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Stupid question: Is the math of the design of cup based on the math of the water? In other words, are the actual curves and angles of things derived from the surface tension properties of water in space?

So, if one wanted to make a capillary cup for custard, it'd be different dimensions? Would love to see a parametric version of this (OpenSCAD?) somehow that can be generated for different liquids ...

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It would need to be different for custard or it just may not work properly past a certain viscosity point where the resistance to flow in the matter overcomes the forces pulling it into the corner. Or it might just flow slowly enough it's not functional but still forms the same shapes eventually not sure. Zero-G fluid dynamics are difficult to work with intuitively.

To expand on your point a bit, it depends on the type of fluid. Newtonian fluids [1] (a good approximation for water and many other "normal" fluids) will be pulled into the groove regardless of how viscous they are. E.g., the cup would probably work for treacle (syrup). Custard is non-Newtonian; more specifically, it's probably a Bingham fluid [2], meaning it acts like a solid until enough shear stress is applied, and then it acts like a liquid. The problem here is that even if there's enough shear stress for the custard far from the walls to move, the custard deep in the corner will likely remain a solid. So the corner capillary effect won't work.

[1] https://en.wikipedia.org/wiki/Newtonian_fluid

[2] https://en.wikipedia.org/wiki/Bingham_plastic

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