The four Supplementary Movies correspond to Fig. 3 of the manuscript.
Movie 1: glass beads, 0.5 mm in diamater. Movie 2: cinnamon powder. Movie 3: pepper powder. Movie 4: foam.
These stroboscopic movies are synchronized with the forcing frequency (one image per circular translation of the container).
The forcing frequency is 2.3 Hz, and the total time of each movie is approximately 5 minutes.
The direction of the forcing is anti-clockwise. The container diameter is 75~mm, filled with water up to height 20~mm.
The surface density is gradually increased by pouring additional material with a spoon.
At low density, all the rafts are in co-rotation (anti-clockwise).
At large density, Movies 1 (glass beads), 2 (cinnamon powder) and 4 (foam) show counter-rotation (clockwise), whereas in Movie 3 (pepper powder) the raft remain in co-rotation.