Accurate Modelling of Defect Creation and Control

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Accurate Modelling of Defect Creation and Control
R. James, E. Willman, S.E. Day and F.A. Fernández
UCL Electronic and Electrical Engineering, Torring
ton Place, London, WC1E 7JE
Abstract In conventional LC displays and in many
other liquid crystal devices, defects or
disclinations are unwanted and traditionally
regarded as detrimental to the device
performance. However, the appearance and
movement of defects are essential to the
operation of certain devices. Furthermore, the
LC structure produced by the presence of a defect
can be used to advantage in the device design.
Consequently, accurate modelling of the creation
and evolution of defects is essential to the
design and analysis of devices that exploit these
effects. Some bistable display structures, like
the Zenithal Bistable Display (ZBD) and the Post
Aligned Bistable Nematic (PABN) display depend on
the creation of defects and their movement to
switch between the stable states. These devices
rely on the distortion of the liquid crystal in
the proximity of a fixed grating on a surface for
the transition between a continuous and a defect
state. In the Pi cell defect lines and loops
separate topologically distinct domains that may
be shrunk or grown by varying the applied
voltage. High resolution devices might be able
to exploit such abrupt changes between
topologically distinct states through the precise
electrical control of defect lines. Precise
control over the process of defect creation and
movement could then be used to create abrupt
changes of bulk liquid crystal orientation that
can be of use in phase modulation devices.
Furthermore, together with the interaction
between defects and particles immersed in the
liquid crystal, a precise control of the
evolution of defects could provide the means to
manipulate these particles within a liquid
crystal cell. Conventional display devices can
be designed and optimized with the use of
numerical modelling, which is made
straightforward by the simplifying assumptions
such geometries permit. Assumptions typically
include a constant order parameter approximation,
which prohibits the proper treatment of defects,
strong anchoring and negligible flow. However,
for cases where defects play an important role,
these assumptions are no longer applicable and
modelling is made difficult. The aim of our work
is to explore some of the possibilities brought
upon by the accurate control of defect creation
and their movement. To this end, we use a finite
element formulation based on the Landau-de Gennes
free energy formulation, to calculate the dynamic
behaviour of the liquid crystal orientation and
degree of order. The completeness of this
description is a necessity when simulating
defects and their associated trajectories.
Included in this model is the flow of the liquid
crystal and the interaction between the liquid
crystal and alignment surfaces, to which the
liquid crystal may weakly or strongly adhere.