Second sound in bursting freely suspended smectic-A films

We describe the bursting of macroscopic spherical bubbles formed by smectic liquid crystals. During rupture, strong light scattering is observed. It is suggested here that peristaltic undulations of the films are responsible for this scattering. This phenomenon distinguishes bursting smectic films from bursting soap films. The dynamics of these mechanical waves are strongly influenced by the internal layered structure of the smectic films, viz. by the elasticity of the molecular layers, expressed by the smectic layer compression modulus B. We study experimentally the optical properties of bursting smectic films by means of optical transmission measurements and laser scattering. The typical wavelength range of the propagating peristaltic waves is in the micrometer range. The wavelength spectrum rather is independent of the initial film thickness delta, but the scattering intensity strongly depends on delta.