Stick-slip phenomenon

The stick-slip phenomenon, also known as the slip-stick phenomenon or simply stick-slip, is the spontaneous jerking motion that can occur while two objects are sliding over each other.

Below is a simple, heuristic description of stick-slip phenomena using classical mechanics that is relevant for engineering descriptions. However, in actuality, there is little consensus in academia regarding the actual physical description of stick-slip which follows the lack of understanding about friction phenomena in general. The generally agreed upon view is that stick-slip behavior results from common phonon modes (at the interface between the substrate and the slider) that are pinned in an undulating potential well landscape that un-pin (slip) and pin (stick) primarily influenced by thermal fluctuations.

The stiffness of the spring (shown in image below), the normal load at the interface (the weight of the slider), the duration of time the interface has existed (influencing chemical mass transport and bond formation), the original rate (velocity) of sliding (when the slider is in the slip phase) – all influence the behavior of the system. A description using common phonons (rather than constitutive laws like Coulomb's friction model) provides explanations for noise that generally accompanies stick-slip through surface acoustic waves. The use of complicated constitutive models that lead to discontinuous solutions (see Painlevé paradox) end up requiring unnecessary mathematical effort (to support non-smooth dynamical systems) and do not represent the true physical description of the system. However, such models are very useful for low fidelity simulations and animation.

Engineering description

Stick-slip can be described as surfaces alternating between sticking to each other and sliding over each other, with a corresponding change in the force of friction. Typically, the static friction coefficient (a heuristic number) between two surfaces is larger than the kinetic friction coefficient. If an applied force is large enough to overcome the static friction, then the reduction of the friction to the kinetic friction can cause a sudden jump in the velocity of the movement. The attached picture shows symbolically an example of stick-slip.

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