Dislocation loop dynamics in freestanding smectic films: the role of the disjoining pressure and of the finite permeability of the meniscus.

When a dislocation loop nucleates in a freestanding film, it collapses or grows depending on whether its radius r is smaller or larger than a critical radius r(c). In this paper, we analyze the growth dynamics of a dislocation loop in the limit of r>>r(c). Experiments with pure octylcyanobiphenyl show that the dislocation velocity is constant in thick films (more than 100 layers) regardless of their thicknesses, and only depends on the pressure in the meniscus. At intermediate thickness (between 100 and 15 layers), the velocity is no longer constant and tends to decrease in time on account of the finite permeability of the meniscus. In very thin films (less than 15 layers), the dislocations move faster than in thick films, although their velocities continue to decrease in time. The thinner the film, the larger the global acceleration is. This effect is linked to a supplementary force acting on the dislocations caused by the attraction between the free surfaces (where the smectic order parameter is enhanced). The progressive deceleration is due to the finite permeability of the meniscus.