Phase Behaviour of Colloidal Systems

torting the helical structure of the phase) and the nematic phase develops when the equilibrium pitch of the cholesteric phase becomes larger than the sample thick­ ness. This transition is first order and can easily be observed when approaching a smectic phase, because the cholesteric pitch diverges at this transition. In this way, it is possible to observe the growth of the cholesteric phase into the nematic one. The main observation is that the tex­ ture of the cholesteric phase varies with the front velocity. An example of direc­ tional growth is given in figure 3. This transition is due to a π-rotation of the cholesteric fingers (stripes) whose ends are different due to the absence of mirror symmmetry in a cholesteric. This chirali­ ty-induced morphological transition is not yet completely understood. In conclusion, these examples show the variety of instabilities and pattern for­ mation that arise during the growth of liquid crystals. Most phenomena ob­ served in liquid crystals are generic and present in other systems such as metals, alloys or polymers. In particular, the questions concerning morphological transitions, confinement effects, wave­ length selection, secondary instabilies and transition to chaos or turbulent states are quite general. By contrast, problems relating to chirality are more specific but could play a role in biology—does DNA have a cholesteric texture in the cell nu­ cleus?