Nice question! Fortunately, it was answered by the group of my friend Sid Nagel at U. Chicago.
https://jfi.uchicago.edu/~tten/Coffee.drops/Nagel.7.4.pdf
The puddle upper surface curves the most near the outer part of the contact with the ground. That causes the evaporation to go fastest out there at the edge. The flow in the puddle is then outward, crring dirt or coffee toward the edge. It builds up in a ring just under the region of fastest evaporation.
Mike W.
(published on 09/01/2020)
