A tiny, levitated glass sphere behaves like the hottest engine ever made

The hottest engine ever created is, by some definitions, not an engine and not particularly hot. But it reveals the weird physics of extreme temperatures at the microscale.

The engine is made of a tiny glass sphere, just 5 micrometers across. It’s levitated in a near-vacuum, using an electric field. A jittery voltage jiggles the sphere so violently that it behaves as if it’s extremely hot, researchers report in a paper in press in Physical Review Letters. The particle reached the equivalent of 13 million Celsius — nearly as hot as the core of the sun.

But you wouldn’t singe your finger if you touched the glass sphere. Its effective temperature is based on the energy of the glass sphere’s overall motion, rather than the jiggling of individual molecules that normally determine temperature. “It is moving as if you had put this object into a gas that was that hot,” says physicist James Millen of King’s College London, a coauthor of the study. “It moves around like crazy.”

That’s especially impressive for such a tiny object, says physicist John Bechhoefer of Simon Fraser University in Burnaby, who was not involved with the work. “Creating effective temperatures that high at that scale is very nice.” He notes that the effective temperature depends on the size of the object, so larger engines might be able to reach even higher effective temperatures.

Reaching high temperatures matters for the performance of an engine. In the parlance of thermodynamics  — the physics field concerned with heat, work and energy — the sphere is what’s known as a heat engine. That’s a machine that performs mechanical work as it takes in heat from a high-temperature source and sends waste heat to a lower-temperature heat sink. A larger ratio of hot to cold temperatures corresponds to a higher efficiency. For commercial engines, that ratio goes as high as about 3. This engine had a ratio of about 100.

The engine allows the researchers to investigate how engines work at extreme temperatures and on small scales. The team found that the engine’s properties fluctuated wildly. Sometimes it was 10 percent efficient, sometimes 200 percent. Sometimes the engine ran in reverse: Instead of heating up, it cooled down.

“Thermodynamics down at the microscale is really, really weird. I really think it’s as unintuitive as something like quantum mechanics,” Millen says.

But it’s crucial to understand: It’s the reality inside cells, where tiny structures like proteins are buffeted around by the random jostling of their surroundings. The researchers hope that their engine could help scientists study tiny, biological engines such as kinesin, a type of motor protein that shuttles cargo around inside cells. 

Unlike conventional machines, the tiny glass sphere doesn’t do anything particularly useful. “No one would really say, ‘Vroom vroom, that’s an engine,’ ” Millen says. Instead, “It’s a perfect analog of an engine,” one that scientists can fiddle with to study how such tiny devices operate.

For example: As the particle moved about the electric field that jostled it, the temperature it experienced changed. This phenomenon, called position-dependent diffusion, is important for biological processes such as protein folding.

Creating an engine with a single one of these attributes — extreme temperature, a large hot-to-cold temperature ratio or position-dependent diffusion — would make for a nice experiment, says physicist Uroš Delić of TU Wien in Vienna, who was not involved with the research. “This work combines all three, so that’s quite cool — or hot.”