Julia Yeomans OBE FRS

Abstract:

We study the static properties of cubic blue phases by numerically minimizing the three-dimensional, Landau-de Gennes free energy for a cholesteric liquid crystal close to the isotropic-cholesteric phase transition. Thus we are able to refine the powerful but approximate, semianalytic frameworks that have been used previously. We obtain the equilibrium phase diagram and discuss it in relation to previous results. We find that the value of the chirality above which blue phases appear is shifted by 20% (toward experimentally more accessible regions) with respect to previous estimates. We also find that the region of stability of the O5 structure-which has not been observed experimentally-shrinks, while that of blue phase I ( O-8 ) increases thus giving the correct order of appearance of blue phases at small chirality. We also study the approach to equilibrium starting from the infinite chirality solutions and we find that in some cases the disclination network has to assemble during the equilibration. In these situations disclinations are formed via the merging of isolated aligned defects.

Abstract:

We present a lattice Boltzmann algorithm to solve the Beris-Edwards equations of motion for a cholesteric liquid crystal. We use our algorithm to investigate permeative flow. We find that, for helices pinned at the boundary, a small body force leads to a huge viscosity increase whereas larger ones induce no increase. This shear thinning is in agreement with experiments. If instead, the helix lies perpendicular to the plates, there is almost no viscosity increase. For strong forcing, we identify a flow-induced double twist in the director field. We compare this texture with the static double twist of cubic blue phases.