J. Phys. II France
Volume 7, Numéro 4, April 1997
Page(s) 649 - 661
DOI: 10.1051/jp2:1997149
J. Phys. II France 7 (1997) 649-661

Travelling Waves in Electroconvection of the Nematic Phase 5: A Test of the Weak Electrolyte Model

Martin Treiber1, Nándor Éber2, Ágnes Buka2 and Lorenz Kramer1

1  Universität Bayreuth, Theoretische Physik II, Universitätsstrasse 30, 95440 Bayreuth, Germany
2  Research Institute for Solid State Physics, Hungarian Academy of Sciences, POB 49, 1525 Budapest, Hungary

(Received 15 July 1996, revised 9 December 1996, accepted 9 January 1997)

We investigated travelling waves appearing as the primary pattern-forming instability in the nematic Phase 5 (Merck) in the planar geometry in order to test the recently developed weak electrolyte model of ac-driven electroconvection [1]. Travelling waves are observed over the full conductive range of applied frequencies for three cells of different layer thickness d. We also measured the elastic constants, the electric conductivity, and the dielectric constant. The other parameters of Phase 5 are known, apart from the (relatively unimportant) viscosity $\alpha_1$ and the two parameters of the weak electrolyte model that are proportional to the geometric mean of the mobilities, and the recombination rate, respectively. Assuming a sufficiently small recombination rate, the predicted dependence of the frequency of the travelling waves at onset (Hopf frequency) on d and on the applied frequency agreed quantitatively with the experiments, essentially without fit parameters. The absolute value of the Hopf frequency implies that the geometric mean of the mobilities amounts to $1.1 \times 10^{-10}~{\rm m}^2$/(Vs).

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