On the Effect of a Rotating Magnetic Field or Sample on the Bend Freedericksz Critical Field in Nematic Slabs

A numerical study of the effect of a continuously rotating field or sample on the magnetic Freedericksz transition in homeotropic nematics slabs is presented. Homeotropic boundary conditions with strong anchoring are considered. The magnetic field is applied parallel to the plates, i.e., in the bend Freedericksz geometry. The behaviour of a low molecular weight liquid crystal (5CB) and of a high molecular weight polymer liquid crystal (PBLG) is compared using the Leslie-Ericksen theory of nematodynamics. The effect of the sample thickness, of the magnetic field strength and of the spinning frequency on the response of both materials is studied. Our results show that, in both cases and for both materials, the value of the Freedericksz critical field H-c(v) increases with the spinning frequency v. The transition H-c(v) -> H-c(0) is found to be continuous in the rotating sample case and to be discontinuous in the rotating field case. The plot of the reduced critical field vs. the reduced spinning frequency gives universal curves for both materials in both cases.