I proposed this unrequited idea to a former Research Lab employer for providing supersonic water droplets to emulate kinetic dew droplets. A number of other rigorous techniques have been implemented to perform this task. The idea presented herein may have provided an easier and more economical facility. The idea is velocity amplification based on conservation of momentum similar to that which occurs when 'cracking a whip'.
A whip is a long slender flexible transmission line that tapers in diameter from the handle to the lash. A momentum wave is launched from the handle which propagates along the braided tapered body toward the lash. Momentum in the braided body must be conserved since very little momentum can escape to the ambient air environment. Since the braided body is tapered, the wave is constrained to propagated by lower and lower mass as it approaches the lash; conservation of momentum therefore transforms the wave into higher and higher velocity. The lash goes supersonic as the momentum wave exits the whip 'lash'. The whip body and lash basically perform a mechanical impedance transformation of the momentum wave launched at the handle.
The water (dew) drop launcher employs a similar impedance transformation scheme. The impedance transformer takes the form of a gradually (conical) tapered rigid pressure vessel likely mounted vertically in a reduced pressure test cabin. The lower large end of the launcher is fitted with a piezo quartz crystal that can launch a fraction of a mm thick plane wave into the conically tapered wave guide. The momentum wave travels at the speed of sound in water at about 4500 Ft/Sec. Conservation of momentum will cause the wave to accelerate to high velocity as the wave diameter is reduced in the conical wave guide. The launched water droplet is still subsonic relative to sound speed in water which reduces concern of shock waves in the wave guide. The area ratio of the piezo to the launch nozzle can be varied to achieve the desired droplet launch velocity.
The piezo pulse width is made short enough to assure the momentum wave thickness is small compared to the taper rate of the wave guide which will help prevent edge distortion of the wave at the wave guide perimeter. The entire apparatus could be operated in a reduced pressure somewhat above the vapor pressure of the launch water. A fairly high rate of fire like 100 droplets per second or much more should be achievable with settling time between shots.
A patent disclosure for the device was submitted some many years ago.