A normal force moves slowly over the surface of a layer which is pressed against a substrate. Two asymmetric slip zones are generated if the force exceeds a friction dependent critical value. The slip is opposite in direction in the two zones, but due to the lack of symmetry, a net tangential shift of the layer is left behind by the passage of the force. This net shift is generally influenced by the coefficient of friction: for high coefficient, the layer shifts in the direction of motion, while for low coefficient, the shift is opposite. The latter result has been observed experimentally.
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