Discrete thinning of free-standing smectic films in the de Gennes "pre-smectic liquid" model

It is shown that the successive discrete thinning of free-standing smectic films (FSSFs), which is observed when the films are heated above the temper ature of the smectic A-nematic bulk phase transition, has a natural explana tion in terms of the de Gennes "pre-smectic liquid'' model, provided that a sufficiently large external compressive force is applied to the free surfa ces of the FSSF. In a real situation this force stems from the curvature of the surrounding miniscus, which plays the role of a volume reservoir. In t his model a superheated FSSF is stabilized by balancing the external compre ssive and elastic forces. When heating takes place the bulk modulus of the pre-smectic lattice decreases, and when the superheating reaches a critical value, the FSSF is subject to a long-wavelength instability in thickness b ecause the external compressive and elastic forces can no longer be balance d for a fixed number of smectic layers. If a superheated FSSF possesses ade quate stability against disruption, the balance of forces, which was disrup ted, and hence the stability of the FSSF can be restored as a result of spo ntaneous thinning of the film to a thickness corresponding to a smaller num ber of smectic layers. In general, heating of a superheated FSSF is accompa nied by a series of such thinning transitions. Near the critical points whe re the balance of the forces breaks down, the dislocation mechanism of spon taneous thinning, which could be responsible for the stratified nature of t he progressive discrete thinning of real FSSFs, can become dangerous. (C) 1 999 American Institute of Physics. [S1063-7761(99)00601-0].