FIELD-INDUCED THICKNESS CHANGE OF FERROELECTRIC LIQUID-CRYSTAL FILMS

We observe an increase of the thickness of a chiral ferroelectric liqu id crystal film when subjected to ac fields. At 1 kHz at voltages slig htly above The threshold for helix,unwinding, the sample thickness inc reases by 0.3 mu m in a few minutes. Under this field the cover plate vibrates vertically with a dominating component at twice the field fre quency of an amplitude of almost 1 nm. This vibration is expected by t he continuum theory of Leslie, Stuart, and Nakagawa [Mol. Cryst. Liq. Cryst. 198, 443 (1991)], but a theoretically unexplained small asymmet ry in the vibration is needed to explain the pumping. The reason for t he asymmetry is not yet understood.