Abstract
The real area of contact of a frictional interface changes rapidly when the normal load is altered, and evolves slowly when normal load is held constant, aging over time. Traditionally, the total area of contact is considered a proxy for the frictional strength of the interface. Here we show that the state of a frictional interface is not entirely defined by the total real area of contact but depends on the geometrical nature of that contact as well. We directly visualize an interface between rough elastomers and smooth glass and identify that normal loading and frictional aging evolve the interface differently, even at a single contact level. We introduce a protocol wherein the real area of contact is held constant in time. Under these conditions, the interface is continually evolving; small contacts shrink and large contacts coarsen.
Highlights
When two ostensibly flat solid bodies are brought into contact, small-scale roughness results in the formation of a multitude of tiny contact patches known as microcontacts [1]
AR and the frictional strength of an interface grow logarithmically for a wide variety of materials including metal [7], plastic [8,9], rock [10,11], sand [12,13,14], and paper [15]. This growth is captured by the rate and state friction laws [16,17,18], in which a phenomenological state variable is often interpreted as being directly related to the instantaneous value of AR/FN, where FN is the normal load
This interfacial memory suggests that aging and an increase in FN affect the interface in different ways
Summary
Tom Pilvelait ,1 Sam Dillavou ,2 and Shmuel M. AR and the frictional strength of an interface grow logarithmically for a wide variety of materials including metal [7], plastic [8,9], rock [10,11], sand [12,13,14], and paper [15] This growth is captured by the rate and state friction laws [16,17,18], in which a phenomenological state variable is often interpreted as being directly related to the instantaneous value of AR/FN , where FN is the normal load. AR by slowly decreasing FN in time continually evolves: Large contacts with complex shapes grow while small, more circular contacts shrink This evolution suggests a clear difference between the effects of aging and changing FN , which we verify systematically using both ordered and randomly rough surfaces.
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