To achieve all-optical-lab-on-a-chip systems, it requires optical manipulation tools for both microparticles and microfluids. Optical tweezers have attracted a great deal of interests in manipulating cells or particles. However, it is not effective in handling microfluid. Its high optical power requirement also limits its application in high throughput bio-analysis system. In this paper, we demonstrate two novel mechanisms, optoelectrowetting (OEW) for handling microdroplets, and optoelectronic tweezers (OET) for optical manipulation of microscopic particles with low optical power actuation. Instead of using direct optical force, both mechanisms rely on light induced electrical force for optical manipulation.
Optoelectrowetting (OEW) enables control of microfluids in droplet form by optical beams. It is based on light induced electrowetting, which changes surface tension at solid-liquid interface at illuminated area. It is realized by integrating a layer of photoconductive material with electrowetting electrodes. By programming the illumination pattern, we have successfully demonstrated various functions for droplet, such as moving, splitting, and merging. A 100 pico-liter droplet was transported at a speed of 785um/sec by an optical beam with an optical power of 100mW.
Optoelectronic tweezers (OET) manipulate cells or particles based on light induced dielectrophoresis (DEP). Trapping or repelling of microscopic particles is achieved with a light intensity of 2W/cm^2, which is 5 orders of magnitudes lower than that required by optical tweezers (10^5~ 10^7 W/cm^2). The liquid containing cells or particles is sandwiched between a photosensitive surface and a transparent ITO glass, with an ac bias between them.. When the laser beam is focused on the photosensitive layer, it creates a virtual electrode on the illuminated area, resulting a non-uniform electric field at the aqueous layer. Cells or particles in the liquid layer are polarized by this non-uniform electric field and driven by the DEP force. The force could be attractive or repulsive, depending on the dielectric properties of the particles and the bias frequency. Using OET, we have demonstrated concentration of polystyrene particles and live E.coli cells using an optical power less than 10uW.