Superhydrophobic surfaces reduce drag

Turbulent flows of a liquid along a surface experience frictional drag, a macroscopic phenomenon that affects the speed and efficiency of marine vessels, the cost of pumping oil through a pipeline, and countless other engineering parameters. The drag arises from shear stress, the rate per unit area of momentum transfer from the flow to the surface. To reduce the flux, engineers could add polymers to the flow, inject bubbles against the surface, or combine the two methods, but those approaches bear their own cost. Jonathan Rothstein and colleagues at the University of Massachusetts Amherst now offer a proof-of-principle demonstration of a new, passive option for reducing drag in turbulent flow. They tailored the microscale structure of a hydrophobic material—polydimethylsiloxane, similar to the rubbery polymer used to caulk bathtubs—to create air pockets at the surface, as shown in the figure, that allow the flow to “slip” (shear free) at the liquid–air interface. The greater the area covered by air pockets, the greater the reduction in shear stress—up to 50%, they estimate, judging from particle-image velocimetry and pressure-drop experiments over a wide range of Reynolds numbers. The researchers found that the critical Reynolds number at which the onset of drag reduction occurs is related to the ratio of two length scales—one associated with the geometry of the hydrophobic surface corrugations, the other with the thickness of the viscous boundary layer there. (R. J. Daniello, N. E. Waterhouse, J. P. Rothstein, Phys. Fluids 21, 085103, 2009.) —R. Mark Wilson