Despite initial scepticism, acceptance of the existence and special properties of nanobubbles is now growing and their formation and characteristics are now becoming the subject of an increasing amount of study, especially in Japan.
Due to the supposed very high pressure within bubbles of such small size and radius of curvature, and thus high surface tension, conventional calculations show that the gas should be ‘pressed out’ of the nanobubbles within microseconds. However, it is now clear that under the right conditions such bubbles can both form freely and remain stable for extended periods of time, sometimes for many months. Explanations as to just why such structures are so stable are focussing on the role of counter-ions forming layers at the nanobubbles surface, explaining claims that they apparently form only in the presence of salts.
Kaneo Chiba and Masayoshi Takahashi of Japan’s famous AIST research centre have shown that in the presence of electrolytes and with the correct physical stimulus, stable nanobubbles can be formed from conventional microbubbles. The latter tend to coalesce to large buoyant bubbles which either float away or collapse under intense surface tension-derived pressure to the point that they vanish, as predicted by theory. However, the addition of salt (electrolytes) is thought to cause the formation of a counter-ion screen around the nanobubbles which effectively blocks the ability of gases within them to diffuse out.
Furthermore, Prof. William Drucker of the University of Melbourne has also used infra-red spectroscopy to show that the pressure of gas within such nanobubbles is not significantly higher than atmospheric pressure, perhaps explaining their stability and resistance to collapse.
Figure 1: The formation of nanobubbles.
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