Dynamic Friction and Lubrication in Soft Hydrogel Interfaces*

The increasing use of biologically-inspired hydrogel material and the unusual mechanical qualities that these gels offer drive the need for a better, more intuitive understanding of the mechanical response of such continua. Using a custom low-force pin-on-disc tribometer, we have probed the single-contact frictional properties of both polyacrylamide and agarose hydrogels under a variety of environmental conditions. Under a constant, physical contact, these hydrogels exhibit a dynamic frictional transition characterized by a precipitous drop in the friction coefficient at a critical velocity. Within a range of speeds near this dynamic frictional transition, transient behavior can be observed. Upon increasing the speed, the coefficient of friction decreases exponentially, with a characteristic decay time of order 10 minutes. We show how our results can be interpreted as a competition between the relaxation time of the polymers and shear rate in the fluid (i.e. Wiessenberg number), and that this transition can be tuned by varying the liquid salt concentration, liquid viscosity,  and sliding geometry.