AIP Physics News - Superhydrophobic Surfaces and Black Holes in the Atmosphere

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 764 February 6, 2006 by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi

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). 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% drag reduction
can be achieved for a channel gap of 10 microns; a 55% reduction for
a 100 micron gap; and a 11% 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)

LOOKING FOR BLACK HOLES IN THE ATMOSPHERE is one of the prominent
missions for the newly built Pierre Auger Observatory. Black holes
can arise from the collapse of heavy stars but might also, according
to theoretical particle physics, be produced when cosmic ray
particles (especially neutrinos) with multi-TeV energies pass very
close to a particle within our atmosphere. The ensuing air shower
of secondary particles would be sensed on the ground in Auger's huge
array of detectors, which began their work in 2003 (see figure). A new analysis of this hypothetical black hole
production process, however, questions whether many such
mini-black-hole events would occur. According to Dejan Stokovic
(Case Western Reserve University) and his colleagues, the same
process that encourages black hole creation in cosmic-ray neutrino
scattering events at the TeV energy level (rather than at the
impossibly inaccessible 10¹⁹GeV level, referred to as the Planck
energy) also should hasten the decay of protons to an extent not
seen in experiments designed to look for them. Therefore, Stokovic
(dejan@balin.phys.cwru.edu) argues, the robust stability of the
proton militates against an expected mini-black-hole production of
several hundred events over the Auger Observatory's active period
from 2003 to 2008. This doesn't necessarily mean that no black hole
events would seen, but probably not as many as were once
anticipated. (Stojkovic et al., Physical Review Letters, 3 February
2006)