Open-loop electrowetting actuation with micro-stepping

Microfluidic-driven mechanical actuation opens new possibilities for positioning and manipulating delicate small components. However, existing microfluidic actuation methods are not well-suited to positioning with high resolution. This paper reports a method for precise, open-loop control of droplet position in finite steps by varying the duty cycle of the input signal in electrowetting actuation. When wetted to a solid object, both the droplet and the solid can be actuated. Unlike conventional electrowetting actuation methods, positioning resolution in our proposed method can be much smaller than the size of the underlying electrodes without requiring closed loop feedback control system. Using a leaky dielectric coating, the electrode/electrolyte combination in our device acts as a simple diode by blocking current in one direction and conducting in the other. Each duty cycle of the applied AC square wave corresponds to a unique position on the electrode. The position - duty cycle relationship is found to be nonlinear but symmetric about the center of the electrodes. This approach provides a method for improving open-loop positioning resolution without adding more electrodes. Positioning is within 0.2 mm (<2.5% of the droplet diameter) of the idealized model and repeatability is <0.07 mm (<0.8% of the droplet diameter).