Just add water. 

Notes: 

• Print in PLA – although the ideal material would be Nylon
• The top reservoir is for reagent, and should stop flowing when it reaches the main branch, unless there is liquid in the main branch
• Use a paper towel as a capillary pump to “pull” the sample and reagent 
• For demos, just use water; perhaps add some food coloring to one or the other (or both)
• This would work better if it were smaller; however, it is hard to create macro-scale capillary demonstrations (that is the intent here)

     Mk1:

• Infill modifier is used to create the capillary pump structure (see image #2)
  • Note that pressing a paper towel over this is still best for good pumping
• I do feel there is room to make effective capillary pumps using infill structures, but for most demonstration / educational purposes, having some kind of capillary network that ends in a paper towel will provide 100x more pumping action. For an actual laboratory application where a filament-based solution is desired, an all-printed solution may be viable for early testing, prototyping, or first-run experiments while the “real” microfluidics are being fabricated. If you don't like the idea of this now, come back in a few years after you have a resin sensitivity. 

     Mk2:

• Open-channel design
• Still works best with a paper towel as a capillary pump

     Mk3:

• Open-channel with an “on” switch for the sample reservoir
  • Wet the printed slug used as the on switch and place in the hole
  • Can be best to drag a bead of water from the sample reservoir
  • This setup lets you prepare the demo ahead of time
• There is a video for this one: 

References/inspiration: 

doi:10.1039/c8lc00458g

doi:10.15760/etd.2914 (ch. 4)

youtube / watch?v=pct3JhVFSLo

and thing:4116612