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
A large slip length is good; and the UCLA team has
observed the largest slip values yet seen, even under pressurized
Choi et al., Physical Review Letters, upcoming
Contact Chang-Hwan Choi, email@example.com