Pulp friction: lubricating properties of soft particle suspensions*

Friction between two sliding surfaces is often reduced with a lubricant. Lubricants can contain particles that help mitigate dissipation. The mechanics of friction is complex, especially when soft substrates/particles and lubricating fluids are involved. We shine new light on the complex mechanics of particle based lubrication by evaluating the lubricating properties of particle suspensions. We synthesize custom soft micron-sized gel particles to create a soft particle suspension that acts as model lubricant. The suspension lubrication deviates from typical Stribeck behavior as we find four frictional flow regimes. These frictional regimes are influenced by the particle size, deformability and the amount of particles in suspension, which allows us to propose mechanisms for the different lubrication regimes. We verify some hypotheses by performing lubrication experiments with soft substrates and hard particles.