  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 the phase diagrams 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.
