Soft surface layers control contact and slip of polyacrylamide hydrogels

Swollen gels with high water content >85% by mass tend to form softer surface layers due to the osmotic pressure mismatch between the bulk gel (Π>0) and that of the open bath (Π=0). These skin layers are locally swollen, and settle with a gradient profile of density that is homogeneous in the bulk, but vanishing at the surface. This experimental work uses broad probes in micro-indentation to characterize the contact mechanics of this gradient layer based on the evolving scaling of the indentation force F with depth F~dⁿ. We find that the contact mechanics at the extreme of the surface scale like a polymer brush, the intermediate surface as a soft laminate, and finally the bulk as a homogeneous half-space. We also use finite-element simulations of indentation into slabs with controlled gradient surfaces to determine the most appropriate profile of the experimental data. The simulations explain how the stiffer bulk layer enforces the softer top layers to extend radially, which causes the observed increase the contact radius of the gradient samples. Finally, we show corresponding slip measurements, and connect the friction to the surface composition. These results facilitate the design the hydrogel with gradient layers, having a predictable contact and friction.