Viscous Fingering in a Smectic Liquid Crystal

To study the effects of a driving force on interfacial-pattern formation, we introduce a novel experimental system, a radial Hele-Shaw cell with nitrogen injected into a smectic liquid crystal. In the absence of initial alignment of the molecules a cross-over can be observed: the effective fractal dimension of the pattern changes from the value D approximately equals 1. 6 (low pressures) to D approximately equals 2 (high pressures). If an external magnetic field is applied to achieve uniaxial ordering, a dramatic change in the direction of the easy growth takes place as a function of the increasing pressure. The results are discussed in terms of the internal structure of smectics.