A tiny, particle-sized engine that runs at temperatures approaching the innermost core of the Sun could open a window into the smallest extremes of thermodynamics.
By levitating a single particle of silica in a vacuum and blasting it with synthetic temperatures higher than 10 million kelvin (10 million °C or 18 million °F), physicists have created a microscopic Stirling heat engine – not to power a tiny machine, but to better understand the physics of heat and energy.
Remarkably, this also offers insight into the complex microscopic processes that take place within our bodies.
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"This experimental platform shows great promise in its ability to simulate and explore not only high temperatures, but also the biologically relevant thermodynamic scenario of position-dependent diffusion," writes a team led by physicist Molly Message of King's College London.
"Position-dependent diffusion is key to understanding, for example, protein folding and mass transport in biological settings."
A Stirling engine works by heating and cooling a sealed gas or fluid so that it expands and contracts in a repeating cycle, converting heat into mechanical energy. A microscopic Stirling engine is a miniature analog, based on the same principles, but operating on a micrometer scale.
Message and her colleagues built their engine around a spherical particle of silica just 4.82 micrometers in diameter – a fraction of the width of a human hair. This particle was levitated in a trap made of electric fields, where it can jiggle about a little bit, but not escape.
Then, they applied electric noise to the particle to simulate temperatures up to 13 million kelvin – far hotter than the 5,800-K temperature of the Sun's surface, and nearing the 15-million-K temperature at its core.
These are effective (not physical) temperatures: The electric noise applied to the system makes the silica particle jiggle about exactly as it would under temperature conditions up to 13 million K.
Meanwhile, the 'cool' environment around the particle stayed about 100 times lower – a temperature contrast that would be unachievable in a real Stirling engine – allowing a probe of thermodynamics far beyond what's possible at full scale.
This is because the second law of thermodynamics can only be applied to averages on the microscopic scale. So while there are moments that seem to break the law, such as a large fluctuation, or efficiency seemingly in excess of 100 percent, once everything is averaged out, the system behaves like it should.
The team ran their system first by applying the noise to 'heat' the particle. Then, they adjusted the electric trap to allow the particle to jiggle about more – the expansion phase of the Stirling cycle. Then, for the contraction phase, the noise was turned off, allowing the particle to 'cool' before the trap was adjusted again to reduce the jiggling.
The researchers ran each experiment for between 700 and 1,400 cycles to study in detail how the system behaved. They found huge fluctuations in heat exchange, as well as brief periods where the particle seemed to produce more work than the heat it consumed, temporarily demonstrating an efficiency rate over 100 percent.
This is just a result of the short-term randomness and giant fluctuations in heat and energy at small scales, and isn't unexpected.
The really interesting part is that the particle didn't jiggle about randomly in the trap, as we might see in normal diffusion in a uniform environment; its movement depended on where in the trap it was.
When the temperature and consistency of a medium change, that alters how particles move through it, a phenomenon known as position-dependent diffusion.
This is important in biological systems, where particles interact with membranes, fluids, and tissues. So the team's setup may be a way to investigate problems like drug transport through the body.
The team now hopes to push their microscopic Stirling engine even further from equilibrium, exploring the strange, fluctuating physics that govern motion and energy at the tiniest scales.
The research has been published in Physical Review Letters.