The standard for indentation and contact measurements is to perform them at moderate indentation rates to avoid long-term creep but account for quick relaxations that occur due to fluid effects. However, low-pressure interfaces of interest such as those in the body are not contacted at moderate timescales, but rather held in contact, such as in the knee joint or the eyelid resting on the cornea when the eye is not blinking. Thus we investigate whether contact models like the Hertz model or Winkler model describe the behavior of long-duration soft, hydrated contacts, and whether all creep relaxation be attributed to poroelastic diffusion, as is reported widely in literature.<br/>In this study we perform creep relaxation experiments in three contacting configurations: hard probe-soft substrate, the inverse, and self-mated soft surfaces. Soft probes and flat substrates were molded either in combination delrin/polyolefin molds or between polystyrene plates. The composition was a chemically-crosslinked polyacrylamide of 7.5% w/w acrylamide and 0.3% w/w bisacrylamide crosslinker. The hard probe was a steel sphere of 1.5 mm diameter. A custom microindenter was used in force-control mode to apply and hold a load of 0.50 to 1.00 mN. Simultaneously, either particle-exclusion or particle-inclusion microscopy was used to view the contact area and record its changes in time up to 10 minutes. All experiments are submerged in water at 25°C.<br/>All configurations exhibited creep relaxation, or increasing contact area under a constant-load contact experiment. The self-mated contact between the hydrogel probe and hydrogel slab increased in contact area ~ 20%. The soft probe-hard substrate area increased by only 10%. Each experiment was repeated at least 3X. The soft substrate configuration had the greatest extent of contact area growth, the self-mated contact a moderate extent, and the soft-probe the least extent.<br/>These results add a new dimension to contact mechanics of soft materials, moving from quasi-static results to time-dependent results that evolve under constant load. The contact pressure evolution indicated that the Hertz model is only valid over short durations of contact, and that self-mated contact approaches a constant-pressure contact model. Additionally, pressure-driven diffusion of water away from contact is confirmed only briefly, and does not fully explain creep relaxation of polyacrylamide under low applied pressures.

Low-pressure contact experiments reveal multiple modes of relaxation in polyacrylamide hydrogels

2019 Fall National Meeting

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