Stability of viscous fingering patterns in liquid crystals

We present experimental data of viscous fingering patterns in a radial Hele-Shaw cell filled with the liquid crystal 4,4′-$n$-octylcyanobiphenyl (8CB). We observe a dense-branching-dendritic-dense-branching morphological phase sequence as a function of temperature. The wave number of the fastest growing mode can be selected by varying experimental parameters, and the number of initially growing fingers on a circular interface is in good agreement with linear-stability analysis which includes the full kinetic term. A mechanism is proposed for tip stabilization by anisotropic viscosity; the critical viscosity ratio for circular tips is ≃2.