The engine uses a colloidal particle that is optically trapped using a laser beam.

Researchers from Bengaluru’s Indian Institute of Science (IISc) and the Jawaharlal Nehru Centre for Advanced Scientific Research achieved a major breakthrough when they designed a microscopic heat engine that operates at 50-60 per cent efficiency by relying on changes in bacterial activity. The results were published on August 29 in the journal Nature Physics.

While a conventional engine relies on very high temperature difference to move the piston back and forth, the microscopic heat engine developed by the researchers relies on very small changes in temperature input to impact bacterial activity to achieve large work done by the engine.

The tiny heat engine uses a colloidal particle of 5 micrometre size (1/50 the thickness of the human hair) that is optically trapped using a laser beam. The extent to which the particle can move is controlled by varying the intensity of the laser beam — the more the intensity the less the particle can move and vice versa.

At high temperature the intensity of the laser beam is reduced so the particle can get displaced more; the intensity is increased at lower temperature.

The colloidal particle is kept in a water bath that contains soil-dwelling bacteria Bacillus licheniformis.

When the temperature of the bath is increased to 40 degree C, the bacterial activity becomes very high as the bacteria tend to move around vigorously. The vigorous movement of the bacteria influences the colloidal particle and it undergoes a large displacement resulting in large work done by the engine.

When the temperature of the bath is changed to 17 degree C the bacterial activity becomes less due to sluggishness of the bacteria resulting in smaller displacement of the colloidal particle.

The change in temperature, which is carried out every four seconds, leads to changes in bacterial activity and hence the work done by the engine.

“The temperature difference is too small to get work done. The efficiency of the engine will be around three per cent at this temperature difference. But due to bacterial activity the efficiency is over 50 per cent,” says Prof. Ajay K. Sood, the corresponding author of the paper from the Department of Physics, IISc.

“When the bacterial activity is high at 40 degree C the effective temperature is around 2000 degree C; at 17 degree C the bacterial activity is significantly diminished leading to low effective temperature.”

“Instead of using high and low temperature, we are exploiting bacterial activity to change the two states,” he says. While the tiny heat engine outperformed a conventional engine in efficiency, it is still far less efficient compared with biological motors that operate in our bodies at 100 per cent efficiency even when the temperature remains constant. “The next step is to connect the heat engine to some nano device or electromechanical device,” Prof. Sood says.