This is a test to check the effectiveness of any Ultrasonic cleaner: suspend a piece of aluminum foil (those used in the kitchen) in the water and let it run for a few minutes. This is how it looked like after running the ultrasonic cleaner for 3 minutes with plain water and no detergent or cleaning solution. Amazing, isn't it? The tiny indentations were created by the microscopic action of the microscopic water bubbles created by the high frequency (42 KHz) of the ultrasonic cleaner. These bubbles constantly formed/exploded on the surface of the object to remove any particles that clung to the surface.