Measuring friction on soft substrates

The relative motion of two interfaces induces dissipation that we experience as friction. Friction is a complex phenomena that involves many length and time scales. Importantly, friction depends on the composition of the two interfaces sliding with respect to each other. It has been challenging to measure between soft substrates such as hydrogels. We describe a methodology to measure friction between hydrogels of arbitrary composition and discuss how this method reveals nontrivial frictional properties, even for example in the rate independent sliding regime. We link the frictional properties of hydrogels to their surface roughness as obtained with cryo-SEM.