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When a falling liquid droplet hits a surface, like a raindrop hitting pavement, it either flattens and spreads out evenly, or produces a sharp-edged splash. Now, a team of Chinese researchers has created a surface which allows the descending liquid to rebound and swirl around like a graceful dancer.
The team, led by Huizeng Li of the Chinese Academy of Sciences, Beijing (CAS), began by coating an aluminum surface with nonstick material, like the kind used on cooking utensils. The researchers then created several designs by covering portions of the nonadhesive area so that it would act like a normal, "wettable" surface – meaning it would cause the falling droplets to spread around. An ultraviolet (UV) light shone over the entire surface completed the setup.
When a water droplet hit the patterned surface, the part of it that hit the wettable areas stayed on the surface, while the portions that came in contact with the waterproof areas sprang up back in the air. According to the researchers, who published their study in the journal Nature on March 5, 2019, this created a set of unbalanced forces that pulled unevenly at the water droplet - more in some parts than in others - and caused it to "dance"
By varying the designs of the wettable and water-repellant areas, the researchers were able to control the motion of the droplet and make it roll, twirl and even perform a combination of different actions. While the dancing water is certainly mesmerizing to watch, controlling how liquid leaps off a surface may also have some practical applications – like creating self-cleaning surfaces or mixing different materials.
Resources: nature.com, Sciencemag.org