This is an exhibit that demonstrates two different physical effects: transverse wave motion in water jet, and the coalescence of water drops in mid-air. The two effects are viewed with the aid of stroboscopy and work one at a time, with the particular effect being chosen by the viewer with a
The unit contains a self-circulating water supply using a pump and holding tank, and a solenoid
to direct the water to one or other of the demonstrations.
Transverse Wave Motion
A laboratory vibrator or loudspeaker is required to produce a vertical oscillation. A small metal post with a hold drilled through it
is mounted on the oscillating part of the vibrator, and a piece of heat-shrink sleeving or fine rubber
tubing around 10cm long is placed through the hole. When the vibrator is fed from an adjustable
signal generator, standing modes are set up in the tubing. When water is passed through the tubing
its density changes and the mode patterns alter, making it possible to generate some rather
interesting mode structures with the signal generator being adjusted from around 5 to 40Hz.
A stobe light is used to illuminate the apparatus with its own flash rate adjustable over roughly
the same range. By adjusting the two frequencies, it is possible to freeze the standing wave
patterns, or cause them to appear to move backwards or forwards at different speeds. By tuning the
vibrator through resonances in the tubing, or even the vibrator's own internal structure, the precise shape of the wave
can be adjusted from a perfect sinusoidal variation, to very complex shapes.
The water is passed through a Y-junction to two fine nozzles with aperture around 1mm diameter.
On its way to the Y-junction the water passes through a piece of fine rubber tubing which is
pinched by a solenoid (such as the hammer of a ticker timer) at an adjustable rate.
The result is to produce two streams of tiny water drops from the two nozzles. The nozzles are
mounted on adjustable joints and the jets are aimed toward each other. By adjusting the relative
phasing of the nozzles, the drops can be made to pass through one another. When this is
illuminated with a strobe of suitable flash rate, the result is to freeze the action of the drops coalescing, which can
then be studied and watched. The resulting exhibit is fascinating to view.