Izabela Piechocka (FOM-AMOLF)
Fibrin as a highly extensible biopolymer
For over 60 years a lot of interest has been put into fibrin, which is the main component of blood clots. While the polymerization and structure of fibrin fibers is fairly well described, the elastic properties of this biopolymer are still poorly understood. We have characterized the non-linear viscoelastic properties of fibrin networks over a large range of protein concentrations (0.1 – 8 mg/ml) by measuring the strain response to an oscillatory stress. We analyzed the full non-linear response by Fourier Transform spectra as well Lissajous plots of the stress-strain behavior. Our results show that fibrin polymers behave as typical semiflexible polymers at small strains, but are much more extensible at large strains.
Peter Eshuis (University of Twente)
Granular Convection: Experiment, Numerics, and Theory
Buoyancy driven granular convection is studied for a shallow, vertically shaken granular bed in a quasi 2-D container. At sufficiently strong shaking counter-rotating convection rolls form with pronounced density variations. These rolls are also found in our Molecular Dynamics simulations. The onset of convection is quantitatively explained through a linear stability analysis of the hydrodynamic continuum model of the granular Leidenfrost state presented in [Phys. Rev. Lett. 95, 258001 (2005)].
Henk Jan van Gerner (University of Twente)
Coarsening of Faraday heaps: Experiment, Simulation, and Theory
Heaps of granular material that are formed on a vibrating plate (Faraday heaps) tend to merge on an ever increasing timescale, a process which is little understood. We report on a model for this coarsening behavior of Faraday heaps, which captures the essential features of this complicated process in two differential equations. The results of the model are in excellent agreement with both experiments an simulations. We deduce from the model that the average life time of a N-heap state decays as N-3.
Andriy Kyrylyuk (Utrecht University)
Jamming of Non-Spherical Particles: from Dense Colloidal Mixtures to Polydisperse Granular Materials
Understanding the properties of amorphous packings of dense colloidal particles and granulates at the jamming point is a key issue for the performance optimization of composite materials, colloidal dispersions and glasses, granular and porous media as well as fiber networks in biological cells. A jammed system usually consists of particles that are polydisperse in size and shape – a packing of pure spheres is rather the exception than the rule. Recently the Mechanical Contraction Method was developed in our group to theoretically investigate the packings of non-spherical particles of different shape such as spherocylinders, spheroids and cut spheres. Here, we investigate the effects of non-sphericity on random close packing by means of mechanical contraction simulations. We find that the mixtures of polydisperse rod-like particles exhibit some universality in packing. The particles pack most efficiently/densely at one unique rod aspect ratio of one component that has a near-spherical shape, independently of the mixture composition and the sizes of the other components. Our simulations also reveal another striking and non-intuitive behavior of packing of near-spheres in binary rod-sphere mixtures – its locality. The dependence of the maximum in packing density of the mixture on the volume fraction of its components is linear, which suggests that the packing is governed by local contacts between the neighboring particles. The plausible explanation for this behavior is that the correlations between the particles are completely lost in the range of distances of several particle sizes.
Vijayakumar Chikkadi (UvA Amsterdam)
Shear banding in colloidal glasses
We perform slow shear of colloidal glasses using a confocal microscope and shear-cell set up. The particles are tracked in time and space to construct the local strain field, which is observed to be non-uniform with high strain and low strain zones interspersed in space. We intend to perform experiments at different shear rates to delineate the role of shear transformation zones in the formation of a shearband and also to establish the regime of their formation. A few preliminary results would be presented.
Liesbeth Huisman (Leiden University)
Compression and stretching in composite networks
Thus far, little is known about the mechanical behavior of biopolymer networks composed out of multiple types of filaments. We build upon our previous computer simulations of single-component biopolymer networks to gain insight in the mechanical behavior of composite networks. By generating and deforming mixed networks consisting of both floppy and stiff filaments we can monitor the macroscopic behavior of the network and the microscopic behavior of individual filaments. This enables us to address the individual roles of composite constituents in the mechanical response during deformation, and eventually to relate composition and structure to the biological function of the network.
Oksana Manyuhina (Radboud University Nijmegen)
Topological defects in soft matter: interplay between molecular order and curvature
Burak Eral (University of Twente)
Confinement-induced effects in concentrated colloidal suspensions
When a molecular fluid is cooled down at sufficient rates, it is unable to reach thermodynamic equilibrium, but is dynamically arrested. This transition from an ergodic to a nonergodic state is known as the glass transition. Such a transition is also known for colloidal particle suspensions. The influence of spatial confinement on the particle dynamics and the liquid to glass transition is far from well understood for these soft materials. We have studied the effect of confinement on particle mobility, structural relaxations and glass transition, using a home built confinement apparatus with a micron gap “sphere-plane geometry” and mounted on a Confocal Scanning Laser Microscope (CSLM). A concentrated suspension of fluorescent core-shell silica particles with a radius of 550 nm was used as a model fluid. The colloidal particles were tracked with the CSLM in two dimensions for various gap sizes. We observed a significant dependence of the particle dynamics as well as the density profiles on the gap spacing. At volume fractions below the glass transition in bulk, the particle mobility decreased with decreasing gap size, which indicates that the system approaches the glass transition.
Jan Hilhorst (Utrecht University)
Disorder-Induced Stacking Order in Sedimentary Colloidal Hard Sphere Crystals
Using confocal microscopy, we found that stacking disorder in colloidal crystals occurs in all <111> directions. Pairs of hexagonal close packed planes at 70 degree angles with the substrate were found to be accompanied by large regions of pure face centred cubic (fcc) crystals. We propose that the surface step created by the stacking faults acts as a non-degenerate nucleation centre for new layers, resulting in pure fcc crystal growth. We are currently investigating the possibility of exploiting this mechanism to selectively incorporate defects in colloidal crystals by substrate templating.
Esther Vermolen (Utrecht University)
Real-space study of colloidal nucleation
We studied the nucleation and growth of a colloidal crystal from a dispersion in real-space by confocal microscopy. The nucleus was induced by a template of tracer-particles fixed by time-shared optical tweezers in the bulk of host particles. By controlling the volume fraction of the dispersion by dielectrophoresis, we can study the nucleation and growth of colloidal crystals from dispersions as a function of the volume fraction and the symmetry and spacing of the template.