Drop of water just before it hits the sheet of plastic.
Test with the Arduino Duemilanove microcontroller to control delay.
Now it's much easier to control the delay as i can program it from my PC.
here is a quick picture of the setup : www.flickr.com/photos/34463171@N04/3601835144/
Now i'm waiting for a solenoid valve for the setup so i can control the drops too and start it all with a push of a button.