Publication
Electrowetting on a Dielectric (EWOD) uses electric fields to control droplet shape and motion on solid insulating surfaces, offering silent manipulation without moving parts. A single EWOD electrode allows manipulation of droplet curvature, enabling liquid lenses and displays. Multiple EWOD electrodes can displace, merge, and split drops on a two-dimensional surface.
The ability to apply forces to droplets makes EWOD a promising mechanism for actuators or fluidic pumps. Rotational and linear actuators based on EWOD have achieved power densities comparable to high-performance electric motors, yet few EWOD pumps have been reported. This thesis explores the limits of EWOD force on a droplet to estimate the potential power of EWOD actuators or pumps. In the second part, I use these insights to build the first tubular EWOD pump and characterize its performance.
To investigate the EWOD force on a droplet, I built a cantilever setup that, for the first time, measures force and contact angle simultaneously at high voltages and at 6000,frames/s. For several liquids and surfaces, I observe that the force saturates at approximately 150,V. Increasing the voltage up to 2,kV (ten times higher than typical) does not significantly increase forces beyond the saturation point. However, the transient dynamics at the front contact line do not saturate with voltage. At higher voltages, the initial front contact line speed increases, the front contact angle temporarily becomes near zero, creating a thin liquid film, and capillary waves form at the liquid-air interface. When the localized EWOD forces exceed the capillary forces, projectile droplets form. I demonstrate that increasing surface tension allows higher droplet forces, as shown with mercury.
The pump is designed as a tube with ring electrodes to minimize drag and maximize pumping performance. The transparent pump is made from 0.9,mm glass tubes, which are coated hydrophobically inside and patterned with ITO on the outside. The ring electrodes are connected to three phases. I demonstrate pumping up to 32 droplets with oil slugs between them. Droplets move at speeds up to 126,mm/s when switching at 42,Hz between phases (4.8,ml/min flow rate). The pressure scales linearly with pump length, reaching 3.1,kPa for every 10,cm section, equivalent to 0.1,kPa per drop. Activating only one phase fixes the droplet in place, even with external pressure as high as 1.7,kPa per droplet. I demonstrate longevity by moving droplets 15 steps forward and backward for 10,000 cycles over 16 hours. The droplets never miss a step, retaining their exact position throughout.
Unlike any existing pump, this EWOD pump moves liquid silently in discrete steps, all within a compact, tubular structure free of moving parts. The solid-state tubular shape makes the pump well suited for integration into soft robotics. For example, weaving a flexible tubular EWOD pump into garments could power pneumatic haptic buttons. Other potent