This is a remix of my capillary toy to create a color gradient in a simple, visible way. Most of the diffusion takes place in the paper towel, so this isn't really a mixing device. I'm interested in capillary-driven open-channel flow at the moment, and this is a fun way to play with that.

I included the 3d model for my old prototype because there may be slight differences to the center channel and output throat that make the old one work more reliably (in case anyone wants to get into the weeds on this). You have to cut the paper towel for it to fit in the old model, because I wasn't thinking that far ahead at the time. I cleaned up the geometry when modifying the output throat, which may have made it more finnicky to start. 

Pro tip: rinse this off in the sink and dry with a paper towel before your demonstration. Try to get most of the liquid out of the three channels. The stuff you can't get sort of acts to “prime” it so it is easier to start.

Designed for PLA, but the ideal material for stuff like this should be Nylon. 

Old version: