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If a drop of water falls on a very hydrophobic surface, then under the effect of surface tension, it can also jump like a flexible ball:
And today we will also see an upgraded version of bouncing water droplets: it will not only bounce off the solid surface, but will also start to rotate:
The surface that allows the water droplets to start rotating spontaneously undergoes some special treatment, and some clues may already be visible on the animation. Simply put, the treatment method is this: first, a very hydrophobic fluorine compound coating is applied to the solid surface, and then a template is coated on the surface, and the ultraviolet light is used for treatment. The result of this treatment is that only the local material properties exposed to ultraviolet light have changed to become hydrophilic, forming a hydrophilic spiral pattern on the hydrophobic surface (should it be called spiral…).
The dyed water droplets in the picture above are turned into orange lines after being photographed, which is the hydrophilic part left after the treatment. The interaction of the hydrophilic part with the water droplets causes an unbalanced force, which causes the water droplets to rotate. Using this method, the droplet speed can reach 7300 revolutions per minute.
Changing the design of the hydrophilic zone can also make the droplets move in other ways, such as rolling to the side like the following picture:
Researchers from the Institute of Chemistry of the Chinese Academy of Sciences and Tsinghua University reported this novel and interesting method of controlling droplet movement in Nature Communications. The control of droplet movement may be applied in fields such as self-cleaning and inkjet printing.
For related reports and papers see:
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