Static and dynamic properties of cholesteric fingers in electric field

In a cholesteric liquid crystal, the rod-like molecules present a helical arrangement in a single direction. The pitch of the helix at equilibrium can be continuously changed by dissolving a chiral molecule in a nematic phase. This helix can also be distorted, and even completely unwound, by confining the liquid crystal between two electrodes treated for homeotropic anchoring. Indeed, this surface treatment imposes a topological frustration which tends to unwind the helical structure and to favor a homeotropic nematic phase. Similarly, applying a voltage between the two electrodes tend to unwind the helix provided that the dielectric anisotropy of the liquid crystal is positive. It is therefore possible to study the cholesteric-nematic transition by changing either the voltage V or the confinement ratio defined as the ratio of the thickness to the pitch (C = d/p). It turns out that this transition is usually first order. In this case, we can define a coexistence line V-2(C) between the two phases in the parameter plane (V, C). The experiment shows that near V-2(C), the cholesteric phase forms localized structures called cholesteric fingers. Three types of fingers exist (at least) and may nucleate, depending on experimental conditions. In a segment of a finger of the first kind (CF-1) the director field is continuous; by contrast, a segment of a finger of the second kind (CF-2) possesses a point defect near each end and a cholesteric finger of the third kind (CF-3) is discontinuous along two twist disclination lines parallel to its axis. Our purpose is to describe the static and dynamical properties of these three kinds of fingers in an AC electric field. In particular, we shall see that due to their symmetry, only the CF-2's can drift perpendicularly to their axes and form spirals or growing loops in an AC electric field. We shall also describe the dynamical properties of the CF-1's and their morphological transitions observed while the voltage is decreased in a free growth experiment. Cholesteric bubbles, that are other metastable configurations, will also be described in close connection with the fingers. (C) 2000 Elsevier Science B.V. All rights reserved.