Tribology of hard particles lubricating soft surfaces

Soft materials often have interesting and unexpected frictional behavior owing to their deformable nature. We use soft polydimethylsiloxane (PDMS) surfaces lubricated by hard glass spheres to study how this deformability influences particle-based lubrication. For particles between 100 and 2000 $\ensuremath{\mu}\mathrm{m}$ in size, we observe a nontrivial rate dependence and three frictional regimes: (I) a rolling friction regime where the rolling particles keep the surfaces apart sufficiently to give low friction coefficients---this is mainly found for large particles and smooth surfaces; (II) a sliding friction regime with high friction coefficients where the surfaces are partially in contact, which is found for small particles, rough surfaces, and high normal forces; (III) a PDMS-PDMS contact regime where the particles are fully inserted into surfaces and the surfaces are in contact. We interpret the friction dynamics in terms of the Hertzian contact deformation effects in the indentation of the PDMS surfaces.