When I first started doing water drop photography, every now and then, I would see a water drop collision with base that looks like onion root. I was amazed by it and really wanted to do something like that. After some blindly experiments, I finally got it and got it repeatedly.
Here are some of the steps to get it.
Step 1 — This is more like a equipment selection than a step, that is, use larger opening nozzles. In this tutorial, I used an 8mm outside diameter (OD) nozzle with 5.5-5.8mm inside diameter (ID) opening. Note, this is very important.
Step 2 — Set number of drops to 1 and follow Guaranteed Water Drop Collision and find the Flash Delay (FD) value where water drop just about hitting the surface and write this value down.
Step 3 — Adjust drop size to try to get a jet as tall as possible, like the first image in the above picture. Again, this is standard exercise shown in Guaranteed Water Drop Collision.
Step 4 — Now, keep increasing drop size 5ms at a time. You will notice that the jet is actually getting smaller and smaller, until certain point, it is almost gone like the second image in the picture above.
Step 5 — Again, keep increasing drop size, but at finer step, 1ms at a time until you start to hear some popping sound from the jet. Important note here: increasing the drop size one unit at the time.
Step 6 — When popping sound get louder, you will notice that the jet starts to go taller as well and with a large bubble at the base, sort of like the third image in the picture above.
Step 7 — Keep increasing the drop size until you get the tallest jet with bubble base that looks like an onion, like the 4th picture shown above.
Step 8 — Record the Flash Delay (FD) value and follow Guaranteed Water Drop Collision to get collisions like the 5th and 6th images in above picture.
To conclude, from my experience, it all starts with bubble popping sound and from that point one, the jet gets taller and taller until it peaks and then you can treat this a your new jet and use the technique described in Guaranteed Water Drop Collision to get a collision.