Abstracts Talks 

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Hans Wyss (TU Eindhoven)
Squishy Particles as Model Glass Formers: Soft Colloids make Strong Glasses
The colloidal glass transition exhibits remarkable analogies to glass formation in molecular systems. The dynamic arrest of hard sphere suspensions as their volume fraction is increased closely mimicks the behavior observed in molecular glass formers as they are cooled down towards their glass transition temperature. But molecular glasses show a wide range of behaviors in terms of their so-called fragility, a measure for how sensitively the viscosity and structural relaxation time changes as the glass transition is approached. In colloidal systems, only highly fragile behavior has been observed. However, our experiments show that by varying the softness of the colloidal particles we can access the entire range of fragilities found in molecular glasses. We use polymeric microgel particles as a model system; their mechanical behavior can be easily controlled through changes in chemical composition. We find that the fragility of the colloidal glasses decreases with increasing softness of the particles; the softest particles exhibit a strong dynamic behavior, where the relaxation time shows a Arrhenius-dependence on concentration. We thus extend the richness found in the behavior of molecular glass formers to colloidal glasses, which opens up new possibilities for studying glass formation in colloidal model systems. Moreover, our experiments suggest that elasticity is a key towards understanding the origin of fragility in glasses, both colloidal and molecular.

Sandra Veen (Utrecht)
Shape transformations in solutions of Polyoxometalates
Inorganic macromolecules known as polyoxometalates (POMs) can spontaneously organize themselves into large hollow spherical superstructures or ‘POM-shells’ in solution. These POM-shells consist of a monolayer of over 1000 of individual POMs and can have diameters ranging from 20-100 nm in water. We have recently found that single-layer POM-shells slowly transform into bilayers and possibly multilayers. Moreover, in concentrated samples, a transition from spherical objects to elongated agglomerates was observed. The elongated objects subsequently grow into large, crystalline, needle-like structures. From these observations we conclude that POMs follow an unusual nucleation route in which the POM-shells are metastable intermediates.

Cees Dekker (TU Delft)
Controlled nanostructures as a tool to study biology

Martin Depken (VU Amsterdam)
Cortical flows and stresses during cell-polarity establishment
Laser ablation is an important tool to analyze stress distributions in e.g. the cell cortex and in the tissues of developing organisms. To describe the response to such a perturbation, we utilize a hydrodynamic description of active viscoelastic materials. Applying this method to the C. elegans cell cortex, we find that the stress can be both anisotropic and inhomogeneous. We relate this to large-scale flows seen during polarity establishment in the zygote.

Valeria Garbin (Twente)
Optical tweezers for the study of microbubble dynamics in ultrasound
Contrast agent microbubbles are widely used in the field of ultrasound medical imaging to enhance contrast of ultrasound images and quantify organ perfusion. A full understanding of the behavior of microbubbles under acoustical excitation is crucial for improving diagnostic imaging protocols and designing new targeted molecular imaging strategies. We designed an optical tweezers instrument for non-contact manipulation of microbubbles, and were able to precisely control the position of individual microbubbles. Combining optical tweezers with the custom ultra-high speed camera Brandaris 128 (25 million frames per second) at the Physics of Fluids group of the University of Twente, we developed new strategies to investigate microbubble behavior in confined geometries, and were able to reveal for the first time the changes in the dynamics of a particular bubble upon changes in the physical boundaries. Examples include interaction of bubbles with confining surfaces and bubble-bubble interactions. Bubble dynamics experiments conducted with the Brandaris camera and optical tweezers will be discussed and compared to theoretical models.

Zorana Zeravcic (Leiden)
From spheres to ellipsoids: the story of the density of states
Disordered packings of soft frictionless spheres close to jamming display an anomalous departure from Debye-like vibrational behavior. Properties of these system have been extensively studied during the past years. We are interested in the extension of these properties onto packings of soft ellipsoids. We look at how the spectra evolve as we transform spheres into ellipsoids by changing their aspect ratio, and as we change the distance to the jamming point of these ellipsoids packings. Spectra show two-band structure. For small aspect ratios the rst band has rotational character and the second band a translational character, with a gap between the bands. As the aspect ratio is increased, the rst band becomes mixed rotational and translational, and for very high aspect ratios the band gap closes leaving the single-band spectra of mixed character.

Abstracts Soundbites

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Haining An (TU Delft)
Hardening of physical gels in response
The development of new magnetoelasts that exhibit larger anisotropic responses and field dependence than those displayed above has immediate technological implications and potential use in actuators, elastomer bearings, vibration absorbers and microfluidic devices. The main goal of this proposal is to develop soft polymer networks that will respond with strong (anisotropic) increase in the elastic modulus in the presence of an external magnetic field.
The idea of ordered magnetic elastomers (magnetoelasts) is the following. Since magnetic particles tend to align and form piles under a magnetic field, magnetoelasts may exhibit a tailor-made anisotropy when prepared in the presence of an external field. Such anisotropy translates into a directionally dependent elastic modulus and a directionally dependent swelling of the material.

Niels Boon (ITF Utrecht)
Charge renormalization for Janus particles
Charge renormalization of spherical colloids gives the possibility of describing their interactions in an electrolyte by usual Debye-Huckel theory. Inspired on this we describe a method to numerically calculate renormalized multipole charges for colloids with a nonspherical charge distribution, such that its far-field potential can be expressed by an analytic function again. We apply this method to Janus particles, which have two differently charged hemispheres. For the situation that both hemispheres bear positive charge densities, our method converges. We found for these particles that in the high charge-limit all renormalized multipoles vanish again.

Wouter den Otter (University of Twente)
Interfacial dynamics of confined binary liquid films
The capillary waves of a liquid-liquid interface confined between two parallel flat walls are studied by molecular dynamics simulations. The decorrelation times of the undulatory modes are shown to depend strongly on the wall-wall separation, with a minor role played by interfacial friction.  A hydrodynamic theory explaining the simulation results is derived.

McGarrity Erin (TU Delft)
Entropic Phase Transition in Polymer Nanocomposite

Silke Henkes (Leiden University)
Soft modes in packings of frictional grains
We investigate simulated packings of two dimensional, spherical particles interacting with frictional Hertz-Mindlin forces. From the dynamical matrix, we are able to extract the spectrum of vibrational modes. Close to zero pressure, approaching the jamming transition, we find an excess density of low energy modes compared to the ordinary Debye behavior found in crystals. The soft modes appear for all values of the friction coefficient, but rely on treating contacts at the Coulomb threshold as sliding contacts.

Sara Jabbari-Farouij (TU Eindhoven)
Para-, ferro- and antiferro-magnetic order in beta-sheet tapes of oligopeptides
Beta-sheet-forming peptides give rise to self-assembled hierarchical structures such as tapes, ribbons and fibrils, which at sufficiently high concentrations form nematic liquid crystalline solutions and gels. Applications of these novel materials are found in nanotechnology, medicine and personal care products. Such aggregates not only appear in the context of desirable biomaterials but also in pathological self-assembly of mis-folded proteins, forming aggregates such as ‘‘amyloids’’. Recently a theoretical model was developed to understand the properties of these self-assembling structures [1]. The question which arises is what happens if we mix different peptide species varying e.g. in length or interaction energy. Do they mix in self-assembled structures or form separate ones? This is of crucial importance as most of industrially produced materials are not monodisperse. To model the simplest polydisperse system, we apply two-component self-assembled Ising model, in which three energy scales are involved. We show that depending upon the relative values of these energy scales and concentrations of the two components, different morphologies of tapes consisting of both components are formed exhibiting paramagnetic, ferromagnetic or antiferromagnetic order.
[1] A. Aggeli, et al; PNAS 2001, 98, 11857

Gijs Katgert (Leiden)
Jamming in Foams
We report on jamming in foams as a function of density. Approaching point "J", we measure scalings and statistics and compare these with theoretical predictions. Our results confirm and extend known predictions

Alexander Korobko (TU Delft)
Single Charge Accuracy in Self-Assembling of Ionic Macrosurfactants
The solution self-assembly of various families of poly(isoprene-block-styrene) diblock copolymer macrosurfactants with precise control of the primary sequence of nonionic and ionic groups along the polymer backbone was studied by small-angle neutron scattering. One or two groups of positive or negatively charged ions were placed at special positions along the chain, namely the chain ends or the junction between blocks. In dilute solution, micelle core sizes follow a universal law as derived from a simple theoretical analysis that integrates effects of both supramolecular architecture and electrostatics. The study demonstrates that in the process of moving towards more complex systems through the integration of multiple interactions at various length scales, one can still devise rational, relatively simple molecular design concepts for understanding supramacromolecular assemblies.

Anke Kuijk (Utrecht University)
Colloidal silica rods
Rod-like systems at high concentrations display rich phase behaviour, including liquid crystal phases. It would be very interesting to study the phase behaviour of these systems in external fields by imaging the system on the particle level.
We followed two strategies to obtain such a rod-like colloidal model system. The first is an anisotropic etching procedure, which is used to etch pores in a silicon wafer. By etching overlapping pores in a hexagonal pattern, rods of silicon remain standing at the wafer surface. The silicon rods that we obtain this way can be modified in several ways, for instance by thermal oxidation which results in silicon dioxide colloids.
The second strategy is a wet chemical synthesis, in which gold nanoparticles and poly(vinyl pyrrolidone) form a soft template in pentanol. Silica grows onto the template so that rod-like colloids are formed.

Andriy Kyrylyuk (Utrecht University)
Packing of Granular Cylinders
Dense packings of granular cylinders with low aspect ratios are simulated by mechanical contraction method and a cubatic phase is discovered for the first time in granular systems. The cubatic phase is observed in the range of cylinder aspect ratio 0.75 < L/D < 1.05. Upon deviation to higher or lower aspect ratios transitions to an amorphous packing are revealed. The amorphous-to-cubatic and cubatic-to-amorphous transitions are accompanied by an increase in packing fraction due to additional order in the system, which is possible because of the presence of flat faces of a cylinder. The absolute maximum in packing fraction of the cubatic phase is found at the aspect ratio L/D = 0.85. A smaller maximum is observed in an amorphous phase upon slight deviation from the nearly equilateral cylinder shape to a prolate, rod-like granule at L/D = 1.15. The appearance of this maximum is attributed to granule elongation, which is somewhat similar to a maximum in random packing of slightly elongated spherocylinders and prolate spheroids.

Kinga Lorincz (UvA Amsterdam)
Visualization of displacement fields in sheared granular systems
The jamming transition, i.e. the transition in a granular system from rest to flow is a fundamental problem of great importance to the understanding of a wide class of disordered materials: grains, clay and glassy materials such as molecular glasses and gels.  We visualize the particles in a sheared three-dimensional granular packing immersed in an index matching liquid using laser sheet imaging. This experimental method allows for an accurate determination of the displacement field of the particles at the onset of flow.

Ethayaraja Mani (Utrecht University)
Stabilization of Hollow Spherical Structures

Ronald Otten (TU Eindhoven)
Continuum percolation of rods: polydispersity effects
By applying continuum connectedness percolation theory we calculate the effect of length and width polydispersity on the percolation threshold of rod-like particles that are randomly dispersed in a host material. We find that this threshold depends on the inverse of the weight average of the rod lengths, implying that it is lowered significantly by the presence of longer rods, even in relatively small quantities. The dependence of the percolation threshold on the width polydispersity is determined by the connectedness criterion, and we find that thicker rods raise the threshold.

Dirk van den Ende  (University of Twente)
Glass transition and aging in dense soft particle suspensions
Using both macroscopic rheology and particle tracking micro-rheology, we studied the glass transition of dense suspensions of thermosensitive microgel particles. These suspensions can be tuned reversibly between the glass state at low temperature and the liquid state at high temperature. Unlike hard spheres, the glass transition is governed not only by the volume fraction but also by the softness of the microgel particles: suspensions of softer particles form a glass at higher effective volume fractions. In the glass state, these suspensions show aging where the relaxation times increase linearly with age, irrespective of the degree of particle softness. This relaxation scaling is in contrast with hard sphere behaviour but consistent with the soft glassy rheology model.

Paul van der Schoot  (TU Eindhoven)
Influence of solvent mixtures on nucleated supramolecular assembly of bisureas
I present a simple model that explains the non-linear dependence of the polymerisation temperature of bisurea compounds on the relative composition of binary solvents.

Tianhui Zhang (Utrecht University)
Structure of colloidal clusters in systems with competing interactions
Mesoscopic structures formed in suspensions of colloids with a short-range attraction and a long-range repulsion is of broad interests in material and biological sciences. Often, gel-like states are observed in these systems while ordered structures, such as lamellar or columnar phases, are theoretically predicted to be more stable. In previous studies, it was found that gelation is usually preceded by the formation of spiral chains which subsequently associate into networks. We have been able to slow down aggregation and show that clusters with a crystalline structure are formed. At longer timescales, the crystalline clusters interconnect to form a dense network. The results in this study offer insight into the kinetics of gelation and a promising experimental approach to the ground state in systems with competing interactions.

Jos Zanikken (Utrecht University)
Nanometer-sized colloidal particles as ideal 'surfactants'
By means of a modified Poisson-Boltzmann theory including Langmuir adsorption, we conclude that water and a slightly reactive oil can spontaneously emulsify in the presence of nanometer-sized colloidal particles. The reactions at the oil interface are stimulated by the particles, and the adsorption of ions at the colloidal surface is stimulated by the charged interface. Due to the stimulated surface activity, the effective interfacial tension can become negative, such that the oil spontaneously emulsifies into a meso-emulsion.

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