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Engineered ridges can make a water drop leave a superhydrophobic surface sooner by redirecting how the liquid spreads and recoils. In a 2013 laboratory study, a macrotextured surface produced a 7.8-millisecond contact time in a representative experiment, compared with 12.4 milliseconds on a control surface. The finding shows how surface shape can change a drop’s motion; it does not establish a commercial coating or a proven aircraft anti-icing system.
Why contact time matters when a drop bounces
After striking a superhydrophobic surface, a drop spreads into a thin shape, retracts, and may rebound. The interval it spends touching the solid is called contact time. Shorter contact can limit the time available for exchanges of mass, momentum, and energy between the drop and surface.
Earlier efforts to shorten contact time focused on reducing interactions and pinning where the drop’s edge meets the surface. The conventional expectation was that symmetric spreading and recoil imposed a hydrodynamic lower limit. Bird, Dhiman, Kwon, and Varanasi showed that changing the surface’s larger-scale shape could alter the flow and bring contact time below that assumed limit.
How a ridge changes the drop’s recoil
A ridge beneath an impacting drop redistributes liquid rather than allowing it to spread and retract uniformly in every direction. The liquid layer over the ridge becomes thinner than nearby portions, and that thinner section retracts faster. The resulting motion is non-axisymmetric: instead of recoiling as one broad, radially symmetric mass, the liquid divides into smaller regions that travel shorter distances before detaching.
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The authors described the principle in the abstract of their paper, “Reducing the contact time of a bouncing drop”: “Here we demonstrate that it is possible to reduce the contact time below this theoretical limit by using superhydrophobic surfaces with a morphology that redistributes the liquid mass and thereby alters the drop hydrodynamics.” The paper’s authors are James C. Bird, Rajeev Dhiman, Hyuk-Min Kwon, and Kripa K. Varanasi.
Contact times reported in the 2013 experiments
Nature’s supplementary material gives representative experimental timings for three surfaces. These are examples from the study’s conditions, not universal performance specifications.
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| Surface | Representative contact time |
|---|---|
| Micropillar array | 18.0 ms (Nature supplementary material, 2013) |
| Control surface | 12.4 ms (Nature supplementary material, 2013) |
| Designed macrotextured surface | 7.8 ms (Nature supplementary material, 2013) |
In the simplest geometry, Varanasi told Chemistry World that contact time fell by approximately 40%. He also described larger reductions as a research goal, not as an achieved result in that statement. The percentage and the timings above refer to different descriptions of the experiments and should not be treated as a general rating for textured surfaces.
What the molten-tin demonstration showed
The supplementary material also describes a specific molten-tin test. A droplet at 250 °C bounced from a macrotextured ridge in 6.8 ms under the reported conditions, while a droplet on the unridged substrate stuck after local solidification. This demonstrates behavior in that experiment; it does not establish performance across other materials, temperatures, or operating environments.
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Could faster rebound help prevent icing?
Reducing the time a drop remains on a surface could be useful where liquid water is a problem. The Nature abstract identifies resistance to icing as a motivation for water-repellent surfaces, and Chemistry World named freezing rain on aircraft engines as a potential application. Those are possible uses, not a validated deployment: the cited reporting does not establish a commercial engine coating or an aircraft anti-icing system based on this work.
What natural surfaces do—and do not—show
The study’s supplementary video description notes non-axisymmetric recoil on a Nasturtium leaf and a Morpho butterfly wing, whose ridges or veins can trigger similar motion. That observation does not show that either natural structure evolved for this particular function; Chemistry World characterized that interpretation as speculative.
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The paper behind the result
James C. Bird, Rajeev Dhiman, Hyuk-Min Kwon, and Kripa K. Varanasi, “Reducing the contact time of a bouncing drop,” Nature 503, 385–388 (2013). Nature lists the paper as published on 20 November 2013, with an issue date of 21 November 2013. View the paper record at Nature.
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