Near-contact interactions between soft, rough and wet particles

Interactions between suspended particles and surfaces are important across applications, including rheology, materials processing, additive manufacturing and soft robotics. These interactions are strongest when particles are in near-contact with each other or with nearby surfaces, such that viscous forces are dominant. In this talk we focus on how the these interactions are modified by surface roughness and elasticity. We first discuss the motion of suspended particles near elastic interfaces and substrates. The interplay between fluid flow and elastic deformation leads to lift forces and torques, which couple the rotation and translation of the particles. We will then discuss the influence of surface roughness on these interactions, interpreting our findings in terms of an effective coefficient of friction of the lubricated elastic contact. Counter-intuitively, we will see that the friction depends non-monotonically on the roughness amplitude. Throughout the talk, we will see that both elasticity and roughness lead to hydrodynamic and particle-kinematic features that resemble frictional interactions, suggesting that these mechanisms may bridge Coulomb friction with traditional particle hydrodynamics.