It may be counter-intuitive, but engineers say by adding a subtle, wrinkle-like texture they can create surfaces that shed liquid much faster than smooth ones.
The team demonstrated that the approach works on a variety of surface materials, and even notes that natural water-shedding surfaces like butterfly wings and nasturtium leaves possess similar properties.
“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,” says James Bird, the paper’s lead author, who directs the Interfacial Fluid Dynamics Laboratory at Boston University.
“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. Furthermore, the approach may help cold surfaces resist icing by shedding liquid drops before they freeze.
37 percent drop in contact time
Adding tiny ridges to a surface, they found, alters the way water drops react when they strike and causes them to bounce off quicker. 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. The drop then split in two before jumping off the surface.
This single innovation 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.
“We reduced the distance the drop had to move by redistributing its mass,” explains Bird, a mechanical engineering and materials science assistant professor. “We introduced larger-scale ridges that were much bigger than the microstructure on the surface, but much smaller than the thickness of the drop.
“The ridges were large enough to influence the hydrodynamics but not so large that they would immediately split the drop.”
The researchers drew upon funds from the National Science Foundation and Defense Advanced Projects Research Agency. Bird and his Massachusetts Institute of Technology collaborators—senior author Kripa Varanasi, Rajeev Dhiman, and Hyuk-Min Kwon—have filed patents on the methods described in the journal Nature.
Source: Boston University