Do try this one at home, Using a syringe, release a single drop of water onto the flat edge of a knife. Notice how the drop sticks to the metal. Now light a candle and apply the flame to the knife for a few seconds, turning it black. Deposit another drop on the soot-covered surface, and you’ll find that the drop bounces off the surface completely, leaving the knife dry.
This bouncing occurs because the soot has a microscopic texture that traps air under the drop, the same mechanism that enables a lotus leaf to repel raindrops. Now engineers at Boston University and MIT have added a slight wrinkle to the drop bouncing challenge—literally. When they augmented micro- or nanostructured surfaces with periodic, wrinkle-like features, drops bounced off at faster rates than previously thought possible.
The engineers, BU Assistant Professor James C. Bird (ME, MSE) and collaborators in the Varanasi group at MIT’s Department of Mechanical Engineering, reported their findings in the cover story of the November 21 issue of Nature.
“We’ve demonstrated that we can use surface texture to reshape a drop as it recoils in such a way that the overall contact time is significantly reduced,” said Bird, the paper’s lead author, who directs the Interfacial Fluid Dynamics Laboratory. “The upshot is that the surface stays drier longer if this contact time is reduced, which has the potential to be useful for a variety of applications.”
Such surfaces may improve the performance of systems that operate better under dry conditions, such as steam turbines or aircraft wings, and enable cold surfaces, such as rooftops, to resist icing by shedding liquid drops before they freeze. Also found in nature—from butterfly wings to nasturtium leaves, which have veins that run through them that minimize contact time with incoming liquid drops—these surfaces may have ecological implications as well.
Irradiating silicon with laser beams, the researchers first engineered micro- and nanostructured surfaces, and then added periodic wrinkle or ridge-like features. Prior to adding the ridges, a drop would spread out to a maximum diameter, retract until the edges of the drop met its stationary center point and bounce off. With the introduction of the ridges, the center point moved to meet the edges as the drop recoiled, heading it off at the pass. The drop then split in two before jumping off the surface.
“On a typical non-wetting surface, the drop spreads and retracts with radial symmetry so that the center of the flattened drop is just sitting there until it is engulfed by the retracting edge,” Bird explained. “In contrast, the new surfaces are designed to activate the center of the drop so that it helps the drop recoil faster, thereby reducing contact time.”
Introducing the ridges to micro- and nanostructured surfaces reduced contact time from 12.4 to 7.8 milliseconds, or about 37 percent. The experiment produced the shortest contact time achieved in the lab under comparable conditions, based on peer-reviewed studies going back to the 1960s.
Bird and his collaborators subsequently duplicated this phenomenon using aluminum and copper oxide surfaces with different microstructures, thus illustrating that the geometry of the enhanced surfaces, not the materials themselves, led to the shorter contact times. They next aim to build on this proof of concept, experimenting with different surface features and optimizing micro- and nanostructured materials for specific applications.
The researchers drew upon funds from the National Science Foundation and Defense Advanced Projects Research Agency. Bird and his MIT collaborators—senior author Kripa Varanasi, Rajeev Dhiman and Hyuk-Min Kwon—have filed patents on the methods described in the Nature paper.
See movie comparing performance of control surface with that of enhanced surface with ridge-like features.