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Title: Origin of the logarithmic aging of static friction
Plain: The longer two surfaces stay in contact at rest, the harder it is to start them sliding, and the increase grows with the logarithm of time. Whether this comes from the contacts slowly squashing and growing or from chemical bonds forming across them is disputed.
Precise: The static friction coefficient grows as $\mu_{s}(t) = \mu_{0} + \beta \ln(1 + t / t_{c})$ with $\beta$ of order 0.01 for rocks, polymers and metals, the basis of rate-and-state friction laws. Determine whether aging comes from creep growth of real contact area, from interfacial chemical bonding (reported in nanoscale silica contacts), or both, and predict $\beta$ and $t_{c}$ from material properties.
Settled by: Simultaneous measurement of real contact area and interfacial bond density during aging for a well-characterized interface, accounting quantitatively for $\beta$.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: (empty)