Investigation into dielectrophoretic force profiles in optoelectronic tweezers using gradient light patterns

Optoelectronic tweezers (OET) is an advanced technology that performs non-contact, scalable, and parallel micromanipulation in various scientific and industrial applications, ranging from biology and medicine to nanotechnology and materials science. Several parameters that affect the forces produced have been well explored, including voltage, light intensity, and frequency, whereas the structure of the light pattern is rarely explored. Often, the movement of a single light spot is used to control an individual particle; however, more elegant control mechanisms can be developed by selectively changing the intensity of different areas of the light pattern used. In this work, we investigate how spatially graded light patterns influence the OET force profile in an optoelectronic tweezers device. Finite-element simulations show that flat illumination produces strong but highly localized electric-field-gradient peaks at the light-dark boundaries, whereas gradient illumination redistributes the field gradient over a wider region with a lower peak magnitude. Peak position analysis further shows that gradient illumination shifts the field-gradient peaks away from the pattern boundary and increases the spatial separation between the horizontal and vertical components. Experimental measurements based on the maximum controllable particle speed show the same general trend for flat and trapezoidal patterns. These results suggest that trapezoidal illumination provides a useful design parameter for tuning the spatial distribution of the OET actuation forces.