Electroosmosis and polymer depletion layers near surface conducting particles are detectable by low frequency electrorotation

Polymer depletion layers adjacent to particle surfaces can be detected by m eans of electrophoresis. The electric field-induced tangential flow within the diffuse part of the electric double layer experiences a decreased visco sity in the depletion layer if the depletion layer thickness is comparable to or larger than the Debye screening length. This results in a much higher electrophoretic mobility than predicted by the classical Smoluchowski form ula. Due to electroosmosis, an analogous effect is expected to occur in the low frequency range of electrorotation (alpha dispersion). To prove the ex istence of the electroosmotic component of low frequency electrorotation, e lectrophoresis and electrorotation of glutaraldehyde fixed red blood cells were measured as a function of ionic strength and dextran molecular weight. The electrophoretic measurements yielded dextran depletion layer thickness es ranging from 4.4 nm (M-w = 71 kDa) to 18.2 nm (M-w = 2400 kDa). The low frequency electrorotation speed was consistent with the viscosity profiles deduced from the electrophoretic experiments. However, at external field fr equencies >10 kHz, the rotation speed gradually became independent of the d epletion layer viscosity. In the beta dispersion range above 100 kHz the ro tation speed depended only on the bulk viscosity. These experiments demonst rate the different mechanisms of low and high frequency electrorotation res ulting in different hydrodynamic flow profiles around the rotating particle s. (C) 1999 Elsevier Science B.V. All rights reserved.