force drives molecular ratchet
Ted Smalley Bowen,
Technology Research News
Before microscopic motors can be put to
work hauling tiny payloads and building infinitesimal structures, suitable
power sources and control mechanisms need to be developed.
Toward that end, Markus Porto, a researcher at the Max Planck Institute
in Dresden, Germany has proposed a way to harness the electromagnetic
interactions between charged particles to drive a molecular ratchet mechanism.
Porto's scheme converts both random and oscillatory motion of charged
particles into linear movement. Molecular motors like Porto's could eventually
transport and arrange minute amounts of substances.
The ratcheting motion of the motor is fully driven by the Coulomb force,
which is the electromagnetic repulsion or attraction of two charged particles,
said Porto. "No ad hoc potential is introduced. The ratchet type potential
is a natural consequence of the charge arrangement," he said. Potential
refers to energy stored in an object, like a coiled spring.
The scheme calls for a composite particle that holds two negative and
two positive charges to move along a track that holds irregularly spaced
When the arrangement of charges within the particle rotates in one direction,
the particle moves along the track. An opposite rotation causes the particle
to remain in place.
"A rotation of the moving particle's charge arrangement in one direction
results in a force acting on the particle... corresponding to [a] ratchet's
open direction," he said. The opposite rotation produces the equivalent
of a closed ratchet, which keeps the particle in place.
At the crux of the design is the conversion of two types of charge movements
-- random, and oscillatory, or rotational -- into controlled movement.
Converting random movement to directed movement will yield only rough
control of speed, however. "That the velocity fluctuates around a mean
value is the price one has to pay to be able to use a random driving,"
Changing the positions of the particle's charges changes the interaction
between the particle and the track, effectively giving the molecular motor
three gears. "The control mechanism relies on the ability to rearrange
the moving particle's charges. By rearranging these charges, one chooses
the `forward,' `reverse,' and `no-load' gear," said Porto.
Researchers could begin experimenting with such motors within about two
years, according to Porto.
"The paper represents good work in that it describes how Coulomb interactions
can be used to make a device that converts rotational, oscillatory, or
random motion into linear motion," said Bernard Yurke, a researcher at
Lucent Technologies. "It provides food for thought for those of us who
have made molecular motors that can execute rotary or oscillatory motion
and would like to use them to make something that can move along a substrate
much like the biological molecular motor, kinesin, moves along microtubules."
Yurke pointed out that a mechanism like the one Porto described would
be built of more than simple charges. "There has to be something that
holds these charges in proper relation to each other and the center of
mass at a proper distance from the track. One might imagine the [track's]
charges attached to a stiff linear molecule," he said. "The charges of
the moving entity rotate in specified ways about their center of mass.
One might imagine charges attached to a wheel shaped molecule that can
slide long the track of the stiff linear molecule."
Similarly, an engine would be needed to drive such a motor. "The power
generator... that rotates, oscillates, or randomly drives the charges
of the moving entity is not described," Yurke said.
Porto detailed his work in the February 27, 2001 issue of the journal
Physical Review E. The research was funded by the Max Planck Foundation.
Timeline: 2 years
TRN Categories: Nanotechnology
Story Type: News
Related Elements: Technical paper "Molecular Motor Based
Entirely on the Coulomb Interaction," Physical Review E, February, 2001.
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