There are many different makes and models of suitable valves you can use to create the water drops. There are a few preferred preferred models though, namely the Shako PU220AR, the AirTac 2V025-08 and the Chuka 2V025-08.
I have a few different types; 24V Shako PU220AR, 12V AirTac 2V025-08 and some that appear to be 12V Chinese Shako copies. I also tried some cheap all-in-one valves but found they were too slow for water drop photography.
I find the AirTac 2V025-08 and the Shako to have very similar performance but they generate a lot more crud than the brass valves. See the photo below. The Airtacs are much cheaper than the Shako valves.
No brand, made in China valve. This was sold as a Shako clone and is currently my preferred valve. I can get fairly small drops and the drops are very reliable.
These are now available under the SLGPC brand. The price seems to be the same.
To separate the plastic part from the metal part, undo the large screw and pull away. Inside the plastic part you will find two connectors, one is positive and the other is negative.
Some valves have the positive and negative terminals marked (+ and -), others do not. The ones that do not have labels tend to have LEDs and all the valves that I have, that have an LED, the positive wire goes to the resistor.
If you get the wires the wrong way a round, the valve will still work but the LED will not light.
Cleaning the valves
The below video shows how to dismantle and clean the Shako valve
and this one shows a neglected Airtac 2V025-08 (not as neglected as the one in the photo below the video though).
When I started I created the leads from twin core wire and phono/RCA sockets. This is great when you need extra long leads. Now, I buy cheap audio cables and cut one of the ends off.
I have tried brass nozzles and plastic nozzles. I find the brass ones give better results with more consistent drops. The plastic nozzles tend to be unreliable, sometimes the drops are good, sometimes they are not. I feel that plastic is too “slippy” and the water drips from the nozzle when it is not active.
The smallest brass nozzles I have found are 6mm. These gives good results but can sometimes be too large. To make the orifice smaller I use a small piece of tube and a short length of drinking straw. Inside the straw there is another small piece of PVC tube. I am still experimenting with different tubes and pipes.
Make sure you get an air tight seal when attaching the nozzles to the valves. If air can get in water will slowly drip out. To make a seal you should use plumbers tape. This is a thin non adhesive tape (the white tape in the photo below). In a pinch you can use electrical tape (the black tape in the photo below) but this can leave an adhesive residue which is difficult to remove. I cut the electrical tape in to thin pieces before using it.