Another important part of water drop photography is making sure that the water flows through the solenoid at a constant, the beauty of using an Arduino to control the setup is that everything is repeatable but this can only be achieved if the water pressure stays constant!!
A mariotte Siphon can be used for this and will ensure that the water flows at the same rate regardless of how much fluid is in the vessel. I decided to use a 50ml syringe to hold the liquid, it might not sound like a lot but you wont need a 500ml drink bottle when all you are doing is releasing a drop of water at a time!
There is plenty of information on the internet about “Mariotte Siphons”, I found these websites useful and it should give you an idea of how to construct something