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.