Mecanochromatic effect in nematics

We consider a nematic slab, with homeotropic boundary orientations (easy axis z) subjected to two simultaneous perturbations = a uniform shear flow along x and a magnetic field H oriented at 45° in the xz-plane. We find that, in a certain range of H-values, the orientation of the molecules in the mid plane of a thick slab does not correspond to a fixed angle = in this region the director rotates by many turns between the walls and the mid plane. In suitable cases, the periodicity may became comparable to an optical wavelength and the sample may become colored.