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a radius of about 0.0000012 inches (0.000003 cm or 30 nm), about three
times less than the larger drop. The curvature of the smaller drop is
greater than the larger drop, which means the smaller drop has greater
tension; when the droplet surfaces touch, this force quickly pushes at-
oms back to the larger drop. At this instant, the oscillator “relaxes” into
its initial state. Electricity then begins to drive them back to the smaller
drop, starting the oscillation cycle again.
At such small scales, the experimenters cannot see the motor work-
ing by any means except an electron microscope. Although the mo-
tor is simple conceptually, its precision is incredible—it operates at the
atomic level, controlling the motion of atoms as they shuffle back and
forth between nanoparticles. B. C. Regan, Zettl, and their colleagues
published the report “Surface-Tension-Driven Nanoelectromechani-
cal Relaxation Oscillator” in
Applied Physics Letters
in 2005. As the
researchers note in their report, “[S]urface tension can be a dominant
force for small systems,” as illustrated in their motor. This is a prime
example of the different forces and situations that must be taken into
account in the nanoworld.
Tiny machines such as Zettl's oscillator may be useful on their own
but also in forming the components of more sophisticated instruments
such as nanobots. Robotic automation is commonly employed in in-
dustrial factories to do jobs that require repetition or extreme preci-
sion, such as spot welding. Nanobots would have the added benefit of
being able to function in otherwise inaccessibly small locations. Tasks
for nanobots include scanning a load-bearing surface and looking for
signs of structural failure that would be impossible for a human inspec-
tor to see.
A large number of nanobots would be required for most jobs, and
that number would probably vary considerably depending on work-
load. Manufacturing this army of molecular machines might become
prohibitively expensive unless the nanobots can self-assemble. An ideal
strategy would have self-replicating nanobots that automatically replace
damaged or lost machines and ramp up production when an especially
demanding task arises.
But the idea of self-replicating nanobots makes some people un-
easy. It is not difficult to imagine a scenario in which the machines
begin to replicate out of control; similar catastrophes occurring in the
human body are called cancers—mutations cause a cell to keep grow-
ing and dividing, producing an abnormal growth that can endanger the
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