We can make water droplets appear to fall in slow motion by pumping them out of a tube using a pump that pushes the droplets out
at regular intervals. Many pumps operate at 60 Hz because AC power in the United States oscillates at 60 Hz. If we force droplets
out of a tube at 60 Hz and illuminate the droplets with a 60 Hz strobe light, the droplets will appear to be essentially motionless.
If you want to make one of these devices, here's what you need.
A reciprocating pump. I used a Gorman-Rupp 14825-690 oscillating pump I picked up on eBay.
Search eBay for "Gorman Rupp oscillating pump" and you should be able to find one that works well.
Make sure it runs off the line voltage in your country. (115 volts, 60 Hz in the USA) You may also
want to get a variac transformer which can adjust the output of the pump. You can often find these
on eBay for $10 - $20.
Tubing. Once you have your pump, pick up a couple meters of clear tubing at your local hardware store.
The inner diameter of the tubing should match your pump.
Fluorescein dye. This is a very, very intense green dye. You can get this from many different
science supply stores, such as ScienceStuff.com.
Please note that a fraction of a gram will turn several gallons of water bright green.
Bucket and strainer. The bucket is used to catch the falling water. The strainer acts as a splash guard
to prevent the fluorescein drops from splattering everywhere and making a big mess. A tube goes from one end
of the pump into the bucket. Tie a weight around this end of the tube so it stays under water.
Water is then pumped out of the bucket, and then dripped back down into the
strainer placed over the bucket.
Strobe light. You need a strobe light that can flash 60 times per second. You can either try to buy an
industrial stobe light (which can be expensive) or you could make your own stobe light using LEDs.
I've used this 555 timer circuit to make an LED strobe. The printed
circuit board is available from www.futurlec.com
for a very reasonable price. You should use a multi-turn 100k potentiometer as you'll want to adjust the
flash rate with high precision. You can get multi-turn pots from
www.circuitspecialists.com although you
can also check out
www.goldmine-elec.com for alternatives. Note that the only thing that really matters for the timing is the
product of the resistance (R1, R2, and the 100k pot) and C1, so if you can only find a 50k multi-turn pot, then
double the value of C1 and you should be OK.
If you would like more step-by-step instructions on how to make an LED strobe, then check out the tutorial on
If you freeze-frame the movie, you can measure the spacing between the water drops and measure the acceleration of
gravity. Each droplet is separated by 1/60th of a second, so the additional distance each droplet moves is due to the
speed that the droplet gains in a 60th of a second. This is approximately
(9.8 m/s^2)*(1s/60)^2 = 0.27 cm
so each pair of droplets will be separated by an additional 1/4 centimeter compared to the pair above it.
It is possible to custom-tune the frequency of a low voltage pump to match the frame rate of either a strobe
or a high-speed digital camera. I've done this using a Gorman-Rupp model 14825-710 pump that normally runs at
19 volts AC but also runs fine at pulsed 24 volts DC when connected to
this circuit based on a 555 timer chip.