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We investigate the hydrodynamic interaction in suspensions of charged colloidal silica spheres. The volume fraction as well as the range of the electrostatic repulsion between the spheres is varied. Using a combination of dynamic x-ray scattering, cross-correlated dynamic light scattering, and small angle x-ray scattering, the hydrodynamic function Hq is determined experimentally. The effective hydrodynamic interactions are found to be screened, if the range of the direct interaction is relatively long and the static density correlations are strong. This observation of effective hydrodynamic screening is in marked contrast to hard-sphere-like systems. Colloidal suspensions have intriguing static and dynamic properties due to their composite nature and the mesoscopic length scales involved. Colloids interact by direct forces, such as electrostatic or hard-sphere repulsions, and hydrodynamic forces mediated by the suspending fluid [1,2]. Whereas the static properties of colloidal suspensions are strikingly similar to those of molecular fluids, the important role of the suspending medium leads to significant differences in the dynamical behavior. The complex viscoelastic and diffusional properties of colloidal suspensions depend on the interplay between direct and hydrodynamic interactions. This interplay is governed by the ratio of the different length scales in the system [3], such as particle radius, interparticle distance, and the range of the interactions. Unraveling these fascinating effects is

The hepatitis C virus (HCV) replicates its genome on a membrane-associated replication complex. These complexes are represented by "dot-like" structures on the endoplasmic reticulum when standard fluorescence microscopy techniques are applied. To screen compound libraries for inhibitors interfering with the formation of the HCV replication complex independent of RNA replication, an image-based high-content screening assay was developed utilizing inducible expression of the HCV non-structural proteins NS3-5B in an U2-OS Tet-On cell line. An eGFP was fused to NS5A for the detection of replication complexes. The cell line was tightly regulated and the eGFP insertion within NS5A did not alter polyprotein processing. The NS5AeGFP signal colocalized with other non-structural proteins in "dot-like" structures. Accompanying image analysis tools were developed enabling the detection of changes in replication complex formation. Finally, the addition of a HCV NS3/4A protease inhibitor resulted in a dose-dependent reduction of "dot-like" structures demonstrating the practicability of the assay.

Dense granular matter has to expand in order to flow, a phenomenon known as dilatancy. Here we perform, by means of Magnetic Resonance Imaging, direct measurements of the evolution of the local packing density of a slow and smooth granular shear flow generated in a split-bottomed geometry. The dilatancy is found to be surprisingly strong. The dilated zone follow the region of large strain rate and slowly spreads as a function of time. This suggests that the local packing density is governed by the total amount of local strain experienced since the start of the experiment., Comment: 4 pages, 3 figs, submitted to PRE

Surface light-scattering measurements have been carried out on thin asymmetric films of pentane on water. We vary the film thickness ? over a wide range (10(-9) m?10(-5) m). Compared with the ripplons wavelength 1/q, thick films of pentane (? ? 1/q) display the same power spectrum as a pure pentane-vapor interface: a single peak. When thinning the film down to ? approximately 1/q, hydrodynamic coupling between the layers of the interface is revealed by the appearance of a second peak beside the first one. We describe the thickness dependence of the two coupled modes through variations of the positions, the widths, and the amplitudes of both peaks.

A transition from local to global shear zones is reported for granular flows in a modified Couette cell. The experimental geometry is a slowly rotating drum which has a stationary disc of radius R_s fixed at its bottom. Granular material, which fills this cell up to height H, forms a wide shear zone which emanates from the discontinuity at the stationary discs edge. For shallow layers (H/R_s < 0.55), the shear zone reaches the free surface, with the core of the material resting on the disc and remaining stationary. In contrast, for deep layers (H/R_s > 0.55), the shear zones meet below the surface and the core starts to precess. A change in the symmetry of the surface velocities reveals that this behavior is associated with a transition from a local to a global shear mode., 4 pages, 7 figures, submitted

We present experiments on slow granular flows in a modified (split-bottomed) Couette geometry in which wide and tunable shear zones are created away from the sidewalls. For increasing layer heights, the zones grow wider (apparently without bound) and evolve towards the inner cylinder according to a simple, particle-independent scaling law. After rescaling, the velocity profiles across the zones fall onto a universal master curve given by an error function. We study the shear zones also inside the material as function of both their local height and the total layer height., Minor corrections, accepted for PRL (4 pages, 6 figures)

We study the thickness of wetting layers in the binary-liquid mixture cyclohexane methanol. Far from the bulk critical point, the wetting layer thickness is independent of temperature, resulting from the competition between van der Waals and gravitational forces. Upon approaching the bulk critical temperature [t=(T(c)-T)/T(c)-->0], we observe that the wetting layer thickness diverges as t(-beta) with effective critical exponent beta=0.23+/-0.06. This is characteristic of a broad, intermediate scaling regime for the crossover from van der Waals wetting to critical scaling. We predict beta=beta/3 approximately 0.11, with beta the usual bulk-order parameter critical exponent, showing a small but significant difference with experiment.

The spectra of interfaeial fluctuations are derived using potential and stream functions for expressing the velocity fields around and in interfaces of films. Fourier transform optic formalism provides a quantitative theory for fitting observed spectra.

We have designed and built a new generation of surface light scattering instrumentation that provides an accurate and non-invasive way to derive the interfacial tension and viscosity of fluids at liquid/vapor and liquid/liquid interfaces.

We have used surface light scattering to study the fluctuations of a film of pentane standing on water where the pentane thickness ranges from nanometers to millimeters at temperatures above and below the wetting temperature.

Granular matter does not flow homogeneously like a fluid when submitted to external stress, but usually forms rigid regions that are separated by narrow shear bands where the material yields and flows1,2,3,4,5,6,7,8,9,10,11,12,13 (examples include geological faults9,10,11, avalanches12 and silo discharges2,13). Shear bands are narrow (five to ten grains in diameter1,2,3,4,5,6,7,8,9,10,11,12,13) and dependent on the particle shape5, and often localize near a boundary4,5,6,7,8,12,13; they hinder mixing and make grain flows difficult to predict or describe1,2,3. Here we show that the shear zones created in the bulk of the material are wider than those near the walls, and that their bulk velocity profiles lie on a universal curve. This finding challenges the accepted picture of shear banding in granular media.

A new generation of vibration-mitigating surface-light-scattering instrumentation has been designed and built. The computational application of an instrument function derived by use of Fourier optics is presented. This instrument and its accompanying suite of analysis software allow us to easily make accurate and noninvasive measurements of the interfacial tension, volume viscosity, and other interfacial parameters of fluids. We derived the necessary surface response function algorithms to study both simple fluids and binary fluids at their wetting transition and near their critical points. These developments can be applied to study systems with liquid-vapor and liquid-liquid interfaces, including spread monolayers, whenever optical access for a laser beam is available.