ScienceWise - Autumn 2010

Nanobubbles:

Article Illustration
Dr Guangming Liu demonstrates the nanobubble cell in his lab
Article Illustration
Electrolytically formed nanobubbles on a conductive surface lift contaminants such as adhered proteins.

The Last Word in Cleaning?

When we wash our dishes, we’re removing protein, fats and all manner of other contaminants from a ceramic surface. If you were to simply run cold water over the plates it’s unlikely that the results would be very satisfactory. However a combination of what’s known as a surfactant (soap),  warm water and a bit of scrubbing with a sponge achieves what most householders would call a nice clean plate. But just how clean is such a surface at the molecular level? Probably not very! There are many proteins that bond extremely well to surfaces leaving a layer perhaps only a molecule or two thick - fine for hygienic dining, but not good enough for some scientific and industrial applications.

Many industrial processes require components to be so clean that their surfaces have no trace of contaminants. To make matters worse some components such as the metallic parts of silicon chips or detectors can’t be scrubbed or abraded in any way. This can necessitate the use of multistage cleaning processes that can be messy and expensive. However a group of scientists at the Australian National University have recently discovered a simple technique that may have widespread potential to revolutionise a whole range of industrial and domestic cleaning applications. The secret is nanobubbles.

When an electric current is passed through water, electrolytic decomposition of the water molecules results in the production of oxygen and hydrogen gas. The process occurs rapidly in salt water but very much more slowly in pure water because of its vastly lower conductivity. This in turn is due to the fact that water has a very limited ability to self ionise, in other words for two H2O molecules to become hydronium H3O+ and hydroxide OH-.

The upshot of all of this is that when a small voltage is applied between a conductive component and a second electrode in a water bath, bubbles of gas begin to form. If the component is used as the cathode (negative electrode) hydrogen gas forms on its surface.  But if the water is pure, the conductivity is very low and the quantity of gas is miniscule. This means the bubbles that form are only a few nanometres across – way too small to see even with a microscope.

However, although the bubbles may be invisible, their effects are not. The gas forms directly on the surface, beneath the contaminants. As the bubble grows it lifts the surface film and carries it off into the water when the bubble leaves the surface. If the process is repeated a few times it is possible to clean a component in a matter of seconds without the use of chemicals or abrasion.

Nanobubbles are a relatively recent discovery; scientists hadn’t looked for them because in theory, they shouldn’t exist. When a bubble of gas in a liquid is very tiny, the quantity of gas it contains is miniscule and it’s surface area is large. Its Laplace pressure (the pressure difference between inside and outside the bubble) is also very high. All this should lead to such tiny bubbles simply dissolving into the surrounding liquid. The researchers believe that reason this doesn’t happen when the bubble is on a surface is that the contact angle of the bubble to the surface is very much lower than would be expected.

Although nanobubbles may have many future applications in cleaning, the potential of this technology isn’t limited to just removing contaminants. It may have a role in preventing the adsorption of materials onto conductive surfaces in the first place. One potentially exciting use of such a system is marine anti-fouling.

When ships are new their hulls are clean and smooth and they slide through the water easily and quickly. However, as they age, marine organisms such as barnacles begin to grow below the waterline creating a rough surface that induces turbulent flow and friction. To counter this, shipbuilders have used a number of techniques. One of the earliest was to attach copper sheets to the bottom of ships, leading to the common phrase “copper bottomed” – meaning solid and trustworthy. The use of copper bottomed ships with their higher speed was one of the deciding factors in the supremacy of the British navy in the eighteenth century.

Modern ships are coated with toxic anti-fouling paint rather than copper sheets, but this is expensive and can lead to environmental problems when the paint needs to be removed and replaced. If one day it were possible to incorporate nanobubble technology into the design of ships, they may be able to clean themselves in the water without the need for toxic coatings.

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