I tested it yesterday with complete setup including board, flash and camera. My observation was if delay between 2 drops is less, both drops used to mix up in single one. Also many times even while firing single drop, i used to get 2 drops with 20ms size.
I suspect on following parameters as well
– water bottle was 40-50cm higher than solenoid valve. I think I also had air leaks in nozzles. (today I will test with bottle and valve at same height with all air leaks sealed)
– maybe I am using very big / very small nozzle size, today i will try standard 6mm nozzle.
Do you suspect anything else?