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Spontaneous chemical reactions between hydrogen and oxygen in nanobubbles

Bulk nanobubbles (NBs) generated electrochemically by short voltage pulses of alternating polarity behave differently from those produced by regular methods. Only bubbles smaller than $200\;$nm are formed in the process and their concentration is very high. Moreover, the bubbles containing both H2 and O2 gases disappear quickly via the combustion reaction, although the reaction in such a small volume cannot happen according to the classical combustion theory. Experimental facts about these unusual NBs are reviewed and current understanding of the observed phenomena is provided. Visualisation methods of a cloud of NBs above the electrodes are briefly discussed. Experimental signatures demonstrating the reaction between the gases in NBs are considered. A surface-assisted mechanism proposed for the combustion reactions in restricted volumes with a high surface-to-volume ratio is discussed. It it explained how the same mechanism may describe the audible explosion of microbubbles, that is observed in certain circumstances.