Contribution of electro-osmosis to electrorotation spectra in the frequency range of the beta dispersion

The theory of electrorotation of uncharged conducting spherical particles s urrounded by an insulating membrane and suspended in an electrolyte solutio n is considered. This system constitutes the simplest model of the electric al properties of liposomes or cells. Its dielectric behaviour is characteri sed by two dispersions, the main one of which (beta dispersion) is determin ed by the finite time of charging the capacity of the membrane. The charge distributions that are formed on both sides of the membrane during the pola rization process: have a finite thickness due to diffusion. This fact was f irst taken into account by Garcia et al. [J. Phys. D Appl. Phys. 18 (1985) 1891] for the calculation of the permittivity, but was never considered in electrorotation theories. Nevertheless, it is in this last case when the th ickness of the field induced charge distributions is particularly important , since it not only influences the value of the dipole moment, but also lea ds to the appearance of an electrsmotic slip velocity of the electrolyte so lution around the particle (this phenomenon has a decisive bearing on the l ow-frequency electrorotation of charged insulating particles [Grosse and Sh ilov, Colloids and Surfaces A 140 (1998) 199]. It is shown that the inclusi on of all the diffusion related phenomena in the theory of electrorotation of uncharged particles with membrane, can lead to a qualitative difference with respect to the classical theories. Depending on the conductivity value s, the beta dispersion counterfield electrorotation peak can split into two , leading to the appearance of a co-field low frequency peak. (C) 1999 Else vier Science B.V. All rights reserved.