Solenoid valves are used to control the flow of liquids and gasses and they are an ideal way to create water drops. To get started I purchased a small range of different solenoids with the intention of testing them out and seeing which was the best.
The first one on the left is a Chinese copy of the Shako PU220AR. The Shako PU220AR is recommend by several people online. See below.
You can see more information about the Shako PU220AR (and also water drop photography) on Kevin Lewis’s site and also on Wayne Fulton’s site. Wayne Fulton has a very good article on solenoid valve precision tests.
The other solenoids are supposedly fast solenoids but I have not tried them yet. All were purchased from Taobao (Taobao is like a Chinese version of ebay but much cheaper). I went for these models because they already had nozzles.
To create a water drop you need to open and close the solenoid valve very quickly. In the range of mill-seconds. Using the Arduino to control the solenoid valve it is simple a case of setting a pin high for the appropriate amount of time. There is, however, a caveat, the solenoid works at a different voltage to the Arduino and you cannot directly connect the two. In this case a TIP120 transistor is used as a bridge.
The TIP120 allows a small voltage (from the Arduino) to switch a larger voltage (12V to the solenoid). It can be thought of as a switch, applying a current to B allows current to flow between C to E.
The two middle solenoids are not suitable for water drops. The nozzles are too small and not enough water comes out.
The ones at the ends are good though.
For everything solenoid related visit http://www.solenoid-valve-info.com/