Dynamic frictional behavior and dilatancy of hydrogels

Hydrogels consist of hydrophilic polymer networks that swell dramatically when immersed in water or other solvents. Due to hydrogels' biphasic nature, their frictional properties often display non-monotonic and time-dependent behavior. During frictional sliding, hydrogels experience shear stresses that can induce dilatancy, commonly known as the Poynting effect. This effect is important since it may strongly affect the surface and bulk transport of solvent in the compressed hydrogel. Here we use a rheometer and a custom-built pin-on-disk tribometer to help examine hydrogels' time-dependent frictional behavior and dilatancy. We observe a positive Poynting effect in polyacrylic acid hydrogels at certain sliding velocities. The role of polymer relaxation was also explored by measuring the friction coefficient and normal stress over several hours, where the timescale for dilatant bulk relaxation is often much longer than the timescale for frictional relaxation.