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 fi eld near the coexistence line with the homeotropic nematic phase. Videomicr oscopy and computer image analysis were used for investigation of the patte rns in polarized light. We show that CF-1s can form stable rectilinear frag ments that crawl at constant velocity along their axes, whereas CF-2s form only unstable curved fragments that drift perpendicularly to their axes. Ob servations of CF-2 staple-shaped fragments which continuously lengthen in t heir centres are also reported. Finally, we describe in detail the experime ntal conditions in which CF-2 loops and spirals grow, collapse and destabil ize.