Coulomb explosion during the early stages of the reaction of alkali metals with water

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. The explosion of alkali metals in water is a typical high-school chemistry experiment, but its mechanism is not fully understood. Using high-speed cameras and molecular simulations it is now shown that a key early step in this reaction is the migration of electrons from the alkali metal into water, leading to a charging of the metal's surface and subsequent Coulomb explosion.