  Swirling a glass of wine induces a rotating gravity wave along with a mean
flow rotating in the direction of the applied swirl. Surprisingly, when the
liquid is covered by a floating cohesive material, for instance a thin layer of
foam in a glass of beer, the mean rotation at the surface can reverse. This
intriguing counter-rotation can also be observed with coffee cream, tea scum,
cohesive powder, provided that the wave amplitude is small and the surface
covering fraction is large. Here we show that the mechanism for
counter-rotation is a fluid analog of the rolling without slipping motion of a
planetary gear train: for sufficiently large density, the covered surface
behaves as a rigid raft transported by the rotating sloshing wave, and friction
with the near-wall low-velocity fluid produces a negative torque which can
overcome the positive Stokes drift rotation induced by the wave.
