  The breakup pathway of Rayleigh fission of a charged drop is unequivocally
demonstrated by first of its kind, continuous, high-speed imaging of a drop
levitated in an AC quadrupole trap. The experimental observations consistently
exhibited asymmetric, sub-critical Rayleigh breakup with an upward (i.e.
opposite to the direction of gravity) ejection of a jet from the levitated
drop. These experiments supported by numerical calculations show that the
gravity induced downward shift of the equilibrium position of the drop in the
trap cause significant, large amplitude shape oscillations superimposed over
the center-of-mass oscillations. The shape oscillations result in sufficient
deformations to act as triggers for the onset of instability well below the
Rayleigh limit (a subcritical instability). At the same time, the
center-of-mass oscillations which are out of phase with the applied voltage,
lead to an asymmetric breakup such that the Rayleigh fission occurs upwards via
the ejection of a jet at the pole of the deformed drop. As an important
application, it follows from corollarial reasoning that the nanodrop generation
in electrospray devices will occur, more as a rule rather than as an exception,
via asymmetric, subcritical Rayleigh fission events of micro drops due to
inherent directionality provided by the external electric fields.
