Nonequilibrium X-Ray Study of the Nematic to Smectic-a Transition Under Shear Flow

Abstract

ABSTRACTWe report on synchrotron x-ray scattering studies of the microscopic structure of the nematic (N) and the smectic-A (SmA) phases in the liquid crystal 8CB under dynamic “steady state” shear flow conditions carried out in an x-ray Couette cell. These measurements directly probe length scales less than 100 Å and as large as 104 Å in the vicinity of the N-SmA phase transition away from equilibrium. The N-SmA transition corresponds to the onset of a one-dimensional mass density wave. In the nematic phase close to the smectic-A phase, there are very large pretransitional smectic-A fluctuations because the transition is almost second order. Under shear flow, we find that the nematic director undergoes a series of abrupt transitions between different orientational states of the director. We discuss the stability regimes of these orientational states by considering the fluctuation induced renormalization of one of the nematic shear viscosities and the fluctuation induced torque on the nematic director.