Using the usual mathematical
model (capillary surface equation with contact angle boundary condition) we discuss
regularity of the equilibrium free surface of a fluid in a cylindrical container in case
the container cross-section has corners.
It is shown that good regularity holds at a corner if the “corner angle” 𝜃 satisfies
0 < 𝜃 < π and 𝜃 + 2β > π, where 0 < β ≤ π∕2 is the contact angle between the fluid
surface and the container wall.
It is known that no regularity holds in case 𝜃 + 2β < π, hence only the borderline
case 𝜃 + 2β = π remains open.
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