Observations of Giant Phospholipid Vesicles Deformed by Elec- Tric Field

External electric field tends to deform a nearly spherical phospholipid vesicle into a rotational ellipsoid, with the rotational axis parallel to the field [1]. The key parameters that govern the extent of deformation are the field strength, its frequency, the size of the vesicle, the membrane tension, the membrane bending elastic constant, and the electric properties of both the membrane and the surrounding medium. The vesicles of average diameter about 20 m were prepared from commercially available 1-palmitoyl-2-oleoyl-sn-3-phosphatidylcholine (POPC) using the standard procedure [2], in both pure water and 0.1 mol/l sucrose solution. The observation was performed by means of a phase contrast microscope and a CCD video camera attached to it. The observed picture of vesicle cross-section was digitized, and the appropriate Fourier coefficients describing the contour were calculated. In the experiment, more than 100 vesicles have been observed. The strength of the applied AC electric field ranged up to 18 kV/m, while the frequency was varied from 1 to 100 kHz. The deformation of a vesicle was measured as the ratio of ellipsoid semiaxes. At low field strengths its value was increasing proportionally to the square of the field strength, whereas at higher field strengths a region with much smaller further