles. We use this technique to reproduce known literature values for the densest packing of several species of particle, in order to analyze the precision that can be achieved. Our results give confidence in the application of this method to nonconvex particle models, for which we will show a preliminary result.
Mieke Kleijn (Wageningen)
Surface forces in apolar media: linear versus cyclic
Interaction between dissimilar surfaces through low dielectric constant media plays an important role in many industrial applications. In the framework of such an application, i.e. dry-cleaning, we study the interaction forces between a cellulose layer and a hydrophilic silica particle through model apolar solvents like hexane and cyclohexane. These low polarity solvents always contain trace amounts of water dissolved in them forming molecular solutions. Using AFM we have shown that this dissolved water (~0.01%) enhances the force of adhesion between cellulose and silica when interacting through hexane. The magnitude of the adhesion force was found to increase with increasing surface delay (0-200 seconds). In contrast to hexane, however, trace amounts of water in cyclohexane do not lead to enhanced adhesion forces. Further to our force measurements, the initial contact angle (water - (cyclo)hexane – cellulose) also displays that there is a difference in behavior between linear and cyclic hexane. The water contact angle through linear hexane shows a time dependence, as opposed to cyclic hexane.
Jissy Jose (Utrecht)
Buckling, loading, and overloading of monodisperse elastic microcapsules
We present a novel method to fabricate bowl shaped microparticles with tunable bowl depth out of spherical PDMS encapsulated silica colloids, prepared via emulsion-templating technique.The method involves controlled buckling of shells by partly dissolving the enclosed PDMS in surfactant solutions.The key innovation of this synthesis route lies in the tunability of bowl depth which is achieved by varying the concentration of surfactant. Furthermore we studied the elastic relaxation of bowls back to microspheres by loading them with octamethylcyclotetrasiloxane,a silicone oil. With hydrocarbon oils, irrespective of particle shape, led to interesting anisotropic particles due to “overloading” of the particles.
Dominik Michler (UvA Amsterdam)
Dynamics of spontaneous emulsification
We investigate the processes induced by contacting an oil phase with a surfactant (AOT) solution by fluorescence and polarisation microscopy. Above the cmc, the AOT solution contains spherical vesicles in micrometersize, which move towards the oil phase. At the interface between the AOT solution and the oil, a spontaneous emulsification takes place, as soon the vesicles arrive there.
Tian Hui Zhang (Utrecht)
Formation and Evolution of Clusters in Colloids with Competing Interaction
We study the formation and the growth of clusters in a colloidal model system with competing short-range attractions and long-range repulsions. In our studies, stable cluster phases are formed at low colloidal volume fractions. The size of stable clusters decrease upon increasing the attraction strength and stronger attractions make the clusters more linear in shape rather than more spherical, being inconsistent to previous studies. The mechanism underlying the inconsistence is explored in terms of nucleation and two-sided effect of short-range attractions. The finding in this study implicates that by tuning the strength of short-range attractions, we can control the growth model, the structure of clusters and thus the route to gels.
Ronald Otten (TU Eindhoven)
Connectivity percolation of carbon nanotubes in an external field
We show how an external field influences the onset of electrical conduction in carbon-nanotube composites in terms of a preferred orientation of the particles that in turn affects the percolation threshold. We find that the percolation threshold is raised significantly and even disappears in a sufficiently strong external field.
Rojman Zargar (UvA Amsterdam)
Thermodynamics of hard sphere glasses
We apply the free volume theory to our experimental colloidal glass and investigate the structure and the thermodynamics of hard sphere glasses.
Maryam Pakpour (UvA Amsterdam)
Flow of saturated sand
We investigate the flow of partially saturated sand under imposed shear rate and large amplitude oscillatory shear (LAOS).
Cecilia Bernardini (Wageningen)
Complex two-dimensional behavior in a mixed polymeric Langmuir film
Thin polymeric films have been widely investigated because of their relevance in the technological development of advanced materials and coatings; in order to achieve full control of this technology, it is crucial that the fundamental physics underlying the behavior of the thin films is known. A critical aspect in thin film fabrication is the control over the structure at the microscopic level, especially when polymer mixtures are to be used. Bulk behavior of polymer blends does not provide relevant information, as the behavior of polymer chains confined to a surface differs strongly. Langmuir monolayers, then, become a suitable model system in order to address issues such as polymeric thin film structural organization, and the lateral phase separation at the air/water interface of polymer mixtures.
We report a system consisting of a mixed Langmuir monolayer, made of the water-insoluble, spreadable, fluid-like polymers polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA) with a minority P(DMS-b-MMA) copolymer. We have performed both Langmuir trough pressure/area isotherm measurements and Brewster angle microscopy (BAM) observations and complemented the experiments with molecularly detailed self-consistent field calculations. PDMS undergoes a layering transition which is difficult to detect by BAM. Addition of PMMA gives contrast in BAM, now showing a two-phase system: if this consisted of separate 2D PMMA and PDMS phases, a PDMS-PMMA diblock should accumulate at the phase boundary. However, the diblock copolymer of PDMS-PMMA failed to show the expected “lineactant” behavior, i.e. failed to accumulate at the phase boundary. The calculations point to a non-trivial arrangement of the polymer chains at the interface: in mixtures of the two homopolymers we find a vertical (with respect to the air/water interfacial plane) configuration, with PMMA sitting preferably at the PDMS/water interface of the thicker PDMS film, during the PDMS layering phase transition. This also explains why the diblock copolymer is not a lineactant. Both PMMA and P(DMS-b-MMA) are depleted from the thin/?thick PDMS film interface, and the line tension between the phases is, consequently, increased, in the binary mixtures as well as in the ternary ones.
Jurriaan Luiken (UvA Amsterdam)
Simulation of protein aggregation
My research is, for now, focused on studying protein aggregation, and in particular fiber formation, with the help of molecular simulation techniques. In the near future I will use the Deserno force field to study the mechanism of detachment and docking of various proteins to higher aggregates, and compare the results to all-atom molecular dynamics simulations. Long term goals are not set in stone, but they will likely involve investigating the effects of crowding and confinement on this aggregation process.
Vinzenz Koning (Leiden)
Frustrated Nematic Order in Spherical Geometries
We investigate elastic instabilities that take place in spherical shells of inhomogeneous thickness filled up with nematic liquid crystals. For shells with a finite thickness there exists an equilibrium defects structure containing two pairs of boojums. By minimizing the elastic free energy, subject to tangential boundary conditions on both bounding surfaces, we determine the locations of the defects in the shells. As the thickness inhomogeneity is increased, we find that the defects make an abrupt confinement transition to the thinnest hemisphere from an initial antipodal arrangement. These results are discussed in the light of experimental studies on nematic double emulsions.
Susanne van Berkum (Utrecht)
Size control of aqueous magnetite nanocrystals
Magnetic particles for biomedical applications are generally based on iron oxide, which is biocompatible, and size control is crucial for the magnetic properties. We have compared different known methods to prepare aqueous iron oxide nanoparticles, both aqueous methods and nonaqueous methods that require subsequent transfer to water. The results vary not only in the obtained size and polydispersity, but also in the shape of the nanoparticles, including the possibility to make truncated cubes and hexagons.
Adrian Cioroianu (TU Eindhoven)
Non-affine collective modes and elasticity in 2D bond-bending networks
Previous work has clearly demonstrated that non-affine deformation modes in elastic (bio)polymer networks greatly affect their mechanics
– both linearly and nonlinearly. We investigate the effect of non-affinity in the linear elastic regime, by using the full dynamical matrix to numerically determine the lowest-energy deformational modes of rigid 2D crosslinked architectures consisting of central force springs with, or without the inclusion of bond-bending contributions.
Andrea Muntean (TU Eindhoven)
Roughness and ordering at oxidized PS-water interfaces
A key challenge in the development of polymeric biological surfaces (e.g. in a biosensor) is the understanding and control of interactions between polymer surfaces and proteins. A very important factor for the applications is the structure of the polymeric surface in water environment. We study the polystyrene surface in water by means of molecular-dynamics simulations, using Gromacs software package. The surface of the atactic polystyrene (aPS) film is computationally oxidized by attaching oxygen atoms to the phenyl rings, in a fashion which is similar to the experimental photo-oxidation of aPS. The aPS films are characterized by their density profile, the surface roughness and the ordering of phenyl rings near the interface. The influence of water and of the chemical modification on the surface properties is discussed. Also the structure of water near the aPS surface with different degrees of oxidation is analyzed.
Triet Dang (UvA Amsterdam)
Phase diagram of gas-liquid equilibria induced by the Critical Casimir effect
Control over particle assembly is essential for the design of structures at the nano scale. Recently, the Critical Casimir effect has been shown to provide an excellent control over particle interactions with temperature. Our own experiments allowed us to observe the “freezing” of colloidal particles from a colloidal gas into a colloidal liquid. In this report, we combine Monte Carlo simulations in the Gibbs ensemble with the experimentally measured particle pair potentials to investigate the phase diagram and to predict the critical temperature of the colloidal gas-liquid transition. Interestingly, the predicted critical temperature from the simulation shows a very good agreement with that of the experiment.
Tom Hunt (Twente)
Simulations of nanorheology of fibrillar networks
As part of a larger project to characterise and model amyloid fibre networks, some initial rheological results are presented from the simulation of semi-flexible fibres in a solvent. The technique we use (multi-particle collision dynamics) is designed to include the hydrodynamic interactions between fibres. Our model for the fibres or polymers allows us to investigate systems of larger molecular weight than other existing methods. By varying the flexibility of the fibres and their concentration we begin to probe the role that structure and hydrodynamic interactions have in the rheological properties of these liquids.
Rob Kortschot (Utrecht)
Differential setup to measure the dielectric dispersion of colloidal semiconductor nanoparticles in a liquid
Whether colloidal semiconductor nanoparticles have an electric dipole moment is controversial, with important ramifications for the electronic properties and interparticle interactions. The dipole moment can be determined from the effect of nanoparticle rotation in a liquid on the frequency-dependent dielectric permittivity. Our first measurements with our differential complex electrical impedance spectroscopy setup on liquids with low concentrations of nanoparticles demonstrate that the dielectric permittivity can be measured sensitively in the range from 100 Hz to 2 MHz, the typical range for thermal rotation of nanoparticles in a liquid.
Jos van Rijssel (Utrecht)
Visualizing nanoparticle interactions with cryo-TEM
For the understanding of nanoparticle self assembly, the knowledge of the interactions involved is of great importance. Using cryo-TEM, we are able to visualize the equilibrium structures of nanoparticles and derive the interaction free energy for these equilibria.