Field-induced thickness change of ferroelectric liquid crystal films

We observe an increase of the thickness of a chiral ferroelectric liquid crystal film when subjected to ac fields. At 1 kHz at voltages slightly above the threshold for helix unwinding, the sample thickness increases by 0.3 {μ}m in a few minutes. Under this field the cover plate vibrates vertically with a dominating component at twice the field frequency of an amplitude of almost 1 nm. This vibration is expected by the continuum theory of Leslie, Stuart, and Nakagawa [Mol. Cryst. Liq. Cryst. 198, 443 (1991)]; but a theoretically unexplained small asymmetry in the vibration is needed to explain the pumping. The reason for the asymmetry is not yet understood.