Gravitational and thermocapillary mechanisms of hydrodynamic motions in liquid crystals induced by laser with a spatially periodic intensity structure

A possibility of excitation of hydrodynamic convective motions of the Rayleigh-Benard and Marangoni type in isotropic liquids and nematic liquid crystals upon absorption of light with a spatially periodic intensity distribution is demonstrated theoretically and experimentally. The opportunity of control and the stability of convective motions are studied. Benard cells become unstable when the light intensity is high. These instabilities are of the thermal origin because the Prandtl number for the medium under study is considerably larger than unity. The competition between the gravitational and thermocapillary mechanisms of photohydrodynamic reorientation of nematic liquid crystal director is also studied. The reorientation of the molecules due to the thermocapillary mechanism causes an optical nonlinearity which is three orders of magnitude stronger than the well-known direct orientational optical nonlinearity.