Importance of Hydration and Surface Structure for Friction of Acrylamide Hydrogels

Abstract

To understand the dissipative mechanisms in soft hydrogel lubrication, polyacrylamide (PAAm) hydrogels with two distinct surface structures were examined under various contact conditions. The characteristic speed-dependent friction of the selfmated, crosslinked hydrogel surfaces could be explained by hydrodynamic shearing of a thin water layer between two rather impermeable bodies. On the other hand, the frictional response of brushy hydrogel surfaces is dependent on the contact conditions and the level of surface hydration. In a migrating contact, brushy hydrogels showed low, speed-independent friction (µ ~ 0.01) likely due to a thick layer of shearing liquid trapped within the sparse surface network. In stationary contact, however, brushy hydrogel surfaces can partially exude water from the near-surface region over time, as shown by timeresolved Fourier-transform infrared (FTIR) spectroscopy. This is assumed to be reflected in a friction increase over time. Interfacial shearing appears to shorten the characteristic exudation times compared to those observed under static loading. Once fluid has been exuded, brushy surfaces were shown to reach similar friction values as their crosslinked analogs. The results thus indicate that the dominating dissipation mechanism during sliding at low contact pressures is shearing of the interfacial liquid film, rather than poro-elastic dissipation within the bulk. Maintenance of surface hydration is therefore crucial, in order to take advantage of the low friction of such systems.

Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures

“…At the same time, the results obtained with such different contact geometries are difficult to compare, if the effects of different contact boundaries (trailing edge of a flat-pin-on-flat compared to a convergent inlet of a sphere-on-flat) are unknown. We have already shown that the continuous contact of a ring-on-flat displays similar friction as a flat-pin-on-flat hydrogel contact at sliding speeds between 0.1-20 mm/s [23]. However, sphere-on-flat contact might facilitate water supply to the contact due to the convergent contact inlet when compared to a flat-on-flat contact geometry [27].…”

“…To produce hydrogel surfaces with different structures, the solutions were gelled in different mold materials. Glass petri dishes were selected to prepare hydrogel discs with a dense, crosslinked surface, while polystyrene (PS) petri dishes were selected to prepare hydrogel discs with a sparse, brushy surface [22,23]. By using spacers between two molding surfaces, we prepared hydrogel sheets of 2-mm and 5-mm thickness, which served to produce flat upper and lower tribological specimens, respectively.…”

“…When a brushy surface is exposed to sufficient constant normal load, it exudes water, increasing the polymer concentration near its surface, and causing it to behave similarly to a hydrogel with a crosslinked surface. Therefore, in typical friction experiments, in which a smaller top hydrogel counterpart is constantly in contact, its surface structure does not play a significant role in friction [23]. The friction seems to be predominantly determined by the surface structure of the larger hydrogel counterpart, where the migrating contact area allows for the surfaces to rehydrate during the out-of-contact period.…”

“…Hydrogel friction has also been shown to be highly dependent on the surface structure of the gels [10,14,15,[22][23][24]. It was found that a friction increase with increasing sliding speed is commonly observed with hydrogels with a crosslinked surface.…”

“…Due to the sparse, brushy surface structure, which traps large amounts of water at the interface, the chances for direct polymer chain contact are lower and the hydrodynamic shearing thickness is presumably larger, reducing the friction. In these brushy, water-rich hydrogel surfaces, however, the friction was demonstrated to depend on water exudation and rehydration in the near-surface region [23]. When a brushy surface is exposed to sufficient constant normal load, it exudes water, increasing the polymer concentration near its surface, and causing it to behave similarly to a hydrogel with a crosslinked surface.…”