The shape dynamics of fluid vesicles is governed by the coupling of the flow within the two-dimensional membrane to the hydrodynamics of the surrounding bulk fluid. We present a numerical scheme which is capable of solving this flow problem for arbitrarily shaped vesicles using the Oseen tensor formalism. For the particular problem of simple shear flow, stationary shapes are found for a large range of parameters. The dependence of the orientation of the vesicle and the membrane velocity on shear rate and vesicle volume can be understood from a simplified model.Comment: 10 pages, LaTeX2e, psfig, 5 eps figures included, to appear in Phys. Rev. Lett., 199

We report on an experimental and theoretical study of phospholipid vesicles at small reduced volume. In this regime, vesicle shapes are non-axisymmetric and resemble starfish. To calculate these shapes, we minimize bending energy under constraints on area, volume and mean curvature on a triangulated surface. We find a multitude of locally stable starfish that have practically the same energy but look very different. Each starfish is build up from basically three different units: a core, several cylindrical arms and spherical caps at the end of these arms.

We investigate the propagation of a suddenly applied tension along a thermally excited semi-flexible polymer using analytical approximations, scaling arguments and numerical simulation. This problem is inherently non-linear.We find sub-diffusive propagation with a dynamical exponent of 1/4. By generalizing the internal elasticity, we show that tense strings exhibit qualitatively different tension profiles and propagation with an exponent of 1/2.Characteristic for soft matter systems such as polymers or membranes is the often subtle interplay between energy and entropy [1,2]. Thermal motion determines the conformations of these systems crucially. For instance, the typical end-to-end distance even of a semiflexible polymer (let alone a Gaussian polymer) is much smaller than its contour length.Likewise for vesicles, thermal fluctuations can store a significant part of the "true" surface area. From the perspective of the projected length or area, this part is hidden. For vesicles or membranes, the hidden area can be pulled out without stretching the true area by application of a localized force such as the suction pressure in a micropipet [3] or the action of optical tweezers [4]. For polymers, recent advances in experimental techniques using magnetic beads, optical tweezers or flow fields made feasible detailed experimental studies of the 1