Experiments show that phospholipid vesicles exposed to AC electric field un
dergo a shape transition from prolate to oblate ellipsoidal shape when the
frequency of the field is increased. A theoretical model, based on the mini
mization of total free energy of the vesicle, was devised to explain this p
henomenon. The model exhibits the same frequency-dependent prolate-to-oblat
e shape transition as observed in the experiment.