Rotation of single crystals of chiral dopants at the top of a nematic droplet: analogy with Lehmann effect.

Detailed studies are reported of the Lehmann-type effect of rotation of small single crystals of chiral substances during their dissolution at the top of a nematic droplet, which was first noted in our previous works. The rotation effect is shown to be essentially dependent on the dopant chirality. The angular velocity of the rotation of the chiral dopant crystals is experimentally shown to be dependent on the crystal dimensions, helical-twisting power of the dopant (determined in independent measurements) and viscosity of the nematic solvent, whereas no rotation was observed for non-chiral dopants or in isotropic solvents under the same conditions. For systematic series of dopants (e.g., cholesterol esters), the angular velocity was proportional to the helical-twisting power; with different nematic matrices, the observed angular velocity was proportional to the rotational-viscosity coefficient gamma(1) ; also, in a certain size range, it was inversely proportional to the characteristic linear dimension of the dissolved crystal. Basing on unit dimensions arguments, a semi-empirical equation is proposed giving a common quantitative description of the whole set of experimental data obtained. It is argued that the obtained results a) suggest a novel method for the evaluation of the helical-twisting power, especially useful for dopants of weak chirality, and b) that they are a direct evidence of transformation of the energy of chiral interactions into the energy of the molecular movement.