Thanks for the help Mathieu!
The whole idea is to take pictures of water drops.
I plan to use:
- a solenoid for controlling flow of water drops eletronically
- a photogate laser trigger (already have this working)
- a loudspeaker and amplifier (my hometheater subwoofer, I need a simpler speaker
) - camera and high speed flashes (have that too)
Steps:
- Arduino activates camera on long exposure (3 seconds), that should be easy, I plan to use a 4N28 optocoupler
- Arduino activates solenoid (another optocoupler), that produces a water drop
- The falling drop crosses a laser beam (it turns on the SCR from my original question here)
- Arduino detects the SCR close, then wait X miliseconds
- Arduino activates the loudspeaker... it has another water drop on top of it
- Arduino waits Y miliseconds
- Arduino activates a wireless flash trigger, that activates multiple flashes (another optocoupler)
By adjusting X and Y miliseconds, I plan to capture the exact moment the 2 waterdrops splash on each other.
I was able to make it work separate... now I want it togheter... Here's some results:
Using a sound trigger, activated by loudspeaker, with a delay to wait the waterdrop formation:
A waterdrop on a US dime 0.10 cent coin, falling from 2 feet, detected by laser:
I did both photos 
The only thing missing for my whole setup is connecting everything togheter on Arduino to get miliseconds precision, I'm tired of adjusting delays on potentiometers... and the solenoid I just started researching.