So, the set up is as above.
The camera is manually focussed at the point at which the droplets enter the water, or just above.
The whole process is done in a near-black room, and the flash is set on it's lowest setting, 1/16th power in this case. The shutter speed, is therefore, irrelevant, as the only image captured is that during the duration of trhe flash, which is about 1/10,000th of a second.
I tend to use F16 for a good depth of focus, and ISO 100 to improve quality when cropping down. The lens is a standard 18-55mm kit lens.
The flow is turned on from the reservoir, at a rate about just SLOWER than that at which it is a constant stream (as apposed to droplets) coming out of the valve, so that a fast steady stream of droplets is coming out, in the range of 4-16 droplets per second. The collisions will occur at a wide range of rates, however.
The jug (bottom left of the picture) is held in the stream of droplets, and the base liquid is allowed to be still. Then the jug is removed, and quickly afterwards, the shutter fired manually, hopefully catching the correct moment when the second droplet collides with the rebounding spout of the first that landed in the base liquid.
My best keeper rate has been about 1/6.5. It is usually MUCH lower, and for some of the "double collisions", we're talking 2-3 keepers in 750 photos.