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SoftMatterMeeting2

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Abstracts Talks 

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Jens-Uwe Sommer (Institute for Polymer Research Dresden)
Polymer Brushes at moderate and high grafting densities: Effects of chain stretching and end-groups

Polymer brushes obtained by end-grafting of linear chains are promising systems for various applications ranging from colloid stabilization over switchable surfaces to biocompatible surfaces. Recent experimental techniques allow high grafting densities implying strong stretching of the individual chains 1. We have investigated static and dynamic properties of polymer brushes at moderate and high grafting densities using molecular dynamical techniques. Severe deviations from the predictions of self-consistent field theories can be related to the finite extensibility of chains which plays an important role for higher grafting densities 2. In accordance with previous simulations we have shown that the distribution of chain ends is strongly peaked at the brush surface and that the overall density profile displays a steep decrease in the surface region. Thus, conformational states of chains are strongly influenced by the surface properties of the polymer brush. As a result small modifications of individual chain properties lead to large effects: Reducing the size of end-monomers leads to a collapse of modified chains, while a corresponding inflation of end-monomers gives rise to considerable stretching. Figure 1 shows a snapshot of a polymer brush with inflated monomers. It can be clearly observed that inflated end-monomers are swimming on top of the brush (high stretching). A similar effect can be achieved by a slight modification of the chain lengths (adding or removing individual monomers). Our results indicate that dense brushes with a stretching ratio larger then about 0.6 lead to a surface dominated behavior which turns out to be highly selective with respect to chain size and types of endmonomers

Abstract_Sommer

Fig. 1 Snapshot of a polymer brush containing chains with inflated end-monomers

Daniela Kraft (Utrecht)
Spontaneous formation of colloidal molecules
A facile and flexible synthesis for colloidal molecules with well-controlled shape and tuneable patchiness is presented. Cross-linked polymer spheres with a liquid protrusion were found to assemble into colloidal molecules by merging of the liquid protrusions. Subsequent polymerization yields patchy particles with a distinct geometry for each number of spheres. A wide topological variety has been achieved by changing the amount and nature of the swelling agent as well as the wetting angle between the liquid and the seed particles. Use of different swelling agents allows for chemical diversity of the patches and the centre.
For small amounts of monomer, the colloidal molecules resemble clusters that minimize the second moment of the mass distribution. Assemblies comprised of a large number of colloids are similar to colloidosomes exhibiting elastic strain relief by scar formation.

Ruud van der Smaan (Wageningen)
Cooking Soft Matter
We have viewed cooking meat from the perspective of soft condensed physics and posed that the moisture transport during cooking can be described by Flory-Rehner theory of swelling/shrinking polymer gels. This theory contains the essential physics to describe the transport of liquid moisture due to the denaturation and shrinkage of the heated protein matrix. The validity of our hypothesis is shown by a numerical model, which comprises a linearization of the Flory-Rehner theory augmented with Darcy’s law for porous media flow, applied to the simulation of cooking experiments performed on a rectangular piece of beef. As the driving force for moisture transport, we take the swelling pressure as derived from Flory-Rehner theory – and which can be correlated to the Water Holding Capacity of meat. Reasonably, comparable results are obtained from simulations and experiments (Bengtson,1974). Further analysis of simulations resolves yet another unexplained phenomena observed during heating of meat (van der Sman, 2007), namely the initial rise in moisture content in the centre of the meat during cooking. Literature review suggests that Flory-Rehner is applicable to cooking of other gel-like foods (van der Sman, 2008).

Alexey V. Lyulin (Eindhoven)
Soft Glasses: Direct Atomistic Simulation of Deformed Polymers
A molecular understanding of aging via structural relaxation and strain softening is still lacking, and even more so for polymers. We [1] use molecular-dynamics computer simulations to explore the influence of thermal and mechanical history of typical glassy polymers, atactic polystyrene (PS) and (bis)phenol A polycarbonate (PC), on their deformation. Polymer stress-strain and energy-strain developments have been followed for different deformation velocities, also in closed extension-recompression loops. The latter simulate for the first time the experimentally observed mechanical rejuvenation and overaging of polymers. Energy partitioning reveals essential differences between mechanical and thermal rejuvenation. The main message of our study is that differences in stress-strain and energy-strain curves between the polymers could be interpreted in terms of ratios between the relevant time scales: the times for cooling down from the high-temperature melts, the times for deformation up to the yield point (forced out-of-cage motion), and the ¥- relaxation times. The results give insight into the energy landscapes and relaxation mechanisms of the deforming polymer glasses, in terms of differences between quenched and annealed samples, between deformations at different rates, and between thermal and mechanical rejuvenation. Qualitatively these conclusions can be translated towards upscaled experimental conditions [2]

[1] A.V. Lyulin and M.A.J. Michels, Phys. Rev. Lett., 99, 085504 (2007).
[2] E.T.J. Klompen, T.A.P. Engels, L.E. Govaert and H.E.H. Meijer, Macromolecules 38, 6997 (2005).

Joris Remmers (Eindhoven)
Jamming and rheology of foams


Allard Mosk (Twente)
Universal optimal transmission of light through disordered materials
Transmission of coherent waves through disordered materials is a subject that has been studied theoretically and experimentally, especially in condensed matter systems. Theorists have predicted that in any non-absorbing disordered sample there exist open eigenchannels: specific linear combinations of incoming waves which experience a transmittance of nearly one. Such eigenchannels have not been directly observed in condensed matter systems, however, they are the cause of phenomena such as universal conductance fluctuations.
By carefully constructing a suitable linear combination of incoming laser light waves on a disordered optical material (zinc oxide powder) we find we can selectively couple light to the open eigenchannels.
We are able to increase both the transmitted intensity in a single point and the total diffuse transmittance. The magnitude of the increase is exactly as predicted by random matrix theory.
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Abstracts Soundbites

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Sandra Veen (Utrecht University)
Shape transitions in concentrated aqueous solutions of Polyoxometalates
Inorganic macromolecules known as polyoxometalates (POMs) can spontaneously and reversible organize themselves into large hollow spherical superstructures in solution. We now show that that these POMs display even more fascinating behaviour. Upon increasing the concentration in aqueous solution these POMs can undergo a shape transition: from single layer shells to long needle-like superstructures. These needle-like structures are about 100 nm thick but can grow up to several micrometers in length. Single needles as well as large clusters of needles can be distinguished in solution. This type of shape transition has not been observed in POM systems before.

Eelco Eggen (Utrecht University)
Boundary conditions in the anisotropic Poisson-Boltzmann cell model
Theorists have been using cell models for solving the Poisson-Boltzmann equation for a while now. However, most of the efforts have been focussed on systems of particles with a homogeneous surface charge. I will show you that the cell model concept can be generalized to particles with an anisotropic charge distribution.

Esther van den Pol (Utrecht University)
Magnetic properties of liquid crystalline goethite
Dispersions of boardlike goethite particles form a colloidal system that both shows rich liquid crystalline phase behaviour and has peculiar magnetic properties. The particles possess a considerable permanent magnetic moment along their long axis, presumably due to uncompensated surface spins within their anti-ferromagnetic crystal structure, combined with an induced moment with an easy axis predominantly along the shortest particle dimension. The interplay between them leads to peculiar field-strength-dependent reorientation phenomena. The behaviour of the different liquid crystalline phases in an external magnetic field was studied by Small Angle X-ray Scattering.

Bob Luigjes (Utrecht University)
Preparation of Magnetic Composite Colloids as Model System for Dipolar Hard Spheres
The synthesis of magnetic PMMA latex microspheres is presented. Thermodynamically stable Pickering emulsions of magnetic nanoparticles in an oil-water mixture are used as templates for colloidal magnetic spheres [1-3]. The properties of these particles can be tuned by varying different experimental parameters, such as the amount of oil or the salt concentration. Such model particles are being developed to study the phase behavior of colloidal dipolar spheres that are dispersed in a liquid.
[1]. S. Sacanna, W. K. Kegel and A. P. Philipse, Phys. Rev. Lett. 98, 158301 (2007)
[2]. S. Sacanna and A. P. Philipse, Langmuir 22 (24), 10209-10216 (2006)
[3]. S. Sacanna and A. P. Philipse, Advanced Materials 19 (22), 3824-3826 (2007)

Sissi de Beer (University of Twente)
Frequency dependence of small amplitude AFM spectroscopy measurements in liquid
In this soundbite we discuss a harmonic oscillator model for AFM cantilever dynamics including the finite amplitude of the driving mechanism.The model is compared to quantitative measurements of the oscillatory forces in Octamethylcyclotetrasiloxane (OMCTS) confined between the AFM cantilever tip and highly oriented pyrolitic graphite (HOPG) surface. The model and the measurements agree quantitatively.

Ivo Peters (University of Twente)
Shape selection of drops sliding down an inclined plane
Experiments have been performed on drops sliding down an inclined plane under partial wetting conditions. Above a critical speed, a cornered shape with a very sharp tip can be observed at the rear of the drop. Here we show that the tip curvature increases exponentially with speed during the formation of the corner.

Peichun Tsai (University of Twente)
Slippage over and Splashing upon “Soft” Microstructures

Bram Verhaagen (University of Twente)
Ultrasonic cleaning of root canals
My work is on the cleaning of root canals using ultrasonically vibrating files, a technique which is already used in dental practice. The exact cleaning mechanism is not known however, but probably involves microstreaming and cavitation in order to remove biofilms and debris from root canal walls.

Julian Martinez Mercado (University of Twente)
Turbulent bubbly flows
In my PhD project we are interested in studying the effect of injecting bubbles in turbulent flows. Dispersed-particle flows either augment or attenuate the kinetic energy of the interstitial fluid. A better understanding of this phenomena is crucial, not only for the physical point of view but for the industrial as well (i.e. scale-up of equipment). Another important issue in dispersed flows is how do the particles (in our case, bubbles) accumulate within the flow? Do they have a preferential orientation? Novel experimental techniques are applied to answer these questions.

Antina  Ghosh (UvA Amsterdam)
Density of states and soft modes of hard sphere colloidal glasses – experimental observations
Recent theories and simulations have predicted the presence of soft modes due to which the density of states (DOS) of glassy materials does not go to zero at zero frequency. We obtain DOS of colloidal hard sphere suspensions from experimental data. The displacement fields of hard sphere colloidal suspensions were studied for a range of volume fractions near the glass transition using confocal microscope. Normal mode frequencies are then computed from the time averaged correlation matrix. The density of vibrational states obtained from normal mode analysis indeed reveals the presence of excess of low frequency anomalous modes in the system.

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.

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