Travelling fingers in two-frequency cholesteric liquid crystals: fragments, loops and spirals

We review the different dynamical patterns that cholesteric fingers of the first type (CF-1) and of the second type (CF-2) form in an a.c. electric field near the coexistence line with the homeotropic nematic phase. Videomicroscopy and computer image analysis were used for investigation of the patterns in polarized light. We show that CF-1s can form stable rectilinear fragments that crawl at constant velocity along their axes, whereas CF-2s form only unstable curved fragments that drift perpendicularly to their axes. Observations of CF-2 staple-shaped fragments which continuously lengthen in their centres are also reported. Finally, we describe in detail the experimental conditions in which CF-2 loops and spirals grow, collapse and destabilize.