Gabriele Albertini (University of Nottingham)

Squeaking is a constant companion in various aspects of our daily lives, whether we are sliding rubber-soled shoes across hardwood floors, scraping chalk on a blackboard, engaging the brakes on a bicycle, or walking with a hip replacement–where a hard metal or ceramic ball slides against a polymer liner. When two rigid bodies slide over each other, squeaking is widely understood to result from self-excited stick-slip oscillations, triggered by a decrease in the friction coefficient with increasing slip velocity [1-2]. However, sliding of extended interfaces can involve crack or slip-pulse propagation [3-5]. This distinction is amplified when a soft body slides on a rigid one, where large deformations and material mismatch can cause detachment via opening slip pulses [6-8]. Previous studies focused mainly on slow sliding, where pulses are slow [4,9-12]–commonly referred to as Schallamach waves–and squeaking is absent. Although squeaking at soft–rigid interfaces has been linked to stick-slip oscillations, the mechanisms remain unclear. In this study [13], we experimentally and numerically investigate soft–rigid interfaces sliding at velocities that produce dynamic opening pulses. High-speed imaging and acoustic analysis reveal that opening pulses propagate at approximately the soft material’s shear-wave speed cs, mediating local slip across diverse materials. In flat samples, these pulses are irregular and generate broadband acoustic emissions. Introducing thin surface ridges confines pulse propagation, yielding a consistent repetition frequency matching the first shear mode of the sliding block and squeaking at that frequency. Numerical simulations further reveal that the mechanism setting the frequency of pulse initiation is trailing-edge oscillations that take place near the first shear eigenmode. This mode produces a periodic modulation of the contact pressure localized at the trailing edge, thereby setting the tempo for the nucleation of opening pulses. These findings reveal a structure-driven mechanism that stabilizes rupture in bimaterial friction. Geometric confinement suppresses competing modes, transforming irregular two-dimensional dynamics into coherent one-dimensional pulse trains, offering new insights into frictional rupture from engineered surfaces to geological faults.

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[13] Djellouli, A., Albertini, G., Wilt, J., Tournat, V., Weitz, D., Rubinstein, S., Bertoldi, K.. Squeaking at soft–rigid frictional interfaces. Nature 650, 891–897 (2026).