User:Nanoboffin/sandbox

A nanobubble is a bubble of diameter <1μm. Nanobubbles are small, negatively charged, neutrally buoyant, stable bubbles which have a high internal pressure. When they burst they emit an ultrasound wave and heat of up to 40,000K for a very brief period of time. Nanobubble properties make them well suited to a number of applications where they can have a large impact, including wastewater treatment, agriculture, aquaculture and certain areas of medicine to name a few. Bubbles which are greater than 1μm but smaller than 1mm are termed microbubbles, and a solution in which both micro- and nanobubbles are present is termed a micronanobubble solution.

Nanobubble Physics
Nanobubble diameters D are calculated as : D= (3σεδ_l^4 k^4)/(2πe^2 ) Where σ is surface tension, ε is the permittivity of the media, δ_l is the radius of a molecule of the surrounding liquid (approximated as a sphere), e is the fundamental unit of charge and kδ_l are the allowed distances between adsorbed ions (i.e. between the centres of adjacent Wigner-Seitz cells) for the following such values of k: k = 2√3, 4, 6, 4√3, 8, 10, 6√3, 12, 8√3, 14, 16, 10√3, 18, 20, … Owing to the fact that nanobubbles are in constant Brownian motion their particle size distribution (PSD) can be measured using dynamic light scattering (DLS) techniques. More recently a technique called Nanoparticle Tracking Analysis (NTA) has also become a popular method for measuring nanobubble PSD as well as their concentration.

Over time the diameter of a nanobubble will shrink due to the surface tension which acts to minimize the area of its surface. This takes place by diffusion of the gas contained within the bubble into the surrounding liquid.

Nanobubble Properties
Nanobubbles have five main useful properties. These are:

- A negative electric surface charge density

- Longevity – they are physically stable and persist for a long time

- Very low (almost neutral) buoyancy

- Multipotency – they may constitute any gas

- Sonoluminescence upon collapse

Negative Charge
It is currently held that the property of negative surface charge density comes about due to ions, derived from inorganic salts dissolved in the surrounding media, which adsorb onto the gas-liquid interface and stabilize against the collapsing effect of surface tension. If the bubbles are placed in a uniform electric field it is straightforward to show that their charge density ρ is related to their diameter D_b as D_b∝ρ^(-1⁄3) and thus is greater at smaller diameters. The ability to manoeuvre the bubbles using their charge is advantageous for example in the application of nanobubbles to drug delivery systems, where IV drugs associated with nanobubbles may be guided to their target location.

Longevity and low buoyancy
This property of longevity is derivative of the fact that the surface of a nanobubble is negatively charged and thus stabilizes against the collapsing effect of surface tension. Their low buoyancy means they do not float quickly to the surface of the surrounding medium. In fact the velocity that nanobubbles rise through the solution has been related as proportional to their radius squared such that a 100 nm bubble is proposed to take at least two weeks to rise 1 cm whereas a 10 µm microbubble would only take 2-3 minutes to rise that far. This means that they can be stored in solution for long periods of time without deteriorating. In 2010 research at Osaka University led to the creation of a high concentration of nanobubbles lasting for up to two weeks. This makes them able to oxygenate water more efficiently than conventional methods since they do not quickly rise to the surface and escape to the atmosphere, but remain in solution for long periods of time constantly supplying oxygen by diffusion into the surrounding water.

Multipotency
An even broader range of applications is made possible by varying the choice of gas which is not limited to air/oxygen. For example ozone, due to its very strong oxidizing effect, is made very effective in sterilization applications (enhanced further by the ability of nanobubbles to infiltrate very small spaces and their sonoluminescence), carbon dioxide nanobubbles can increase the rate of plant growth or be used effectively to extinguish fires, and hydrogen nanobubbles are proposed to have an anti-tumour effect.

Sonoluminescence
Though nanobubbles are stable they may collapse upon collision. When they do they exhibit a remarkable phenomenon called sonoluminescence – the radiation of light and the emission of a powerful shockwave. The radiation spectrum depends strongly on the type of gas contained within the bubble, but in most cases the inferred temperature in the centre of the bubble at the point of collapse is of the order ~(10)^4 K. Combining this high temperature with the powerful shockwave and potentially oxidizing properties of the contained gas makes nanobubbles a very effective method for cleaning/sterilization purposes.

Applications
Nanobubbles (and microbubbles) have many current applications, but there are many more which have not been thoroughly explored to date.

Industrial Cleaning
The powerful cleaning properties of nanobubbles, particularly when made with ozone or oxygen makes them an obvious natural choice vs some of the harsh chemicals used for cleaning. Nanobubbles are used for cleaning wastewater from a variety of industries, cleaning factory equipment, and for precision cleaning of semiconductors to name a few examples.

Aquaculture
Aquaculture is increasingly being relied upon to fill the global demand for seafood. Most types of aquaculture involve a limited volume of water in which the maximum number of organisms are reared. The unnatural close quarters cause a number of problems such as increased incidence of pests and disease, but also a depletion of oxygen in the water. Oxygen or even air nanobubble water has a large surface area for dissolving oxygen into the water, and can alleviate bacterial and viral disease problems, and increase the dissolved oxygen level of the water, leading to healthier fish and more productive farms.

Medical
Nanobubbles are being used in medical research, particularly in the area of cancer theranostics. Plasmonic nanobubbles which are not themselves bubbles, but a transient nanoexplosion that occurs when a gold nanoparticle is excited with a laser. These allow for much more targeted methods for killing cancerous cells whilst not harming healthy cells in the body. A different cancer theranostics approach uses pluronic nanobubbles as an ultrasound contrast agent to also more accurately target the cancerous cells.

Agriculture
Agricultural applications for nanobubbles are most prominent in plants, which have been shown to grow ~30% more mass when watered with oxygenated nanobubble water over regular water without nanobubbles. This has large consequences both for normal land-based farming, but even more so for the hydroponic systems for vegetable growing which are becoming more prevalent worldwide as demand for food increases. Another agricultural application for nanobubbles is for livestock. Oxygenated nanobubble water has been shown to improve the health of livestock and greatly reduce the need for pesticides and antibiotics in their rearing. A study has shown that oxygenated nanobubble water can also promote growth in both fish and mice, so potentially we can see similar effects in other livestock.

Home
Nanobubble cleaning properties have applications in the home too. Some cosmetics companies have begun putting nanobubbles into their products to enhance the cleaning action, whilst you can buy home spa units which create nano- and microbubble baths which aid in hydration and exfoliation of the skin. Nanobubbles are small enough to penetrate skin pores, so they can really get deep into the skin and remove dirt/kill bacteria. The ultrasound waves from the nanoexplosion of the bubble are also thought to have some therapeutic properties.

Technology History
Nanobubble technology has existed for over 50 years in Asia, particularly Japan. Their prominence has risen globally in the past decade as more scientific research has begun to use them. The Fine Bubble Industries Association is a Japanese collective that is now attempting to set standards for the nano- and microbubble industry, founded in July 2012.

Category: Nanotechnology Category: Environment Category: Water