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.

International audienceWe measure the angles produced in the Plateau border region of " dry " soap films. In a simple experimental geometry, we demonstrate that a negative line tension can be attributed to these regions. This result has important consequences for the theoretical description of foams approaching the dry limit

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A smectic-A free-standing film is always connected by a meniscus to the frame on which it has been stretched. The meniscus acts as a dissipative reservoir and is characterized by its permeability. We propose a method to measure directly this quantity by equilibrating two menisci in correspondence with the same free-standing film. The permeability is shown to depend on the film thickness, in full agreement with previous indirect measurements obtained by analyzing the growth dynamics of dislocation loops. An improved model of the meniscus is proposed to interpret all the data.