The dynamics of two-dimensional viscous vesicles in shear flow, with different fluid viscosities etain and etaout inside and outside, respectively, is studied using mesoscale simulation techniques. Besides the well-known tank-treading and tumbling motions, an oscillatory swinging motion is observed in the simulations for large shear rate. The existence of this swinging motion requires the excitation of higher-order undulation modes (beyond elliptical deformations) in two dimensions. Keller-Skalak theory is extended to deformable two-dimensional vesicles, such that a dynamical phase diagram can be predicted for the reduced shear rate and the viscosity contrast etain/etaout. The simulation results are found to be in good agreement with the theoretical predictions, when thermal fluctuations are incorporated in the theory. Moreover, the hydrodynamic lift force, acting on vesicles under shear close to a wall, is determined from simulations for various viscosity contrasts. For comparison, the lift force is calculated numerically in the absence of thermal fluctuations using the boundary-integral method for equal inside and outside viscosities. Both methods show that the dependence of the lift force on the distance ycm of the vesicle center of mass from the wall is well described by an effective power law ycm(-2) for intermediate distances 0.8Rp< approximately ycm< approximately 3Rp with vesicle radius Rp. The boundary-integral calculation indicates that the lift force decays asymptotically as 1/[ycm ln(ycm)] far from the wall.

Ripple patterns forming on Pt͑111͒ due to 5 keV Ar + grazing-incidence ion bombardment were investigated by scanning tunneling microscopy in a broad temperature range from 100 to 720 K and for ion fluences up to 3 ϫ 10 20 ions/ m 2 . A detailed morphological analysis together with molecular dynamics simulations of single ion impacts allow us to develop atomic scale models for the formation of these patterns. The large difference in step edge versus terrace damage is shown to be crucial for ripple formation under grazing incidence. The importance of distinct diffusion processes-step adatom generation at kinks and adatom lattice gas formation-for temperature dependent transitions in the surface morphology is highlighted. Surprisingly, ion bombardment effects like thermal spike induced adatom production and planar subsurface channeling are important for pattern ordering.

The effect of hydrodynamic interactions on the diffusion of rods in the nematic phase is studied, both experimentally by time-resolved fluorescence video microscopy and theoretically by mesoscalehydrodynamics simulations. The aspect ratio of the rods and the relative importance of hydrodynamic interactions-compared to direct interactions-are varied independently. This is achieved in experiments by using charged rod-like viruses (fd-virus) with varying ionic strength, both for the wildtype virus and viruses coated with a brush of polymers. In computer simulations, hydrodynamic interactions are incorporated by a particle-based mesoscopic simulation technique. It is found that translational long-time diffusion coefficients for parallel motion along the nematic director, scaled with the diffusion coefficient at infinite dilution, are significantly affected by hydrodynamic interactions, but are insensitive to the aspect ratio. In contrast, the diffusion anisotropy-defined as the ratio of the diffusion coefficients parallel and perpendicular to the nematic director-shows only a weak dependence on hydrodynamic interactions, but strongly varies with the aspect ratio.

Ripple patterns formed on Pt(111) through grazing incidence ion beam erosion coarsen rapidly. At and below 450 K coarsening of the patterns is athermal and kinetic, unrelated to diffusion and surface free energy. Similar to the situation for sand dunes, coarsening takes place through annihilation reactions of mobile defects in the pattern. The defect velocity derived on the basis of a simple model agrees quantitatively with the velocity of monatomic steps illuminated by the ion beam.

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