So this clever chap Nate True whipped up this nifty little device that uses a UV pigment called Fluorescein combined with strobing LED’s and an oscilloscope to make the drops appear to slow, stop, or even flow backwards. The drops can even be interacted with. Have a look at the clip and pop on over to Nate’s site to see how he made it.
P.S. While you are at Nate True’s site make sure you check out the comments at the bottom for things like:
You can also buy a highlighter, open the bottom of it, and squeeze out its delicious highlighter-juice. I’m pretty sure that’s fluorescein.