Normal Load Scaling of Friction in Gemini Hydrogels

Abstract

Health and physiology are critically dependent on the ability of soft, permeable, and aqueous materials (e.g. cartilage, cells, and extracellular matrix) to provide lubrication over a wide range of speeds and contact stresses. Living cells and tissues present tremendous handling and experimental challenges for fundamental biotribology studies. Synthetic high water content hydrogels, designed to share similar mechanical and transport properties of biomaterials, can provide fundamental insights into the basic dissipative mechanisms associated with aqueous lubrication. Recent studies investigating the response of self-mated (Gemini) hydrogels to a wide range of sliding speeds under constant load conditions revealed transitions in friction behavior that may be associated with polymer relaxation time and contact time for a surface mesh during sliding (mesh size divided by the sliding speed). Here, the extent to which contact pressure and contact area affect hydrogel friction behavior was explored by changing the applied load over two orders of magnitude (0.1-20 mN) and the sliding speed over four orders of magnitude (10 m/s-100 mm/s). Oscillating pin-on-disk microtribological experiments were performed in ultrapure water for Gemini polyacrylamide hydrogels (average mesh size ~7 nm). Friction coefficient decreased across all ranges of sliding speed with increasing applied load, consistent with predictions of contact area scaling non-linearly with applied load and pressureindependent surface shear stresses. The contact area for Gemini hydrogel interfaces under these conditions has been shown to follow Hertzian contact mechanics theory, and supports the scaling of friction coefficient in the speed-independent regime that follows ~ Fn −1/3 .

Pore-size dependence and slow relaxation of hydrogel friction on smooth surfaces

“…This result is quite general and has recently been confirmed for hydrogel-hydrogel interfaces (56). Fig.…”

“…The soft hydrogel is deformed by the lubrication pressure generated in the thin film, leading to a net lift force that maintains the film thickness (35,36,39,40). Under a soft sphere on a hard, flat surface, numerical EHL calculations lead to the following expression for the film thickness h (12,56,70),…”

Uncovering friction dynamics using hydrogel particles as soft ball bearings

“…Hydrogels have become immensely popular in the past decades and these materials have, more recently, been the subject of a broad range of frictional studies. 1,[12][13][14][15] Hydrogels exhibit unique frictional behavior and friction coefficients as low as 0.01 have been reported for hydrogel-on-hydrogel friction (Fig. 1b).…”

“…Each measurement consisted of four measuring intervals, in which the rotation speed was logarithmically increased from 0.1 to 1000 rpm (approximately 0.05 to 500 mm s À1 ) in the first and third interval and decreased from 1000 to 0.1 rpm in the second and fourth interval, respectively. The duration of one Table 1 An overview of the characteristics of the hydrogel microparticle suspension preparation for suspensions varying in particle content, particle deformability (Young's modulus) and particle size interval was 300 s, a complete measurement thus lasted 1200 s. The friction coefficient (m) is determined using the frictional force (F F ) and the normal force (F N ) as shown in eqn (1).…”

“…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.…”