Active microrheology of lyotropic chromonic liquid crystals using optical tweezers

We investigate the rheological behavior of lyotropic chromonic liquid crystals in the nematic phase by moving a microparticle and directly measure the force acting on it using laser tweezers. Two liquid crystal materials, sunset yellow (SSY) and disodium cromoglycate (DSCG), show distinct behavior when the particle is dragged along the director field under constant velocities. In SSY, the particle motion does not distort the director, and the drag force rapidly relaxes to zero after the particle stops, similar to an isotropic medium. In contrast, in DSCG, particle motion distorts the director field in front of it, and the relaxation is 10 times slower. When the particle is moved perpendicular to the director, both liquid crystals show strong director deformation behind the particle and slow force relaxation after particle stops. By oscillating the particle at different frequencies, we measure the storage and loss moduli, G’ and G” respectively, of these materials. G’ measured along the director of SSY is nearly zero at low frequencies, consistent with the dragging experiments. G’ is non-zero and increases with the frequency in all other combinations. To shed light on the observations, continuum simulations accounting for disparate elastic constants and tumbling character of the nematic materials are compared to the experimental data. We discuss our current understanding of the unknown nematodynamic mechanism behind this odd rheological behavior.