AsianScientist (Aug. 18, 2015) – Studies of the impact a droplet makes on solid surfaces hark back more than a century. Until now, it was generally believed that a droplet’s impact on a solid surface could always be separated into two phases: spreading and retracting.
But it’s much more complex than that, as a team of researchers from City University of Hong Kong, Ariel University in Israel, and Dalian University of Technology in China report in the journal Applied Physics Letters.
“During the spreading phase, the droplet undergoes an inertia-dominant acceleration and spreads into a ‘pancake’ shape,” explained Professor Wang Zuankai from the Department of Mechanical and Biomedical Engineering at the City University of Hong Kong. “And during the retraction phase, the drop minimizes its surface energy and pulls back inward.”
As the team previously reported in the journal Nature Physics, it’s possible to shape a droplet into the pancake shape directly at the end of the spreading stage without going through the receding process, if it is bounced off a superhydrophobic surface. The water-repelling ability of hydrophobic surfaces like lotus leaves are attributed to the presence of an air cushion within the rough surface.
Because of the ‘pancake bounce’ off superhydrophobic surfaces, the droplet spends much less contact time with the surface and can be shed away much faster.
“Interestingly, the contact time is constant under a wide range of impact velocities,” said Wang. “In other words: the contact time reduction is very efficient and robust, so the novel surface behaves like an elastic spring. But the real magic lies within the surface texture itself.”
To prevent the air cushion from collapsing or water from penetrating into the surface, conventional wisdom suggests the use of nanoscale posts with small inter-post spacings.
“The smaller the inter-post spacings, the greater the impact velocity the small inter-post can withstand,” he elaborated. “By contrast, designing a surface with macrostructures—tapered sub-millimeter post arrays with a wide spacing—means that a droplet will shed from it much faster than any previously engineered materials.”
Despite exciting progress, rationally controlling the contact time and quantitatively predicting the critical Weber number—a number used in fluid mechanics to describe the ratio between deforming inertial forces and stabilizing cohesive forces for liquids flowing through a fluid medium—for the occurrence of pancake bouncing remained elusive.
So the team experimentally demonstrated that the drop bouncing is intricately influenced by the surface morphology. They showed that pointed posts give rise to more pancake bouncing than straight ones, significantly reducing the contact time and critical Weber number while increasing the Weber number range.
Wang and colleagues went on to develop simple harmonic spring models to reveal the dependence of timescales associated with the impinging drop and the critical Weber number for pancake bouncing on the surface morphology.
“Our new surface structure can be used to help prevent aircraft wings and engines from icing,” he said.
Beyond anti-icing for aircraft, “turbine blades in power stations and wind farms can also benefit from an anti-icing surface by gaining a boost in efficiency,” he added.
As you can imagine, this type of nature-inspired surface shows potential for a tremendous range of other applications as well—everything from water and oil separation to disease transmission prevention.
The next step for the team? To “develop bioinspired ‘active’ materials that are adaptive to their environments and capable of self-healing,” said Wang.
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Source: American Institute of Physics.
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