The Physics Of Ultrasonic Cleaning

As someone who uses ultrasonic cleaning quite often, it is fascinating to me that we are still learning on what actually goes on when we put something in an ultrasonic cleaner. This study, published in PRL no less, examines the physics of how things actually are cleaned on the surface using an ultrasonic cleaning process.

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.

Zz.