Effect of aerosil gels on de Gennes nematic-smectic coupling.

There is a well-known de Gennes coupling between the scalar nematic order parameter S and the smectic density-wave amplitude /Psi/, which causes the nematic-smectic-A phase transition to cross over from second order to first order via a tricritical point. This sort of crossover is observed for numerous liquid crystals (LCs) with small to moderate nematic ranges. Recent studies of LC+aerosil systems, where the aerosil particles form a soft thixotropic gel, have shown that such aerosil gels can greatly reduce or completely suppress this coupling. The underlying LC critical exponents once de Gennes coupling is absent are discussed and compared with theory.