Nanobubble. A nanobubble is a small sub-micrometer gas-containing cavity, or bubble, in aqueous solutions with unique properties caused by high internal pressure, small size and surface charge. Nanobubbles generally measure between 70-150 nanometers in size  and less than 200 nanometers in diameter and are known for their longevity and stability, low buoyancy, negative surface charge, high surface area per volume, high internal pressure, and high gas transfer rates.

Nanobubbles can be formed by injecting any gas into a liquid. Because of their unique properties, they can interact with and affect physical, chemical, and biological processes. They have been used in technology applications for industries such as wastewater, environmental engineering, agriculture, aquaculture, medicine and biomedicine, and others.

Background 
Nanobubbles are nanoscopic and generally too small to be observed using the naked eye or a standard microscope, but can be observed using backscattering of light using tools such as green laser pointers. Stable nanobubbles in bulk about 30-400 nanometers in diameter were first reported in the British scientific journal Nature in 1982. Scientists found them in deep water breaks using sonar observation.

In 1994, a study by Phil Attard, John L. Parker, and Per M. Claesson further theorized about the existence of nano-sized bubbles, proposing that stable nanobubbles can form on the surface of both hydrophilic and hydrophobic surfaces depending on factors such as the level of saturation and surface tension.

Nanobubbles can be generated using techniques such as solvent exchange, electrochemical reactions, and immersing a hydrophobic substrate into water while increasing or decreasing the water’s temperature.

Nanobubbles and nanoparticles are often found together in certain circumstances, but they differ in that nanoparticles have different properties such as density and resonance frequency.

The study of nanobubbles faces challenges in understanding their stability and the mechanisms behind their formation and dissolution.

Properties 
Nanobubbles possess several distinctive properties:

 Stability: Nanobubbles are more stable than larger bubbles due to factors such as surface charge and contaminants that reduce interfacial tension, allowing them to remain in liquids for extended periods.
 High Internal Pressure: The small size of nanobubbles leads to high internal pressure, which influences their behavior and interactions with the surrounding liquid.
 Large Surface-to-Volume Ratio: This property is crucial for efficient gas transfer between the nanobubbles and the liquid, which is beneficial for various applications.

Usage 
In aquaculture, nanobubbles have been used to improve fish health and growth rates and to enhance oxidation. Nanobubbles can improve health outcomes for fish by increasing the dissolved oxygen concentration of water, reducing the concentration of bacteria and viruses in water, and triggering the nonspecific defense system of species such as the Nile tilapia, improving survivability during bacterial infections. The use of nanobubbles to increase dissolved oxygen levels can also promote plant growth and reduce the need for chemicals. Nanobubbles have also been shown as effective in increasing the metabolism of living organisms including plants. In regards to oxidation, nanobubbles are known for generating reactive oxygen species, giving them oxidative properties exceeding hydrogen peroxide. Researchers have also proposed nanobubbles as a low-chemical alternative to chemical-based oxidants such as chlorine and ozone.

References 

Fluid mechanics
Bubbles (physics)