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

Abstract

Polyacrylamide (PAAm) hydrogels with brush-covered or crosslinked surfaces were produced and their tribological behavior was studied over a wide range of sliding speeds for two different contact geometries: sphere-on-flat and flat-pin-on-flat. Irrespective of the contact geometry, the brushy hydrogel surfaces displayed up to an order of magnitude lower coefficients of friction μ (COF) compared to the crosslinked surfaces, even achieving superlubricity (μ < 0.01). In general, a hydrogel sphere showed a lower coefficient of friction than a flat hydrogel pin at a similar contact pressure over the entire range of sliding speeds. However, after normalizing the friction force by the contact area, the shear stress of hydrogels with either crosslinked or brushy surfaces was found to be similar for both contact geometries at low speeds, indicating that hydrogel friction is unaffected by the contact geometry at these speeds. At high sliding speeds, the shear stress was found to be lower for a sphere-on-flat configuration compared to a flat-pin-on-flat configuration. This can be attributed to the larger equivalent hydrodynamic thickness due to the convergent inlet zone ahead of the sphere-on-flat contact, which presumably enhances the water supply in the contact, promotes rehydration, and thus reduces the friction at high sliding speeds compared to that measured for the flat-pin-on-flat contact.

Oxygen inhibition of free-radical polymerization is the dominant mechanism behind the “mold effect” on hydrogels

“…33 It is important to note, however, that despite the differences at the surface glass-and PS-molded gels have a similar bulk structure. 30 As expected, we again confirmed that different molding materials result in hydrogels with different surface structures, however, surface treatment and thus the hydrophobicity of the molding surface appears to have no significant effect on the surface structure of the synthesized hydrogels.…”

“…The extracted elastic modulus was 30 kPa ± 1 kPa for both glass-molded hydrogel surfaces. In the past, the elastic moduli of similar glass-molded hydrogels were also measured using microindentation 29 and macroindentation 30 , which showed a good agreement with both the Hertzian and the Winkler contact models. With both these models assuming a homogenous structure, glass-molded hydrogels appear to have a homogeneous polymer density profile throughout their thickness.…”

“…In the past, we have evaluated differently molded hydrogel surfaces using various characterization techniques, including infrared spectroscopy 8 , neutron reflectometry 29 , rheology 22 , tribology 8,22,29,30 as well as various indentation methods 8,22,29,30 . All the obtained data correlated well with each other, and enabled us to elucidate the surface structures of differently molded hydrogels.…”

“…Open indentation data, where softer, sparser hydrogel surfaces always result in lower friction 8,22,23,30,34 .…”

Lubrication Effect of Polyvinyl Alcohol/Polyethylene Glycol Gel for Artificial Joints

“…erefore, the development of alternative lubrication methods to increase the service life of artificial joints has attracted considerable attention [26][27][28][29].…”