Directional Transport Behavior of Droplets on Wedge-Shaped Functional Surfaces

Functional surfaces attract considerable research interest because of many practical applications. A wedge-shaped functional surface is a typical example, which can be designed to achieve self-cleaning, water collection, heat dissipation, or separation of mixed droplets, depending on the wettability of wedge-shaped areas inset in a hydrophobic substrate surface. In this work, a simple technology is proposed to achieve a wedge-shaped functional surface, which could realize directional transport of droplets. It is found that the transport behavior of droplets depends significantly on the wedge angle, the static contact angle of the wedge-shaped hydrophilic region, and the droplet volume. An approximate theoretical model is established, which can predict the most important parameter, that is, the transport displacement. The theoretical prediction can be verified experimentally. Furthermore, an interesting phenomenon of multistep acceleration is also observed in the transport process. With the aid of the present simple technology, several functional surfaces for the directional transport of droplets can be well-designed, including root-like patterned surfaces and nonlinear spiral- or curve-patterned surfaces. All the results should be helpful for the flexible design of functional surfaces for droplet transport in microfluidics or lab-on-a-chip applications.