Laser-Driven Vortex Flow in a Nematic Droplet: Experimental and Numerical Results

The dynamic evolution of an optically induced vortex flow in nematic microliter droplets caused by exposure to a focused laser beam has been studied both experimentally using polarized optical microscopy and numerically within the framework of a corresponding nonlinear extension of the Ericksen–Leslie theory supplemented by thermomechanical correction of the stress tensor and the entropy balance equation. The vortex flow in nematic droplets consisting of 4-pentyl-4′-cyanobiphenyl molecules spreading over the functionalized surface was visualized in microliter droplets doped with monodisperse polystyrene tracers, under exposure to a laser beam with an optical power equal to 8.0 mW. Using the computer vision detection algorithm, we have identified radial symmetry in the tracers motion, where comet-like tracks are aligned along the rays emanating from the center of the resulting structure. At the same time, some of the “comets” are flying towards the center, while others are moving away from it. This allowed us to estimated the average value of tracer flows, which is of 17 µm/s. All these observations indicate that vortex currents are excited in the droplet under the action of the focused laser beam. The nature of thermally excited vortex flows in the microliter hybrid aligned nematic droplet with a free upper LC/air interface and spreading over the solid surface under the influence of the heat flux directed through the lower bounding surface is also numerically investigated. It was shown that due to the interaction between ∇T and the gradient of the director field ∇n^, the thermally driven bi-vortical flow is maintained in nematic microvolume.