Micro engine works on liquid power

Researchers have demonstrated that a sphere five times smaller than a red blood cell can operate as a tiny engine, rotating 1000 times a minute.

Although not the first microscopic engine ever invented, its creators claim it is the only one so far that does not rely on the transference of angular momentum from sources such as high-powered lasers, magnetic fields or the flow of energy from hot to cold reservoirs.

Instead, it exploits the minuscule energy release created by a tiny temperature rise that prompts the liquid in which it is embedded to separate into two components.

The micron-size engine turning at increasing speed as the laser increases the temperature of the surrounding liquid.
The micron-size engine turning at increasing speed as the laser increases the temperature of the surrounding liquid.
Credit: Schmidt et al

The engine has been designed by a team led by physicist Falko Schmidt from Gothenburg University in Germany.

The heart of the system is a micron-wide sphere made of silica and iron oxide. The tiny ball is suspended in a liquid and held in place by a low-power near-infrared laser beam that acts as an optical tweezer. (In the paper describing their work, Schmidt and his colleagues note that their set-up rests on “a homemade inverted microscope”.)

It is the liquid that represents the key innovation. It comprises a mixture of water and an aromatic organic compound called 2,6-lutidine. The power for the engine derives from the different temperature sensitivity of the two components.

When the combination is held at a temperature of 26 degrees Celsius, it behaves like a normal liquid, and the sphere simply lingers around the centre of the optical trap established by the laser beam.

Things change, however, when the laser power is increased. This heats up the side of the sphere nearest the laser beam, which in turn warms the liquid it is touching past the “critical temperature” of 34 degrees. At that point, the two components of the liquid separate – they “demix”, in the jargon – which creates a concentration gradient and induces an effect known as diffusiophoresis, in which the sphere is pushed out from the centre.

While doing so, the researchers observe, small impurities and asymmetries in the sphere induce it to rotate around the axis of the laser beam.

“Sometimes it can be observed that the rotation stops,” they write. “The particle moves towards the centre of the trap, and subsequently starts rotating again in the opposite direction.”

The efficiency of the engine is critically dependent on temperature. The researchers note that raising the heat above certain levels causes the sphere to rotate in unstable patterns.

Schmidt and his colleagues say their engine matches the power output of other microscopic motors but has the advantage that it is simpler and may be a model for biological processes.

They suggest it could serve as a prototype for future “biocompatible engines” that could be able perform surgeries on a cell-by-cell level.

The paper will soon be published in the journal Physical Review Letters. A version on the preprint server arXiv is available here.

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