In firing Ink Jet printheads, we have observed from time to time that it is possible to create a drop much smaller than the nozzle. In normal operation of Xaar Printheads, the drop ejected is first extruded through the nozzle by a positive pressure. This results in a forward moving plug having a certain momentum and of the diameter of the nozzle. At the end of a time period (typically between 2 and 10usec, according to detail design), the pressure is reversed and a neck begins to form at the nozzle. The momentum of the plug carries it forward, extending the neck to eventually become a long ligature, which finally breaks off at the nozzle. Break-off occurs between 10 and 100usec after the pressure reversal, and this time is easily predicted with a slight modification to the Rayleigh break off criterion.
However, under certain circumstances, not really known in the sense of my being able to repeat them, we have seen a quite different behaviour in which a much smaller drop - eg 1/4 dia of the nozzle emerges rather suddenly from the centre of the nozzle and flies off at typically 2x the normal firing velocity. We believe that this is associated with very short pressure pulses - of the order of 1usec - but usually it happens as a result of reflected acoustic waves and not as a result of controlling the applied pressure - hence the difficulty of reproducing the circumstances.
In a paper "Micro-machined Acoustic-Wave Liquid Ejector" Journal of MicroMechanical Systems, Vol 10 No 3, Sept 2001, is described a focussed acoustic wave technique which gives rise to a droplet in a similar fashion from the centre of a free surface. I believe this may be relevant, though there is no actuator element in our system corresponding to the one described.
Our actuator consists of a piezo-electric channel of a defined length which squeezes the ink and creates a nominally plane acoustic wave which impinges on the nozzle. Any focussing which takes place must be due entirely to the shape of the nozzle.
I believe that radial acoustic waves are dispersive, and it might be that we accidentally create a soliton, which propagates radially within the nozzle causing the energetic ejection. Normally such waves would disperse, hence my suggestion that a soliton may be involved.
Really, this exhausts my current knowledge on the subject (and I do not understand solitons!). I can describe our current actuator in more detail, but I feel that it is probably better at this stage to keep the minds open and allow an element of imagination to take charge!