I like it when articles ask me questions. right at the beginning, i usually throw out an answer and then reading the rest of the article is like a mystery novel... Let's see what kind of guesses we can come up with for this one.
A team of researchers from Hong Kong have just published their article, "Do droplets always move following the wettability gradient?" appearing today in Applied Physics Letters, the authors look at liquids on a superhydrophobic surface. To get an idea of how hydrophobic a superhydrophobe is, check out this picture of a drop of water on a lotus leaf. The standard definition is that the contact angles of a drop of water exceeds 150° and the roll-off angle is less than 10°. Interestingly enough, this is called the Lotus Effect and has some interesting properties.
The authors summarize:
[W]e systematically investigated droplet impacting dynamics on a nonuniform superhydrophobic surface with a wettability gradient. Different from previous reports that the droplet moves toward the direction of decreasing CA, interestingly, we found that the droplet can fashion strikingly different self-migration patterns (toward or against the wettability gradient) depending on the competition of the capillary pressure and the effective water hammer pressure. Our findings highlight the importance of controlling surface roughness (wettability) and impact condition in precise manipulation of droplet placement and trajectory in microfluidics, heat transfer, and water harvesting systems.