Experimental test of the Warren-Langer model in nematic-isotropic planar interfaces

In a directional solidification apparatus, the recoil of the nonsteady plan ar nematic-isotropic interface of the liquid crystal 8CB doped with hexachl oroethane was measured, for different pulling velocities. Results agree ver y well with the predictions of our two-sided extension of Warren and Langer 's one-sided model [Phys. Rev. E 47, 2702 (1993)],therefore supporting the validity of their ansatz about the evolution of the dopant concentration he ld. From the comparison between experiment and theory we obtain values for the segregation and diffusion coefficients of hexachloroethane in 8CB compa rable to those found in the literature and measured by other methods. Using the same procedure, we measured the value of the segregation coefficient o f 8CB doped with water as a function of applied sinusoidal electric held pe rpendicular to the sample along the homeotropic direction. The segregation coefficient increases with electric field. In addition, preliminary results on the cellular instability in this system show that the capillary length of the pattern also increases with electric field. To our knowledge, this i s the first binary system with continuously tunable segregation coefficient and capillary length. [S1063-651X(99)01405-4].