Directional Sliding Behavior of a Water Droplet on a Wedge-Shape Patterned Functional Surface

Wedge-shape patterned functional surfaces have attracted considerable interests, due to the application of directional water droplet transport. It has been found that the directional sliding or rolling of water droplets rather than water film spreading on a functional surface is more popular in many applications due to its efficiency. In order to realize the directional sliding of water droplets, a patterned functional surface is well fabricated in the present study, which consists of a hydrophobic wedge-shaped region and a superhydrophobic region. The directional sliding behavior and its influencing factors of water droplet transport on the functional surface are studied experimentally and theoretically. It is found that the three-phase contact line of the water droplet presents three different shapes in the whole sliding process, which are mainly affected by the instant contact angle at the front and rear edges of the water droplet. The sliding velocity of the water droplet is significantly affected by the wedge angle, while the maximum transport displacement depends not only on the wedge angle but also on the static contact angle of the wedge-shape patterned surface and the volume of the water droplet itself. Further comparison shows that the transport displacement of a sliding water droplet is much larger than that of a spreading one. The results should be helpful for the design of functional surfaces with highly efficient water droplet transport, which are needed in the fields of heat transfer, water collection, and drug delivery.