Cholesteric-nematic transition in an asymmetric strong-weak anchoring cell

We discuss the cholesteric-nematic transition induced by an external field in a cell in the shape of a slab of finite thickness whose anchoring energy is infinitely strong on one surface and negligible on the other. The case where the surface orientations of the cholesteric director are parallel in the ground state is considered. In a sample of thickness l, multiple of the natural periodicity of the system Lambda, the number of complete twists in the cholesteric liquid crystal is reduced by increasing the strength of the external distorting field. The sequence of the field values for expelling complete twists accumulate as the strength of the field approaches a critical value. Beyond this critical value the system assumes an unwinding configuration going toward a uniform structure, corresponding to nematic phase. Generalization of the results to the weak-weak anchoring cell case is also discussed. In our analysis the anisotropic part of the surface energy is assumed or very large or very small with respect to the total energy per unit surface due to the bulk distortion induced by the magnetic field.