Spreading characteristics of droplets impact on microstructured surfaces with gradient wettability

Manipulating droplet motion using gradient wettability surfaces has attracted much attention in recent years. In this paper, four kinds of microstructured surfaces with gradient wettability were fabricated by photolithography using silicon materials. The spreading characteristics of a droplet impacting on these surfaces were studied by a high-speed camera. Experimental results show that the curved topography has a better guiding effect on the droplet spreading direction than the straight topography. In addition, by changing the Weber (We) number and impact position, the spreading characteristics of the droplet during the impact are analyzed. The results indicate that the gradient wettability has no effect on the spreading diameter when the We number is small. However, with the increase in We number, the spreading diameter is significantly affected by surface topography. The maximum spreading diameter parallel to the gradient microstructures is greater than that perpendicular to the gradient microstructures, in other words, the surface topography plays a guiding role in the spreading direction of the droplet, especially for larger We numbers.