Slow frictional waves

Koushik Viswanathan, Purdue University


When modeling physical phenomena, it is common to lump the effects of friction into one or a few parameters. Historically, this phenomenological view of frictional processes was driven by purely practical considerations. As a result of independent developments in miniaturization of components, tribology and improved geophysical observations, the microscopic aspects of friction are now becoming better understood. In particular, the occurrence of stick-slip instabilities has been explained in many systems within the asperity framework. However, in highly adhesive soft polymers, strong van der Waals forces mask the effects of asperity deformation. Hence such surfaces are in more intimate contact and the interface dynamics is governed by pinning and de-pinning of polymer chains. Additionally, the extended nature of the contact surface allows elastic effects to be manifest on the mesoscale (several micrometers). Hence stick-slip in soft polymer surfaces is a complex phenomenon that has hitherto escaped a unified description. I n this work, we isolate individual stick-slip events in a soft polymer surface. Using high-resolution high-speed in situ imaging methods, it is shown that stick-slip in such adhesive surfaces is fundamentally of three kinds, corresponding to the propagation of three different surface waves—detachment pulses, slip pulses and Schallamach waves. Each of theses waves has basic differentiating properties and occurs under certain experimental conditions. The nucleation and propagation of these waves are studied quantitatively, revealing a fundamental relation between stick-slip and slow surface waves. The three observed surface waves have direct analogues in muscular locomotion of many soft-bodied invertebrates. The results raise an important question about the origin and subsequent development of specialized physiology in these organisms. A theoretical description is presented to explain the observations. A unified elastic theory for slow waves involving detachment is developed and is capable of quantitatively reproducing experimental observations. This theory is independent of any assumed friction model. Other simple models are also presented for describing the nucleation and propagation of Schallamach waves. It is finally shown that Coulomb friction cannot account for the existence of waves without detachment—assuming Coulomb friction to hold at the interface results in a completely inconsistent solution. Hence new statistical or microscopic models are necessary for fully explaining wave propagation without interface detachment. Important implications of the results for the phenomenon of slow earthquakes, as well as other recently observed slow fronts is briefly discussed. This work outlines the fundamental link between the propagation of slow surface waves and friction. It is hoped that the findings will also help drive efforts to control stick-slip in soft polymer surfaces.




Chandrasekar, Purdue University.

Subject Area

Mechanics|Mechanical engineering|Materials science

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