| Summary: | The origin of the squeal acoustic emissions when a chalk is rubbed on a
blackboard or better on a ceramic plate, and those when a wet finger is rubbed on a
smooth surface, such as a glass surface, is sought in the stick-slip effect between the
rubbing surfaces. The elastic agency is sought in a shear band between the two
surfaces characterized by very low shear modulus. In the case of the squealing chalk,
it can be argued that the shear band is a layer of chalk powder, about 0.3 mm thick,
forced to slide over the ceramic plate surface. In the case of the wet finger on a glass
surface, it can be argued that the shear band is the layer of soft tissue between the
epidermis and the finger bone, and that the water layer simply facilitates the
stick-slip effect.
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