World's smallest steam engine comes to life

  • German physicists say they've built a heat engine measuring only a few micrometers across which works as well as a normal-sized version - although it sputters, they admit.

    Researchers at the University of Stuttgart and the Max Planck Institute for Intelligent Systems say that the engine does basically work, meaning there's nothing, in principle, to prevent the construction of highly efficient, small heat engines.

    "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," says Clemens Bechinger of the University of Stuttgart.


    The researchers couldn't construct the tiny engine in the same way as a normal-sized one. In the heat engine invented almost 200 years ago by Robert Stirling, a gas-filled cylinder is periodically heated and cooled so that the gas expands and contracts. This makes a piston execute a motion with which it can drive a wheel, for example.

    However, the working gas in the new engine consists of just one individual plastic bead measuring three micrometers, which floats in water. Since the colloid particle is around 10,000 times larger than an atom, researchers can observe its motion directly in a microscope.

    The physicists replaced the engine's piston with a focused laser beam whose intensity is periodically varied. The optical forces of the laser limit the motion of the plastic particle to a greater and a lesser degree, like the compression and expansion of the gas in the cylinder of a large heat engine. The particle then does work on the optical laser field.



    "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," explains Stuttgart's Valentin Blickle. Since macroscopic machines convert around 20 orders of magnitude more energy, the tiny collision energies of the smallest particles in them aren't important, he says.

    "Our experiments provide us with an initial insight into the energy balance of a heat engine operating in microscopic dimensions," says Bechinger. "Although our machine does not provide any useful work as yet, there are no thermodynamic obstacles, in principle, which prohibit this in small dimensions."

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