Propulsion of laser printed polymer micro-rods by a low frequency electric field in nematic liquid crystals

We use polarized optical microscopy and confocal fluorescence microscopy to explore electric-field induced swimming of direct laser written polymer microrods in a nematic liquid crystal in the regime of very low frequencies. The rods are of variable aspect ratio and swim in a liquid crystal layer with a thickness comparable to that of the longest rods. We observe significant spatial reorientation of the microrods under an applied electric field, which is characterized by their up and down movement along the applied electric field, oscillation in their tilting with respect to the field, sidewise wobbling of their center of mass and propulsion along the direction perpendicular to the electric field. The velocity of propulsion shows a power law behaviour on the electric field magnitude, v_x ∝ E^α, where α is between 3 and 5 for different aspect ratio rods and can be partially explained by the shear thinning of the viscosity at higher velocity. The time analysis of 3D trajectories of swimming microrods shows a linear coupling of the microrod's center of mass to the applied electric field, and quadratic (i.e. dielectric) coupling of the microrod's tilt to the field, which appears to be the main driving mechanism for microrod propulsion.