Alkali metals can react explosively with water and it is textbook knowledge that this vigorous behaviour results from heat release, steam formation and ignition of the hydrogen gas that is produced. Here we suggest that the initial process enabling the alkali metal explosion in water is, however, of a completely different nature. High-speed camera imaging of liquid drops of a sodium/potassium alloy in water reveals submillisecond formation of metal spikes that protrude from the surface of the drop. Molecular dynamics simulations demonstrate that on immersion in water there is an almost immediate release of electrons from the metal surface. The system thus quickly reaches the Rayleigh instability limit, which leads to a ‘coulomb explosion’ of the alkali metal drop. Consequently, a new metal surface in contact with water is formed, which explains why the reaction does not become self-quenched by its products, but can rather lead to explosive behaviour.
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We thank J. Jiráček for boldly making his chemical laboratory available to us for the initial experiments in liquid ammonia. P.J. acknowledges the Czech Science Foundation (Grant P208/12/G016) for support and thanks the Academy of Sciences for the Praemium Academiae award. S.B. acknowledges support from the Deutsche Forschungsgemeinschaft (Grants BA 2176/3–2 and BA 2176/4–1). P.E.M. acknowledges support from the viewers of his YouTube popular science channel.
The authors declare no competing financial interests.
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Mason, P., Uhlig, F., Vaněk, V. et al. Coulomb explosion during the early stages of the reaction of alkali metals with water. Nature Chem 7, 250–254 (2015). https://doi.org/10.1038/nchem.2161
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