World’s smallest stirling engine developed

Technology that works on a large scale can cause spluttering in a smaller one.

Robert Stirling invented a heat engine nearly 200 years ago. Known as stirling engines, they are powered by the expansion of gas 

A macro steam engine

when heated, making a piston execute a motion such as driving a wheel.

Replicating this on a micro-world scale created some serious challenges, as the different laws of the micro-world meant that the thermal motions of a few particles could cause the engine to splutter. Researchers at the University of Stuttgart and the Max Planck Institute for Intelligent Systems, Germany, have succeeded in developing a micro-world stirling 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,” Professor Clemens 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.”

In the micro-engine, the working gas consists of a three micrometre individual plastic bead which floats in water, rather than countless gas molecules. The researchers replaced the piston with a focused laser beam, which controls the motion of the particle.

In a macro-world engine, the system is heated from the outside during the expansion process, to prevent the contributions to the work from cancelling each other out during compression and expansion. In the micro-world engine, the researchers replaced the coal-fire with another laser, which heats the water surrounding the plastic bead.

However, the water molecules are causing the spluttering, as their constant motion causes them to collide with the plastic bead. These collisions cause the plastic particle to exchange energy with its surroundings on the same order of magnitude as the micromachine converts energy into work.

“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,” Valentin Blickle said. These collisions would not matter in a macro machine, as it converts 20 orders of magnitude more energy to work.

Bechinger explained the experiments have provided 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.”

Source: Eureka Alert