Reports the completion of four fundamental fluidic operations considered essential to build digital microfluidic circuits, which can be used for lab-on-a-chip or micro total analysis system (μTAS): 1) creating, 2) transporting, 3) cutting, and 4) merging liquid droplets, all by electrowetting, i.e., controlling the wetting property of the surface through electric potential. The surface used in this report is, more specifically, an electrode covered with dielectrics, hence, called electrowetting-on-dielectric (EWOD). All the fluidic movement is confined between two plates, which we call parallel-plate channel, rather than through closed channels or on open surfaces. While transporting and merging droplets are easily verified, we discover that there exists a design criterion for a given set of materials beyond which the droplet simply cannot be cut by EWOD mechanism. The condition for successful cutting is theoretically analyzed by examining the channel gap, the droplet size and the degree of contact angle change by electrowetting on dielectric (EWOD). A series of experiments is run and verifies the criterion.

Superhydrophobic and superoleophobic surfaces have so far been made by roughening a hydrophobic material. However, no surfaces were able to repel extremely-low-energy liquids such as fluorinated solvents, which completely wet even the most hydrophobic material. We show how roughness alone, if made of a specific doubly reentrant structure that enables very low liquid-solid contact fraction, can render the surface of any material superrepellent. Starting from a completely wettable material (silica), we micro- and nanostructure its surface to make it superomniphobic and bounce off all available liquids, including perfluorohexane. The same superomniphobicity is further confirmed with identical surfaces of a metal and a polymer. Free of any hydrophobic coating, the superomniphobic silica surface also withstands temperatures over 1000°C and resists biofouling.

While many recent studies have confirmed the existence of liquid slip over certain solid surfaces, there has not been a deliberate effort to design and fabricate a surface that would maximize the slip under practical conditions. Here, we have engineered a nanostructured superhydrophobic surface that minimizes the liquid-solid contact area so that the liquid flows predominantly over a layer of air. Measured through a cone-and-plate rheometer system, the surface has demonstrated dramatic slip effects: a slip length of $\sim 20\mu \mathrm{m}$ for water flow and $\sim 50\mu \mathrm{m}$ for 30 wt % glycerin. The essential geometrical characteristics lie with the nanoposts populated on the surface: tall and slender (i.e., needlelike) profile and submicron periodicity (i.e., pitch).

We have obtained interfacial properties of Galinstan, a nontoxic liquid-metal alloy, to help replace mercury in miniature devices. To prevent formation of an oxide skin that severely hinders the fluidic behavior of small Galinstan droplets and leads to inaccurate property data, we performed our experiments in a nitrogen-filled glove box. It was found that only if never exposed to oxygen levels above 1 part per million (ppm) would Galinstan droplets behave like a liquid. Two key properties were then investigated: contact angles and surface tension. Advancing and receding contact angles of Galinstan were measured from sessile droplets on various materials: for example, 146.8 and 121.5, respectively, on glass. Surface tension was measured by the pendant-drop method to be 534.6 10.7 mN/m. All the measurements were done in nitrogen at 28 with oxygen and moisture levels below 0.5 ppm. To help design droplet-based microfluidic devices, we tested the response of Galinstan to electrowetting-on-dielectric actuation.

This article discusses and experimentally verifies how to lower the operating voltage that drives liquid droplets by the principle of electrowetting on dielectric (EWOD). A significant contact angle change (120°→80°) is desired to reliably pump the droplet in microchannels for applications such as lab-on-a-chip or micrototal analysis systems. Typically, much higher voltages (>100 V) are used to change the wettability of an electrolyte droplet on a dielectric layer compared with a conductive layer. The required voltage can be reduced by increasing the dielectric constant and decreasing the thickness of the dielectric layer, thus increasing the capacitance of the insulating layer. This dependence of applied voltage on dielectric thickness is confirmed through EWOD experiments for three different dielectric materials of varying thickness: Amorphous fluoropolymer (Teflon® AF, Dupont), silicon dioxide (SiO2) and parylene. The dependence on the dielectric constant is confirmed with two different dielectric materials of similar thickness: SiO2 and barium strontium titanate. In all cases, the surface is coated with a very thin (200 Å) layer of amorphous fluoropolymer to provide initial hydrophobicity. Limiting factors such as the dielectric breakdown and electrolysis are also discussed. By using very thin (700 Å) and high dielectric constant (∼180) materials, a significant contact angle change (120°→80°) has been achieved with voltages as low as 15 V. Based on these results, a microfluidic device has been fabricated and tested, demonstrating successful transporting (pumping) of a 460 nL water droplet with only 15 V.