Hydrodynamic Instabilities in Nematic Liquids Under ac Electric Fields

We present some measurements on the instability threshold of $p-\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y}-n$, $p-\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{e}$ butylaniline (MBBA) under ac fields of frequencies 10-1000 Hz. The threshold curves, in both the low- and high-frequency regimes, are interpreted by a simple extension of the Helfrich theory to time-dependent phenomena.