Deformation of giant vesicles in AC electric fields —Dependence of the prolate-to-oblate transition frequency on vesicle radius

The electrodeformation of giant vesicles is studied as a function of their radii and the frequency of the applied AC field. At low frequency the shape is prolate, at sufficiently high frequency it is oblate and at some frequency, fc, the shape changes from prolate to oblate. A linear dependence of the prolate-to-oblate transition inverse frequency, 1/fc, on the vesicle radius is found. The nature of this phenomenon does not change with the variation of both the solution conductivity, ?, and the type of the fluid enclosed by the lipid membrane (water, sucrose or glucose aqueous solution). When ? increases, the value of fc increases while the slope of the line 1/fc(r) decreases. For vesicles in symmetrical conditions (the same conductivity of the inner and the outer solution) a linear dependence between ? and the critical frequency, fc, is obtained for conductivities up to ?=114??S/cm. For vesicles with sizes below a certain minimum radius, depending on the solution conductivity, no shape transition could be observed.