Phys. Rev. B 52, R13075.
Fingers of phase.
Propagating phase transition in a liquid
crystal driven by an electric field looks
similar to those observed by Wang et al.
Liquid crystals make the numbers on your digital watch, but
they are also a fascination to physicists who study phase
transitions. This unique class of molecules has several electrical
and optical properties that vary with temperature and make it easy
to monitor the various phases of liquid crystals in the lab. Most
phase transitions, such as salt crystallizing out of solution,
propagate with dendritic, finger-like patterns in two or three
dimensions, but theorists are hard-pressed to completely describe
such processes. In the 18 May PRL, a team
describes a liquid crystal phase transition that propagates in one
dimension at uniform temperature--the first of its kind in any
system, and one that may allow deeper understanding of such growth
phenomena in higher dimensions.
Most liquid crystal molecules are roughly cigar-shaped,
typically five times longer than their diameter. At high
temperatures the cigars are oriented randomly, but cooling down
causes a series of phase transitions, as the cigars become more
ordered. In the smectic C phase, the molecules are in layers in
which they flow freely, liquid-like, but cannot easily move from
one layer to the next. At the same time, they remain oriented at a
specific tilt angle, not parallel to the layers, nor perpendicular
to them. At lower temperatures some liquid crystals can enter the
smectic CA phase, where molecules in alternate layers
point in directions that differ by a 180 degree rotation about the
Xin-Yi Wang, of the University of Akron, Charles Rosenblatt, of
Case Western Reserve University, and their colleagues, studied
this smectic C to CA phase transition. They started with a sample
in the smectic C phase, just above the transition temperature
(about 100 degrees Celsius), and suddenly brought it to a
temperature below the transition. The phase change propagated as
narrow, parallel "fingers," a few µm wide, easily viewed in a
microscope because of the differing optical properties of the two
phases. The fingers grew at a constant speed that was essentially
independent of the size of the temperature change.
Rosenblatt says the process is "different from any other
[one-dimensional] phase transition in nature that we know of,"
because it does not require a temperature gradient or any other
special methods. The simplicity of the system allowed the authors
to describe the basic properties of the finger growth with a
relatively simple equation and derive the speed of propagation.
They hope to further refine their model of the non-linear,
non-equilibrium growth phenomenon in a way that is not possible
with the theoretical complications of two or three dimensions.
Kinetics of Phase Transition in an Anticlinic Liquid
Crystal Induced by a Uniform Temperature Field: Growth in
X. Y. Wang, Jian-feng Li, Eliezer Gurarie, S. Fan, T. Kyu,
M. E. Neubert, S. S. Keast, and Charles Rosenblatt
Rev. Lett. 80, 4478
(issue of 18 May 1998)