electroconvection overview - modulation experiments - domain growth
THIS WORK IS FUNDED PRIMARILY BY: NSF award DMR 9975479
with additional support from Research Corporation and the Sloan Foundation.
Nematic liquid crystals are long rod-like molecules that
tend to align with each other. The direction of the average alignment is
referred to as the director. For the study of electroconvection, the nematic
liquid crystal is confined between two glass plates with a thickness of
25 - 100 microns. The glass is treated so the that the director is aligned
in a uniform directions parallel to the plates. The glass also is coated
with transparent conductors, and an ac voltage is applied perpendicular
to the plates. Above a critical value of the voltage, a periodic distortion
of the director and an associated flow in the form of convection rolls
develops. This is referred to as a pattern. The pattern is imaged from
above using a microscope (see photograph on the
left) and generally consists of a single set of stripes or a
superposition of a small number of stripe with different orientations.
In electroconvection, the director selects out a direction, so it is possible
to have oblique rolls. These are patterns where the wavevector of the pattern
forms a nonzero angle with respect to the director. An example of such
a pattern is show in the image on the right.
Electronconvection is useful for studying a number of
interesting nonlinear and pattern forming problems.
These include (but are not limited to):
Our recent work has focused on using modulations of the control parameter to stabilize regular dynamics. Movies of the
impact of resonant modulation can be found at the picture page. These movies illustrate the establishment of regular
standing wave patterns from irregular traveling waves. This work has been published in
Temporal Modulation of the Control Parameter in Electroconvection in the Nematic Liquid Crystal I52, M. Dennin, Phys. Rev. E 62, 7842 (2000).
We have also used modulation to study the dynamics of
localized states, known as worms. This work has been submitted
to Phys. Rev. E.
Domain growth deals with the study
of systems after a sudden change in an external parameter. In the case
one can make a sudden change in the driving voltage from below the critical voltage to above the critical voltage. If the steady
state consists of oblique rolls, a pattern of small domains of zig and zag rolls forms. As a function of time, these domains
grow in size, that is the pattern "coarsens". We study the scaling behavior of the growth of these domains. Currently, we
find that the size of the domains grows as t1/4. There is currently no theoretical explanation for this behavior. We can
study the domain coarsening in two systems: I52 and N4. I52 is the system used for the modulation experiments. For domain
coarsening, we make sudden changes in the modulation strength to generate standing waves of zig and zag domains that
coarsen. In N4, the patterns are stationary zig and zag rolls, and we make sudden changes in the applied voltage. The
coarsening behavior in both systems appears to be similar. However, one difference is the structure of the domain walls.
For an examples of the coarsening, see the movie page.