Nanobubbles are the tiniest bubbles you’ve never seen,” says Niall English, a chemical engineering researcher at Univ. College Dublin.

With diameters from 50 to 100 nm — less than the wavelength of visible light — nanobubbles are too small to be seen with the naked eye. Yet despite their invisibility, these bubbles have the potential to revolutionize a variety of environmental and industrial processes, including agriculture, wastewater treatment, and ecosystem management.

One of the key properties of nanobubbles is their extremely high surface-area-to-volume ratio. This makes them less subject to the phenomena of buoyancy, meaning they can stay suspended in liquids for long periods of time. While larger bubbles rise to the atmosphere very quickly, nanobubbles have a lifetime of hours or even days.

Up until now, these curious bubbles have typically been generated using a method called cavitation, where gas is forced through a membrane at high pressure. But this process has proved both energy expensive and error-prone, whereby the membrane gets fouled and breaks down.

To address these issues, English and his colleagues invented a novel method to generate nanobubbles using static electric fields. When an electric field is applied to a liquid, it induces electrostriction, a phenomenon that increases the density of the liquid by packing molecules more closely together, creating a temporary “negative pressure region” at the interface between the liquid and gas. As a result, the liquid will suck in the gas like a vacuum, generating nanobubbles instantly.

According to English, electrostriction is five to ten times more energy efficient than other fine bubble generation techniques. His generators utilize a sheathed electrode through which a direct current flows, necessitating a power consumption of only a few watts.

The method developed by English produces nanobubbles that are denser, which increases longevity, and have thick, quasi-liquid layers rather than membranes. Additionally, the bubbles have what the inventor calls an “electrostatic surface personality,” allowing them to electrostatically absorb anything with a charge in the surrounding liquid. This makes them useful as delivery vehicles or cleaning agents in a range of sustainable applications.

For example, in wastewater treatment, oxygen-enriched nanobubbles could improve aeration, the process of putting air or oxygen into the water to break down organic matter and pollutants. Currently, aeration accounts for two thirds of the energy demand for wastewater treatment. Delivering oxygen via long-lasting nanobubbles could reduce that by three- to four-fold.

In agriculture, nanobubbles could increase the growth and yield of crops. Because of their electrostatic surface, nanobubbles can absorb nutrients and serve as nanosized carriers in irrigation water. They are effective at delivering particulates directly into plant roots, meaning less fertilizer would be needed. This could have wider benefits for the ecosystem at large. Runoff from overfertilized fields is the reason why water bodies become contaminated by harmful algal blooms, claims English.

To further prevent eutrophication, the process by which water bodies become overrun with excessive nutrients, English imagines a low-maintenance nanobubble generator that could be installed into lakes and reservoirs, powered by solar energy. Such a nanobubble generator would deploy dissolved oxygen into the water, supporting abundant aquatic animal populations.

Greater dissolved oxygen content would help reduce seasonal fish kills. During the summer, high temperatures can cause water oxygen levels to drop, which can suffocate fish. “We can sidestep that problem neatly by generating nanobubbles,” says English. “And these nanobubbles effectively become reservoirs, or stores of gas, that give up their bounty or treasure.”

According to English, the list of potential applications for nanobubbles goes on, including desalination, oil recovery, algae cultivation, mining flocculation, and drug delivery in medicine. “No one has really thought about the massive, platform-wide possibilities that this has,” he says. “That’s why, to me, this is the biggest development in chemical engineering in a century.”

English, N. J., “Environmental Exploration of Ultra-Dense Nanobubbles: Rethinking Sustainability,” Environments, 9 (3), p. 33, doi: 10.3390/environments9030033 (2022).

English, N. J., “Sustainable Exploitation and Commercialization of Ultradense Nanobubbles: Reinventing Liquidity,” ACS Sustainable Chemistry & Engineering, 10 (11), pp. 3383–3386, doi: 10.1021/acssuschemeng.2c01058 (2022).

Read the full article here