Shrinking an engine down to a tiny device capable of powering micromachines is no longer just a flight of fancy. The world's smallest steam engine combines a small plastic bead with lasers to replicate the same basic idea envisioned by inventor Robert Stirling almost 200 years ago.
The tiny engine re-creates the working idea of the Stirling engine — a gas-filled cylinder that drives a piston with heated gas expansion or cooled gas contraction. Instead of a piston, a plastic bead floating in water has its motion controlled by two lasers: one laser changes intensity periodically to allow greater or lesser degrees of motion; the other laser switches on and off to heat or cool the water.
"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," said Clemens Bechinger, a physicist at the University of Stuttgart and Fellow of the Max Planck Institute for Intelligent Systems in Germany.
The achievement surprised even the researchers. That's because the tiny engine — just 3 micrometers in size (1 micrometer being 0.001 of a millimeter) — runs into new problems in its microscopic world that cause it to sputter.
Such sputtering arises when surrounding water molecules move around and continually collide with the plastic bead. The collisions lead to drastic swings in energy gain or loss that wouldn't affect a full-size heat engine.
"This effect means that the amount of energy gained varies greatly from cycle to cycle, and even brings the machine to a standstill in the extreme case," said Valentin Blickle, a postdoctoral physics researcher at the University of Stuttgart.
Still, the physicists remained impressed by how well the tiny engine converted energy per cycle on average. The engine even ran with the same efficiency as its full-size counterparts under full load.
"Although our machine does not provide any useful work as yet, there are no thermodynamic obstacles, in principle, which prohibit this in small dimensions," Bechinger said.
The work is detailed in Sunday's issue of the journal Nature Physics.
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