Laboratoire Matière et Systèmes Complexes

"Waltzing defects in cholesteric liquid crystal shells: interplay between topology and elasticity".

Alexandre Darmon (Effets Collectifs et Matière Molle, Gulliver, ESPCI; Paris).

Confining rod-like molecules, such as nematogens of liquid crystals, on a curved surface inevitably yields topological defects, which are singular points where the alignment between molecules can no longer be fulfilled. Remarkably, in the specific case of a nematic sphere, Poincaré stated that the sum of the topological charges of the defects must always be +2. Recent studies, led both at experimental and theoretical levels [1,2], have shown that in the case of nematic shells, three kinds of configuration are possible. Each of these configurations corresponds to different arrangements of defects that satisfy Poincaré’s theorem. But much richer scenarios appear when playing with the shell thickness, since bulk effects start competing with surface effects. In particular, we show that inducing chirality in the liquid crystal can have a dramatic effect in the defect structure of the shell, where the ratio c between the cholesteric pitch p and the shell thickness h becomes an additional control parameter.

[1] Lopez-Leon, T., Koning, V., Devaiah, K. B. S., Vitelli, V., & Fernandez-Nieves, A., Nature Phys. 7 (2011).
[2] Vitelli, V. & Nelson D. R., Phys Rev. E 74 (2006).

Mercredi 20 janvier 2016, 18h, 646A.