Berlin 2005 – scientific programme
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AKB: Biologische Physik
AKB 15: Cell Mechanics and Rheology
AKB 15.6: Talk
Friday, March 4, 2005, 13:00–13:15, TU H2013
Mapping Vortex Diffusion in Viscous and Visoelastic Fluids — •Maryam Atakhorrami1, Gijsberta H. Koenderink2, Daisuke Mizuno1, Frederick C. MacKintosh1, and Christoph F. Schmidt1 — 1Dept. Physics, Vrije Universiteit, Amsterdam, NL — 2Dept. Physics, Harvard University, Cambridge, MA, USA
One of the fundamental questions in hydrodynamics is the response of liquids to the displacement of a small immersed object. At low Reynolds numbers the velocity response of a simple liquid at a distance r from a point force is long-ranged, varying as 1/r. This No-dqStokesNo-dq flow accurately describes the motion of micron-size objects in water on time scales longer than a few microseconds. Over short times, however, the inertia of the liquid prevents the long-range stress propagation implicit in Stokes flow, and the disturbance of the fluid remains confined to a small region. In incompressible liquids this must involve back flow, i.e. a ring vortex is set up, which diffuses away from the point disturbance leaving in its wake the usual Stokes flow. Simulations and theoretical studies have demonstrated this phenomenon, while experiments have only observed indirect consequences, e.g. the No-dqlong-time tailNo-dq in scattering experiments. We have directly resolved the spatio-temporal structure of such vortices by measuring correlated thermal fluctuations and driven motions of micron sized particles in viscous and viscoelastic media at high frequency (100kHz). We find good agreement between experimental flow patterns and theoretical calculations for simple viscous fluids. Furthermore, we show how the vortex-like propagation is modified in viscoelastic media.