Periodic Director Patterns in Twist and Splay Frederiks Transitions in Tobacco Mosaic Virus Liouid Crystals

When a magnetic field is applied in the plane and perpendicular to the unperturbed director of a uniform planar-aligned liquid crystal, a twist Frederiks transition occurs. For fields greater than a critical value, periodic bend-modulated twist distortions are dynamically favored over the uniform twist distortion. This is because the periodic flow pattern associated with the bend twist distortion lowers the effective viscosity more than it raises the elastic restoring torque. Under crossed polarizers the sample has a striped texture with the stripes perpendicular to the initial director. The wavelength of the stripe texture is field dependent.

When the magnetic field is applied perpendicular to the uniform planar sample, a similar striped distortion occurs above a critical field value, dominated by splay rather than twist. The most striking difference from the twist transition is that there is a thickness-dependent angle between the stripes and the original director whereas in thinner samples, the stripes tend to be more parallel to the original director. The initial development of the structure appears as a lattice of diffuse spots on a diamond-shaped grid, a precursor that is not seen in the twist instability, which nucleates as stripes at the boundaries of the cell. The splay Frederiks transition also exhibits a field-dependent wavelength distortion.