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Temporal and spatial properties of fluctuations below a supercritical primary bifurcation to traveling oblique-roll electroconvection
 

Summary: Temporal and spatial properties of fluctuations below a supercritical primary bifurcation
to traveling oblique-roll electroconvection
Michael A. Scherer and Guenter Ahlers
Department of Physics and iQUEST, University of California, Santa Barbara, California 93106
Received 3 December 2001; published 26 April 2002
We present measurements of thermally-induced oblique-roll traveling-wave TW fluctuations below the
supercritical primary bifurcation to electroconvection EC in the nematic liquid crystal
4-ethyl-2-fluoro-4 - 2- trans-4-pentylcyclohexyl ethyl -biphenyl I52 . First we analyze time sequences of one-
dimensional shadowgraph images taken parallel to the director to obtain the TW frequency and the fluctua-
tion lifetime . Within our resolution we find that is independent of V/Vc 1 (V is the applied voltage
amplitude and Vc its value at the onset of convection . Contrary to linear theory, the relaxation rate 1/ remains
finite at the bifurcation. Next we present the analysis of temporally uncorrelated two-dimensional shadowgraph
images of the fluctuations for several values of the electrical conductivity . We fitted an anisotropic two-
dimensional Lorentzian function, corresponding to oblique-roll EC, to the time-averaged structure factors S(k)
derived from the images. This yielded information about the components of the mean wave vector k0 and about
the correlation length as a function of and . The angle of obliqueness of the roll patterns was
independent of but decreased anomalously as approached zero. The modulus k0 of k0 depended on . It
also showed an anomalous reduction close to onset. The anomalous dependence of k0 and disagrees with
linear theory, which predicts a smooth, essentially linear dependence on , and presumably is caused by
nonlinear interactions between the fluctuations.

  

Source: Ahlers, Guenter - Department of Physics, University of California at Santa Barbara

 

Collections: Physics