Capillary and winding transitions in a confined cholesteric liquid crystal
D. de las Heras, E. Velasco, and Y. Martínez-Ratón
Soft Matter, 11, 7038, (2015) DOI: 10.1039/C5SM01036E
Full text: journal, pdf
Abstract:
We consider a Lebwohl-Lasher model of chiral particles confined in a planar cell (slit pore) with different boundary conditions, and solve it
using mean-field theory. The phase behaviour of the system with respect to temperature and pore width is studied.
Two phenomena are observed: (i) an isotropic-cholesteric transition which exhibits an oscillatory structure with respect to pore width, and (ii)
an infinite set of winding transitions caused by commensuration effects between cholesteric pitch and pore width. The latter transitions
have been predicted and analysed by other authors for cholesterics confined in a fixed pore and subject to
an external field promoting the uniaxial nematic phase; here we induce winding transitions solely from geometry by changing the pore width
at zero external field (a setup recently explored in Atomic-Force Microscopy experiments). In contrast with previous studies, we obtain
{color{blue}the phase diagram} in the temperature vs pore width plane, including the isotropic-cholesteric transition, the winding transitions and
their complex relationship. In particular, the structure of winding transitions terminates at the capillary isotropic-cholesteric transition
via triple points where the confined isotropic phase coexists with two cholesterics with different helix indices.
For symmetric and asymmetric monostable plate anchorings the phase diagram are qualitatively
similar.
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