Regensburg 2004 – scientific programme

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DY: Dynamik und Statistische Physik

DY 51: Polymers and Colloids

DY 51.2: Talk

Friday, March 12, 2004, 11:00–11:15, H3

Motion of colloidal crystals and fluids in homogeneous electric fields — •Thomas Palberg and Martin Medebach — Johannes Gutenberg Universität Mainz, Institut f. Physik, KOMET 336, Staudinger Weg 7, D - 55128 Mainz

A charged particle subjected to an electric field E will acquire a constant velocity v due to the balance of accelerating electrostatic forces and friction forces. Within the standard electrokinetic model the mobility u = v/E depends on the surface or Zeta-potential and thus on the particle charge. In this contribution we present a super-heterodyne reference beam Doppler Velocimetry experiment to accurately measure the mobility u under conditions of strong particle interaction. Integral flow measurements are used for fluid samples, while spatially resolved measurements are needed for crystalline suspensions. The rich flow scenario observed there is presented in some detail. A prescription to determine the true mobility in the presence of non-parabolic flow profiles (e.g. partial plug-flow or shear banding)is proposed. The mobility is observed to coincide with expectations based on the standard electrokinetic model and a numerically determined effective renormalised charge only in the crystalline state. For fluid order or isolated spheres significant deviations towards lower values are observed.