A Superhydrophobic Surface

A superhydrophobic surface, devised by scientists at UCLA, greatly reduces the friction felt by a fluid as it moves across the surface. It does this by inducing a blanket of air to lodge between nano-posts built onto the surface; the air keeps the fluid from coming into contact with the solid surface (see figure at Physics News Graphics).

This arrangement is a sort of upside-down hydrofoil, the marine design in which the friction between ship and liquid is lessened by minimizing the contact area, and this in turn is accomplished by keeping the larger part of the hull above the water on pylons. The UCLA scheme is also a bit like an "air-hockey" game, in which a quasi-frictional effect is achieved by having pucks float across a table pierced by holes feeding forced air under the puck.

In the new work, a forest of posts one micron in height are etched across the substrate surface. The posts will thereafter trap air, which in turn permits fluid flow above with greatly reduced friction.

Such a scheme has been tried before, but the UCLA researchers have the sharpest posts and the highest yet density of posts so far.

This is important for certain areas for fluid research and for prospective microfluidic applications; the fluid levitation is maintained even when the fluid is pressurized. Applications are also likely at the macroscopic level. For example, submarines and torpedoes coated with the slippery nanoengineered material would glide through the sea under much less propulsion.

How effective is this approach? It's difficult to specify a single drag-reduction amount since so many factors are at play: the surface area, the liquid speed, the viscosity, the fluid pressure, the gap width of the channel, and so on. For instance, a 90 percent drag reduction can be achieved for a channel gap of 10 microns; a 55 percent reduction for a 100 micron gap; and an 11 percent reduction for a 1-mm gap. Therefore, a figure of merit often used by the researchers is the "slip length," which is roughly the extrapolated distance beneath a solid surface at which a no-slip boundary condition would hold true (again, see the figure). A large slip length is good; and the UCLA team has observed the largest slip values yet seen, even under pressurized conditions.

Choi et al., Physical Review Letters, upcoming article
Contact Chang-Hwan Choi, chchoi@ucla.edu