In the second half of the washday blues column, Cecil addresses the phenomenon of bubbles moving towards the center of the cup when solid-body rotation is induced by a stirring spoon. The master speaks of “denser coffee crowding its way toward the rim,” when in fact even the fluid itself undergoes center-seeking motion due to viscous friction near the bottom of the cup. The balance between pressure gradient force and centrifugal force tilts in favor of pressure gradient force (directed radially inward) when frictional effects in the bottom “Ekman layer” are considered. The center-seeking flow near the bottom then gives rise to a secondary circulation (vertically upward) due to mass continuity, and (if coffee is not your hot beverage of choice) you can see this upward motion reflected in the rising of tea leaves near the center of a tea cup.
References:
Stirring Flows in Teacups
Holton, James R. An Introduction to Dynamic Meteorology. ISBN 0123540151. (pp. 133ff.)