Natural and induced surface roughness determine frictional regimes in hydrogel pairs

Abstract

Hydrogels display extremely complex frictional behavior with surprisingly slippery surfaces. We measure the sliding behavior of hydrogels submerged in water using a custom-made tribotool. Samples with an imposed surface roughness give two distinct frictional regimes. Friction coefficients in the first regime change with asperity sizes and Young's moduli. Under increased normal force, a second frictional regime emerges likely due to smoothening of asperities. Friction coefficients in the second regime remain constant across length scales of roughness and appear to be material specific. The hydrogel polymer network also directly influences the surface topography, and with that, the frictional behavior of hydrogels. We highlight the tribological importance of surface roughness at different length scales, which provides potential to engineer functional frictional behavior.

Uncovering friction dynamics using hydrogel particles as soft ball bearings

“…1b). 2,16 This is similar to friction coefficients in synovial joints. The exact mechanism by which these low friction coefficients in hydrogels come about is still under debate with many proposed mechanisms.…”

“…This is not surprising, as biopolymer solutions are known to give low friction coefficients. 2,13,[32][33][34] As can be seen, the friction coefficients for both gelatin samples are similarly low, although the shape of the curves are rather different.…”

“…The Young's modulus of the chemically cross-linked samples (YM500) is 0.5 kPa as we described elsewhere. 2 This value is a factor five higher than the modulus for the original 15 wt% gelatin gel (YM90). The chemical cross-linking of gelatin did not change the particle size ( Table 2).…”

“…To further increase the hardness of certain microgel particles, we chemically cross-linked gelatin using the treatment described in previously published work. 2 After filtration, we obtained a densely packed hydrogel particle suspension, which was used for rheological and tribological measurements. We refer to this densely packed suspension as ''100%'', which refers here to maximum packing.…”

“…In the case of hydrogels, it has been shown that a decrease in friction can be obtained with increased contact areas. 1,2 To decrease the high frictional values caused by increased contact, the contact area between surfaces in motion can be minimized in many ways. The simple presence of a thin lubricating fluid layer or polymer coating can already decrease the friction coefficient by orders of magnitude by preventing the surfaces to come into direct contact.…”