One day, someone asked me a question.
“Can you see a flow inside a concave polyhedron like a sphere?”
I responded “Sure, you can. Get a glass sphere, and immerse it in a glycerol. They have the same refractive index so one could see the flow inside the sphere with proper tools.”
The person was intrigued by this idea. One could, for example, probe the flow of the mantle of the earth by this technique.

Yet, scientists are demanding species so he followed up by saying “Can we extend this idea for an arbitrary concave polyhedron?”
The answer was naturally yes but everyone knows how difficult to morph the glass into a shape that one wants.
Later, I came up with a solution. One could use 3D printing to print any shape that s/he wants. As for the index-matched fluid, I found one after a few trials and errors.
Below, I immersed a 3D-printed capsule with a hole in the fluid. I agitated the fluid to generate some backgorund flow.
Also, I filled the fluid with fluorescent particles that glow when I shine a laser.
Check the footage below. You can clearly see the flow inside the capsule.

This technique is not immediately useful for my research but could be useful for someone else’s. For instance, it could provide a means to conduct a table-top experiment on the flow of the mantle.

The footage provides food for thought.