Enhanced Automatic Sorting of Liquid Crystal Droplets via a Rotating Radial Light Pattern in an Optoelectronic Tweezers System

This study presents an advanced approach for the precise manipulation and automated sorting of liquid crystal (LC) droplets using an optoelectronic tweezers (OET) system. By employing a dynamically rotating radial light pattern to create virtual electrodes, we successfully achieved the controlled trapping, collection, and size-based sorting of LC droplets. A comprehensive analysis of the droplet-light interaction reveals a unique "hopping" mechanism, which is governed by the disparity between the terminal angular velocity of the LC droplets and the angular velocity of the rotating light pattern. This phenomenon facilitates the selective detachment of droplets from OET traps, enabling efficient sorting. Numerical simulations validate that the strongest electric field gradient occurs at the edges of the virtual electrodes, where the dielectrophoretic (DEP) force is maximized, ensuring efficient droplet manipulation. To optimize the sorting process, we systematically investigate the effects of various parameters, including driving voltage, operating frequency, and light pattern width, on the dynamic response of LC droplets. Our experimental results demonstrate that at a driving voltage of 20 Vpp and frequency of 100 kHz, LC droplets within the size range of 7 to 15 μm can be successfully sorted into four distinct regions. The sorting resolution reaches 2 μm, enabling precise classification of droplets based on their sizes. Furthermore, we observed that when the light pattern rotates at 90 rad/s, LC droplets of 28 μm exhibit the hopping mechanism, transitioning into the next sorting region. The critical angular velocity required to induce the hopping mechanism increases with droplet size, reaching 200 rad/s for droplets exceeding 54 μm when using a 24 μm-wide light pattern.