It sounds implausible, yet scientists have managed to create a functioning engine, analogous to a Stirling engine, just three micrometers wide and made of a single particle. The minuscule engine was created by Clemens Bechinger and Valentin Blickle at the University of Stuttgart, and though it has its quirks, the pair have apparently demonstrated the engine's ability to do work.
"We've developed the world's smallest steam engine, or to be more precise the smallest Stirling engine, and found that the machine really does perform work," Bechinger said. "This was not necessarily to be expected, because the machine is so small that its motion is hindered by microscopic processes which are of no consequence in the macroworld."
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But before we conjure up images of a microscopic machine with fully operational furnaces and pistons, it's important to realize that this engine literally consists of a simple particle, a plastic colloidal bead, large enough to be observed by microscope.
Of course, even traditional Stirling engines aren't reliant on furnaces in the way a regular steam engine is. A temperature differential is all that is required. And unlike a steam engine, no gas is vented during operation. Instead a Stirling engine uses hot and cold chambers to expand and compress gas, driving pistons in the process. It's a machine that is remarkably quiet and efficient - where an application can be found.
With their particle engine, Bechinger and Blickle have effectively replaced the pistons with a laser, the intensity of which is cyclically fluctuated, impeding then enabling the movement of the particle in controlled bursts. A second laser heats the surrounding water, inducing the particle to movement when the first laser allows. In motion, the particle performs work on the laser field.
The afore-mentioned quirks arise from the fact that the particle is suspended in water. As such, the particle receives a constant buffeting from the motion of water molecules, and these collisions dampen the useful work that the engine can perform. The amount of energy lost to such collisions varies cycle by cycle.
Despite the dampening effect, the University of Stuttgart researchers claim that their engine is as efficient as a regular Stirling engine. Though there is no immediate practical application so far, the researchers claim that it demonstrates there is no principle preventing the construction of tiny, efficient engines.
"Our experiments provide us with an initial insight into the energy balance of a heat engine operating in microscopic dimensions," Bechinger added. "Although our machine does not provide any useful work as yet, there are no thermodynamic obstacles, in principle, which prohibit this in small dimensions."
Is this a Stirling engine in the strictest sense? One might argue not, but at the very least it's an illuminating analogy with which to describe a notable breakthrough in micro-engineering.