Spontaneous Fission of Charged Water Nanodrops: Unveiling the Stochastic Nature of Fission Pathways and Dynamics

Fission of highly charged micrometer-sized and larger droplets has been investigated using optical methods, but until recently, direct measurements of spontaneous fission of submicrometer droplets have not been possible. Charge detection mass spectrometry is used to track the mass, charge, and energy per charge of aqueous nanodrops that undergo evaporative water loss while they are trapped for up to 4 s. 154 of the 846 trapped nanodrops (18.2%) with charges ranging from 44 to 158% of the Rayleigh limit underwent fission. Although these spontaneous fission processes are highly heterogeneous, four distinct fission pathways that occur over times ranging from a few ms to 100s of ms with ejection of just a few to hundreds of progeny droplets were identified. One is a "continuous" pathway in which many small progeny droplets with progressively less charge are sequentially emitted over the course of ∼25 to 150 ms. Prompt and sequential prompt pathways in which one or a limited number of progeny droplets carry away a significant fraction of the precursor charge are the most common. "Prefission" events in which emission of just a few charges prior to a larger prompt fission event occur for some nanodrops charged above the Rayleigh limit, and these events appear to have similarities to "foreshocks" that often occur shortly prior to major earthquakes.