Dresden 2009 – wissenschaftliches Programm

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 38: POSTERS Micro- and Nanofluidics

CPP 38.3: Poster

Donnerstag, 26. März 2009, 17:00–19:30, P3

Motion of an elastic capsule in time dependent shear flow — •Steffen Kessler, Reimar Finken, and Udo Seifert — II. Institute of Theoretical Physics, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart

The motion of an elastic 3d capsule in a linear shear flow with time-dependent shear rate is studied both numerically and analytically using an extended Keller-Skalak model. Since the ellipsoidal shape of the capsule is fixed here, only two degrees of freedom remain: the orientation of the capsule with respect to the shear flow, and the internal motion of the membrane with respect to the fixed shape.

For a given shape, the capsule dynamics in time-constant shear flow depends on the applied shear rate and the viscosity contrast between inner and outer fluid. In the corresponding phase diagram tumbling and tank-treading regimes are separated by an "intermittent" regime where the capsule is simultaneously tank-treading and tumbling.

In the case of a time-oscillating shear rate, numerical examples show that tumbling motions can be induced from the tank-treading regime if the shear rate enters the tank-treading regime during its oscillation. Furthermore, numerical results for the mean tumbling rate show an interesting resonance dependence on the applied oscillation frequency, superimposed onto a smooth background vanishing at a cut-off frequency. These findings can be understood analytically for quasispherical capsules and parameters close to the critical point of the phase diagram. A mapping of the system onto a discrete model for low frequencies explains the resonance pattern of the tumbling-rate.