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The Amazing Bubble

The Amazing Bubble

By Franz Grieser

Bubbles may seem fleeting and fragile but scientists are getting closer to finding the right conditions to turn them into tiny fusion reactors and to recreate the genesis of life itself.

The ephemeral nature of bubbles is perhaps the reason they catch our eye, be they simple and often beautifully iridescent soap bubbles or streams of bubbles rising rapidly in a glass of carbonated drink.

One remarkable feature of bubbles has captured the attention of physicists and chemists for some time. Under the right conditions they can be converted into transient spherical “mini-pistons: that, when they collapse, produce localised hot spots with core temperatures in excess of 5000°C and pressures of several hundred atmospheres. In the specific case of bubbles in water, the hot spots produced are above water’s super-critical point – where liquid and vapour are indistinguishable and the fluid is more akin to an oil rather than being water-like.

To turn a bubble into one of these thermal reactors requires the interaction of ultrsound with a micrometre-sized bubble in a liquid. Sound is basically a periodic undulation of high and low pressure, and can make a bubble oscillate, expand rapidly and then implode in a fraction of a microsecond. Because the implosion is so rapid, the gases and the vapour of the liquid are trapped inside the bubble, and the mechanical energy of the collapsing bubble goes into heating the bubble’s contents (Fig. 1).

Chemists have made use of these cavitation hot spots in two quite distinct ways. The first involves taking advantage of the extreme temperatures created in the core of collapsing bubbles.

A number of volatile organic molecules can enter a cavitation bubble during its expansion phase. Once trapped, the bubble’s collapse provides the thermal conditions needed to initiate pyrolysis reactions that effectively destroy the original molecules. For example, simple organic molecules like ethanol and acetic acid can be completely converted into carbon dioxide and water.

The second way the hot spots have been used is by exploiting free radicals that are created within the bubble. In the case of acoustic bubbles in water, the primary radicals generated are from the dissociation of water vapour into hydrogen and hydroxyl radicals.

Due to the confined conditions in the core of a bubble, most of these radicals recombine to give water, hydrogen gas and hydrogen peroxide. Some, however, escape into the water surrounding the bubble where they can react with whatever solute is present, all at room temperature and pressure. Here a host of reactions are possible.

These free radicals are the basis of many chemical and oxidation reactions that are suitable for degrading a variety of common environmental pollutants, including textile dyes, water-borne pharmaceuticals and pesticides. Unfortunately, the sono­chemical degradation of pollutants in contaminated wastewater streams and natural waterways is costly, and engineering processes are needed to concentrate contaminants before sonochemistry is applied. Nevertheless, several pilot plants have proven the feasibility for sonochemical decontamination of organic pollutants on a large scale.

Another remarkable consequence of a bubble’s collapse is the emission of a burst of light at the very last stages of compression of the bubble’s contents. The light has a broad wavelength that ranges from the ultraviolet to well into the infrared, and lasts less than a nanosecond.

There has been considerable debate about what causes this “sonoluminescence”. While the best explanation is that it comes from a process called bremsstrahlung (i.e. energy is released as light when electrons are decelerated by electrostatic interactions with other ions or molecules), there may be some conditions where the light is equivalent to black-body radiation (i.e. the light produced from the thermal excitation of molecules in the bubble’s core is not only emitted but some is readsorbed and then emitted at lower energies). If the light is indeed being produced by ion interactions, this implies that a plasma is produced in the core. This has some very interesting possibilities.

Physicists have long thought that under the right conditions it might be possible to fuse deuterium atoms caught within a collapsing bubble. Indeed a United States patent filed in 1982 proposed that a fusion reactor could be built if a bubble was created in a molten metal doped with some deuterium gas. The metals suggested for the reactor are at best difficult to deal with (e.g. molten lithium, sodium) so the search for a user-friendly reactor has been ongoing.

Then in March 2002, US scientists from the Oak Ridge National Laboratory claimed in the journal Science that they had detected 2.5 MeV neutrons. The production of tritium, most likely as a by-product of the fusion of two deuterium nuclei, seemed to confirm that the right conditions had indeed been found with an easy to handle liquid: deuterated acetone. Unfortunately, a number of attempts by other researchers failed to reproduce the original claim, and after a close inspection of the original experiment the data has now been declared to have been be falsified.

However, the search for the right conditions has not stopped, and very recent and reliable work has provided experimental evidence that core temperatures in cavitation bubbles in concentrated sulfuric acid appear to be in excess of 15,000°C. This is still well below the temperatures needed for the fusion of deuterium, but the results show that the core material is a partial plasma – just what is needed if fusion in this type of system is to be realised.

Another type of reaction that can occur inside a bubble is the formation of relatively complex molecules from smaller, and hence simpler, molecules. Our research has briefly looked at some of the products formed by applying ultrasound to water in three separate systems:

  1. acetic acid, nitrogen and hydrogen;
  2. methane, nitrogen and hydrogen; and
  3. carbon dioxide, nitrogen and hydrogen.

After an hour of sonication in the first two systems we detected two amino acids – alanine and glycine. This means that both the nitrogen and hydrogen molecules have had their bonds broken, and the radicals formed from this have subsequently reacted to make the larger amino acid products.

In the third system we also detected the two amino acids but at a lower relative yield. This system is not dissimilar to the Earth’s atmosphere and oceans before life began 3.5 billion years ago. This begs the question: could bubbles have been responsible for creating the building blocks of life?

The conditions that create cavitation bubbles are not restricted to the action of ultrasound on bubbles. For example, forcing water through a small opening will develop a pressure drop in the water, and this is sufficient to produce cavitation bubbles that generate chemical reactions. Likewise waves crashing on the crevices of rocky shores can generate cavitation bubbles.

So the conditions were certainly around in prebiotic times to support the idea that bubbles could have been responsible for creating complex molecules in an ongoing way, providing the ideal bioreactor for the chemistry for life.

These examples illustrates that the simple bubble is actually a remarkable little engine that is capable of generating quite extra­ordinary phenomena. There can be little doubt there is still much to be gained from harnessing the power of the cavitation bubble.

Franz Grieser is a Professor of Chemistry and ARC Professorial Fellow in the Particulate Fluids Processing Centre at the University of Melbourne’ s School of Chemistry.