Wrinkled surfaces shed water to improve dry performance

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Systems that operate better in dry conditions have received a boost with the development of a wrinkled surface that sheds water more quickly than one that is smooth.

By adding a subtle, wrinkle-like texture Boston University’s James C. Bird and collaborators from the Massachusetts Institute of Technology created surfaces that appear to shed drops faster than any previously engineered material. The team have reported their findings in 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 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 could improve the performance of systems including steam turbines or aircraft wings and help cold surfaces resist icing by shedding liquid drops before they freeze.

According to a statement, adding tiny ridges to a surface 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 centre 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 per cent. 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,’ Bird said. ‘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 team demonstrated that this approach works on a variety of surface materials, and even noted that natural water-shedding surfaces like butterfly wings and nasturtium leaves possess similar properties.