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

Abstract

Polymer depletion layers adjacent to particle surfaces can be detected by means of electrophoresis. The electric field-induced tangential flow within the diffuse part of the electric double layer experiences a decreased viscosity 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 formula. Due to electroosmosis, an analogous effect is expected to occur in the low frequency range of electrorotation ( α dispersion). To prove the existence of the electroosmotic component of low frequency electrorotation, electrophoresis 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 thicknesses 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 frequencies >10 kHz, the rotation speed gradually became independent of the depletion layer viscosity. In the β dispersion range above 100 kHz the rotation speed depended only on the bulk viscosity. These experiments demonstrate the different mechanisms of low and high frequency electrorotation resulting in different hydrodynamic flow profiles around the rotating particles.