Active microrheology of liquid crystals: anisotropic moduli and shear thinning

We investigate the rheological behavior of the lyotropic chromonic liquid crystal (LC) Sunset Yellow (SSY) in the nematic phase by measuring the displacement-force response of a microparticle using laser tweezers. The frequency dependences of the storage and loss moduli of the LC exhibit large anisotropy and strong frequency dependence. For loss modulus, G”_⊥/G”_∥ is close to 3 at low frequency but drops below unity in the 10-100 Hz range. For the storage modulus, G'_⊥/G'_∥ is over 10 at low frequency, and decays as frequency increases, but remains larger than unity. The effective viscosity when the particle oscillates perpendicular to the director decays over one order of magnitude as the frequency increases, showing strong shear-thinning behavior. In comparison, when the particle oscillates along the director, the effective viscosity remains close to constant, showing a Newtonian-like behavior. When dragging the particle with constant velocity, particle motion perpendicular to the director induces large-range distortion, while parallel motion triggers very little change. We study the effect of particle size, material concentration, and temperature on these features. We attribute our findings to the different nematodynamic coupling modes between particle induced flow and the LC director.