Going Against the Flow
Particles strive for the life of a couch potato—sinking into a
spot that has the least energy, where gravity can't pull them down
any farther and movement is at a minimum. Getting a particle moving
requires keeping it off kilter, out of equilibrium. But particles in
such a state tend to bounce all over; harnessing their movement in a
single desired direction is the goal of many nanoscale devices.
One way to do this is with a ratchet effect—a mechanism that
uses spatial asymmetry and energy gradients to make movement easier
in one direction than another. It turns out that in some cases,
ratchets not only control movement, but can also move particles in
unexpected directions—away from a minimum energy state, the
molecular equivalent of a creek climbing uphill under its own power.
Indeed, physicist Heiner Linke and his colleagues at the University
of Oregon, along with collaborators at Oregon State University and
the University of New South Wales in Australia, have used a ratchet
mechanism to make water droplets propel themselves up an incline.
Linke and his colleagues employed what's called the Leidenfrost
effect, which many people have seen in their own kitchens.
If a skillet is heated to an extremely hot temperature, between 200
and 300 degrees Celsius, drops of water flicked into the pan will
skitter across the surface, remaining intact for a minute or so. A
surface not quite so hot will boil away the water droplets
instantly, but the superheated surface instead instantly turns the
bottom of the droplets into a layer of steam. Vapor is a poor heat
conductor, so the steam insulates the drops from further boiling. It
also provides them with a means of movement: The water drops bounce
around like hovercraft on a cushion of air.
Linke and his colleagues did not use a smooth metal surface, but one
covered with a sawtooth pattern. The teeth inclined more steeply in
one direction than the other—an asymmetrical surface, and
therefore a ratchet mechanism. Millimeter-sized water droplets piped
onto the superheated sawtooth surface zip off in one direction like
airport passengers on a moving walkway, reaching speeds of up to 5
centimeters per second, even if the surface is tilted so that the
droplets have to climb uphill. As the investigators reported in the
April 21 issue of Physical Review Letters, the phenomenon
works for many other liquids, such as ethanol and liquid nitrogen,
although the temperature at which the Leidenfrost effect kicks in
varies from 50 to 150 degrees above the boiling temperature of the liquid.
Linke believes that the sawtooth shape of the surface ultimately
gives the droplets their staunchly preferred direction. "On a
smooth surface, the steam escapes evenly in all directions, so the
droplets glide around randomly," he says. "We think the
asymmetry of the surface gives direction to the steam." He used
particles of glitter to image the puff of steam, which can be seen
to exit preferentially from the advancing side of the droplet. Linke
believes that this vapor release pulls along the droplet by viscous
forces, much like a boat floating with the current of a river.
Such a flow might be useful in cooling systems for electronics:
Waste heat from a microprocessor could power the pumping
mechanism without any moving parts. The vapor layer prevents the
liquid in a ratchet pump from being a good heat sink, but the
ratchet could be used to pump liquid through a heat exchanger.
Linke is generally more interested in ratchets than in fluid flows.
A few years ago he and his colleagues fabricated a ratchet mechanism
out of a series of triangle-shaped semiconductor wells to control
the random motion of electrons. The triangular well forced electrons
into its pointy end and thus created an electrical gradient, making
a current travel through the ratchet more easily in one direction
than the other.
Another research group, however, reporting in the March 30 issue of
Nature, has found that in some cases charged particles
prefer to move in the "hard" direction created by a
Victor Moshchalkov and his fellow physicists at the Catholic
University of Leuven in Belgium have found that the number of
charged particles in a ratchet mechanism can be a significant factor
in how the ratchet works, particularly if the particles repel one
another. With an odd number of particles, the ratchet worked as
expected, but with an even number of particles, flow was in the
Moshchalkov's group studied fluxons, tiny vortices of magnetic flux
that repel one another, often found in sensitive superconducting
measurement devices. The fluxons were trapped in a ratchet potential
made of two nanofabricated wells, one larger than the other, forming
an asymmetrical shape that is essential for ratchets.
A fluxon put into the ratchet will fall into the deeper well, but if
an oscillating force is turned on, the fluxon will find it
"easier" to pop into the shallower well and then flow over
the top of the step into the next pair of wells, providing movement
of fluxons in only one direction.
However, if two fluxons are put in the ratchet, one will stay in the
shallower well while the other falls into the deeper well. This
time, the external force makes the fluxon in the shallower well push
into the deeper well, repelling the fluxon that's already there and
popping it out of the step in what is supposed to be the
"hard" direction of the ratchet.
"If you have more than one particle locked into an asymmetric
potential, due to the repelling interaction of particles there is a
sort of mini-billiard effect between the particles," says
Moshchalkov. "Then you have some strange knockout effects, and
particles move opposite to the prescribed direction."
Biological membranes also act as a kind of ratchet, allowing the
flow of ions in only one direction, in order to control such vital
functions as heart beat and nerve firing. Moshchalkov speculates
that his group's study might explain why some medications, in
improper dosages, can cause poisoning. Overloading the medication
may cause a membrane's ratcheting mechanism to reverse, pumping ions
in the wrong direction.
"The lesson is that you have to be careful with interacting
particles," says Moshchalkov. "We hope our work may be
able to help specify concentration limits."