DIRECT ABSOLUTE MEASUREMENTS OF THE 2-DIMENSIONAL ELASTIC-CONSTANTS IN SMECTIC C-ASTERISK LIQUID-CRYSTAL FILMS

The orientational elastic constants of free-standing films in a tilted smectic phase are directly measured in absolute units with a new expe rimental method similar to light scattering in the heterodyne regime. Basically, the amplitude of the thermodynamically excited orientationa l fluctuations, which modulate polarized light, is measured in the dir ect space by means of a microscope and a video camera connected to a m icrocomputer. Then, with a Fourier transform, we go into the reciproca l space and deduce the two-dimensional bend and splay elastic constant s of the film in a range of wave vectors simultaneously. The method, w hich only needs light experimental means, is simple, fast, and accurat e. It can be particularly helpful in the ferroelectric smectic C liqu id crystals for determining the elastic constants and indirectly the f erroelectric polarizations, without problems for orienting the samples . (C) 1997 American Institute of Physics.