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.^[1][2][3][4] Nanobubbles generally measure between 70-150 nanometers in size ^[5][6] and less than 200 nanometers in diameter^[7][8] 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.^{[4][9][10][11]}

Nanobubbles can be formed by injecting any gas into a liquid.^[12][13] Because of their unique properties, they can interact with and affect physical, chemical, and biological processes.^[14] They have been used in technology applications for industries such as wastewater, environmental engineering, agriculture, aquaculture, medicine and biomedicine, and others.^[9][15][16]

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.^[14] Stable nanobubbles in bulk about 30-400 nanometers in diameter were first reported in the British scientific journal Nature in 1982.^[14] Scientists found them in deep water breaks using sonar observation.^[14]

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.^[17]

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.^[15]

Nanobubbles and nanoparticles are often found together in certain circumstances,^[18] but they differ in that nanoparticles have different properties such as density and resonance frequency.^[19][20]

The study of nanobubbles faces challenges in understanding their stability and the mechanisms behind their formation and dissolution.^[21]

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.^[21][22]
• High Internal Pressure: The small size of nanobubbles leads to high internal pressure, which influences their behavior and interactions with the surrounding liquid.^[21]
• 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.^[21]

Usage

In aquaculture, nanobubbles have been used to improve fish health and growth rates^[23][24][25] and to enhance oxidation.^[26][27][28] Nanobubbles can improve health outcomes for fish by increasing the dissolved oxygen concentration of water,^[23] reducing the concentration of bacteria and viruses in water,^[24] and triggering the nonspecific defense system of species such as the Nile tilapia, improving survivability during bacterial infections.^[29] The use of nanobubbles to increase dissolved oxygen levels can also promote plant growth and reduce the need for chemicals.^[30] Nanobubbles have also been shown as effective in increasing the metabolism of living organisms including plants.^[28] In regards to oxidation, nanobubbles are known for generating reactive oxygen species, giving them oxidative properties exceeding hydrogen peroxide.^[27] Researchers have also proposed nanobubbles as a low-chemical alternative to chemical-based oxidants such as chlorine and ozone.^[28][29]