(430b) Equilibrium Forces, Twist Transitions and Metastable States in Cholesteric Liquid Crystal Films Probed with a Surface Force Apparatus

Liquid crystals are widely used in optics and
photonics due to their large optical anisotropy and strong electro-optical response
(e.g. in displays, spatial light modulators, smart windows). It is relatively
less appreciated that liquid crystals can also generate large structural forces,
e.g. they can stabilize long-range periodic colloidal dispersions and produce
surface forces under confinement.

Using a Surface Force Apparatus, we have studied
the mechanical response of cholesteric (chiral nematic) films with a 1d-periodic
structure and stimuli-responsive photonic band-gap, which are used in
applications such as thermal sensors, dichroic optical mirrors, mirror-less
lasers and active surface coatings. The cholesteric film was confined between
two crossed cylindrical surfaces of muscovite mica that induced strong planar
anchoring and normal alignment of the cholesteric helix. The film thickness and
total twist angle of the chiral molecular structure were simultaneously
determined via careful analysis of multiple-beam interference in the
anisotropic mica-cholesteric-mica optical multilayer. As the surfaces were
separated and the film thickness increased, the twist angle remained almost
unchanged until discontinuous transitions occurred at critical distances that
were equally spaced by one cholesteric half-pitch length. Structural
deformations generated small, repulsive and oscillatory elastic forces with
periodically spaced maxima corresponding to twist transitions. These findings
were reproduced using an equilibrium model of cholesteric deformation and force
generation, revealing a surprisingly strong azimuthal anchoring on mica surface
.

On the other hand, the repulsive forces measured
during surface approach were orders of magnitude larger than those predicted by
the equilibrium model, i.e. a very large force hysteresis was observed during a
surface approach/retraction cycle. This behavior was due to an array of
topological defects, namely dislocation loops, induced by surface curvature in
the 1d periodic structure. The large defect nucleation energy acted as an
effectively energy barrier that prevented the cholesteric to reach equilibrium
via twist transitions and trapped the liquid crystal in a metastable state well
beyond the equilibrium twist transition point, producing a large elastic force.
We anticipate that the Surface Force Apparatus can be used as a general
platform to study confinement-induced and dynamic phenomena in anisotropic
periodic materials such as chiral, smectic and lamellar fluids,  including
helix inversion, defect nucleation/annihilation and defect-mediated surface
interactions, and permeative
flow.