By therandomsci / February 18, 2021
There are many events when rubbing action becomes converted to a high-frequency vibration due to friction.
When you start to push one surface to another, friction resists the movement. But if the pushing force increased, it reaches the threshold at which friction is overcome and slipping starts. During this force drop and object skid over one another. Skin is elastic, it will respond to the increasing force by distorting.
Due to friction, a finger is put over a glass. The skin is initially pulled out of shape by the force attempting to move it and then spring back closer to its normal shape once sliding starts. Because of the change in the shape of the finger, the friction again increases. and the movement of the finger momentarily stops while the skin is pulled into a distorted shape. A pull will create hundreds of distortion per second, setting up audible sound waves.
All the surfaces are rough under the microscope and when they come in contact with the highest points, known as asperities, actual touch. these asperities interlock and oppose any movement. As you push harder, the area of true contact increases, because any soft surface, such as skin, deforms to fit more closely into irregularities of the other surface. The friction force opposing motion increases. As soon as the finger starts to slide, the asperities bounce off each other and interfere less with the movement.
Hann, R 2005, Does Anything Eat Wasps?, New Scientist, London.
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