World’s smallest steam engine

In a normal-sized engine, a gas expands and contracts at different temperatures, an action that moves a piston in a cylinder. Physicists in Stuttgart have created this work cycle with a tiny plastic bead that they trapped in the focus of a laser field.

Researchers at the University of Stuttgart (Germany) and the Stuttgart-based Max Planck Institute for Intelligent Systems have developed a micrometer-scale heat engine.

“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, professor at the University of Stuttgart and Fellow of the Max Planck Institute for Intelligent Systems. “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 macro world.” The disturbances cause the micromachine to run rough and, in a sense, sputter.

A technology that works at a large scale can cause unexpected problems at a small one. And these can be of a fundamental nature, reported Bechinger and his fellow researcher, Valentin Blickle.  This is because different laws prevail in the micro and the macro worlds. Despite the different laws, some physical processes are surprisingly similar on both large and small scales.

The laws of the micro world dictated that the researchers were not able to construct the tiny engine according to the blueprint of 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.

“We successfully decreased the size of the essential parts of a heat engine, such as the working gas and piston, to only a few micrometers and then assembled them on a machine," said Blickle. Therefore, working gas in the Stuttgart-based experiment no longer consists of countless molecules, but of only one individual plastic bead measuring a 3µm that floats in water. Since the colloid particle is around 10,000 times larger than an atom, researchers can observe its motion directly with a microscope.

The physicists replaced the piston, which moves periodically up and down in a cylinder, by 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 or 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. In order for the contributions to the work not to cancel each other out during compression and expansion, they must take place at different temperatures. This is done by heating the system from the outside during the expansion process, just like the boiler of a steam engine. The researchers replaced the coal fire of an old-fashioned steam engine with a laser beam that heats the water suddenly, but also lets it cool down as soon as it is switched off.

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