In some experiments, the researchers coated one side of every bead with gold, so they could see the beads' rotation. The beads rolled back and forth during a single cycle of the bubble's expansion and contraction, some at a rate corresponding to 150,000 rotations per second. This rapid rotation implies a large twisting force (torque) on the beads. The torque arises because the beads rest partially in a thin, stationary layer of fluid at the glass surface and partially in the fluid above, which moves rapidly in response to the bubble. "The rotation is a very clear indication that this boundary layer is important," says Ohl. Similarly, in ultrasonic cleaning, a dirt particle stuck to the surface will extend partly out of the boundary layer and feel a large torque that can break its connection to the surface.The link also has a video of the action.