A team of engineers from the Indian Institutes of Technology (IIT) Ropar and IIT Kanpur has found that injecting microscopic nanobubbles into common liquid fuels can significantly alter the way they burn. Using a novel temperature-mixing technique to create stable suspensions of oxygen and nitrogen bubbles, the team demonstrated that these tiny gas pockets, each thousands of times smaller than a grain of sand, could either speed up or slow down combustion depending on the fuel type. This discovery provides a potential new roadmap for making everything from car engines to jet turbines more efficient.

The new method relies on a fundamental principle of chemistry: gases are generally more soluble in cold liquids than in hot ones. The researchers used a method called hot-cold solvent mixing to produce these fuels. They took a sample of fuel, saturated it with gas at a very low temperature, and then rapidly mixed it with a hot sample of the same fuel. This sudden temperature change creates supersaturation, in which the liquid holds more gas than it can accommodate. The excess gas is forced out of the solution, forming bulk nanobubbles that are less than 1,000 nanometers in diameter. These bubbles remain suspended in the fuel rather than floating to the top and popping because they carry a slight negative electrical charge. Known as the zeta potential, their charge causes them to repel one another and stay spread out.

To see how these nanobubble-infused fuels performed, the team used high-speed cameras capable of capturing a single burning droplet at 100 frames per second. They tested four different liquids: isopropyl alcohol (IPA), ethanol, petrol, and Jet A-1. While the nanobubbles slightly slowed the burning rate of alcohols and petrol, they boosted the burning rate of Jet A-1, the fuel used in commercial aircraft. The high-speed footage revealed that the nanobubbles act as nucleation sites, or tiny seeds, that encourage larger bubbles to grow and burst inside the droplet. This creates a more chaotic, intense burning process, with frequent micro-explosions that disturb the fuel surface and alter its interaction with the surrounding air.

This research is the first study to systematically report how nanobubbles affect the droplet combustion of fuels like petrol and ethanol. However, the researchers noted that the experiments were conducted on relatively large droplets, about 2.5 millimetres wide, suspended on a quartz rod. While this allows for clear filming and precise measurements, it does not perfectly replicate the conditions inside a real engine, where fuel is sprayed into a fine mist of droplets much smaller than a human hair. Despite these challenges, the work offers a promising path toward future clean technologies. By tuning fuels with nanobubbles, engineers may be able to customise how a fuel burns without adding complex or expensive chemical additives. This could lead to the development of engines that produce fewer harmful emissions and extract more energy from every drop of fuel.