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

Abstract

<jats:p>Hydrogels consist of a cross-linked polymer matrix imbibed with a solvent such as water at volume fractions that can exceed 90%. They are important in many scientific and engineering applications due to their tunable physiochemical properties, biocompatibility, and ultralow friction. Their multiphase structure leads to a complex interfacial rheology, yet a detailed, microscopic understanding of hydrogel friction is still emerging. Using a custom-built tribometer, here we identify three distinct regimes of frictional behavior for polyacrylic acid (PAA), polyacrylamide (PAAm), and agarose hydrogel spheres on smooth surfaces. We find that at low velocities, friction is controlled by hydrodynamic flow through the porous hydrogel network and is inversely proportional to the characteristic pore size. At high velocities, a mesoscopic, lubricating liquid film forms between the gel and surface that obeys elastohydrodynamic theory. Between these regimes, the frictional force decreases by an order of magnitude and displays slow relaxation over several minutes. Our results can be interpreted as an interfacial shear thinning of the polymers with an increasing relaxation time due to the confinement of entanglements. This transition can be tuned by varying the solvent salt concentration, solvent viscosity, and sliding geometry at the interface.</jats:p>

Review: Friction and Lubrication with High Water Content Crosslinked Hydrogels

“…The two models were tested extensively and found to describe systems tested under moderate pressures. More recent papers by the Espinosa-Marzal group, Dunn group, and Burton group [38][39][40][41][42] expand the two-regime model into multiple regimes along a continuum of conditions such as the driving speed of the sliding component ( Fig. 1).…”

“…Rheometers have been modified to include a hydrogel disk adhered to both top and bottom plates, allowing for the center to slip in rotation [44]. Also, significant work has been done to fabricate soft hemispherical probes, constant-pressure probes, and spheres as probes [20,42]. Removing the stiff, impermeable surface from the interface can allow more direct probing of the inherent lubricating abilities of hydrogels because there is no obvious direct contact with a dissimilar countersurface.…”

“…As the sliding speed continues to increase, viscous drag of the fluid film increases, and increases friction slightly. While many groups report a hydrodynamic-like regime at high slip speeds [42,68,69], the ability to pressurize a film between porous surfaces is counteracted by the possibility of pressure-driven flow through the pores. Multiple groups now use dimensionless parameters like the Péclet number to show how competitive effects can describe transitions in friction [40,41].…”

“…If pressures are high enough, the dwell time causes pressure-driven flow, or local drainage [18,40,41,72], which increases both contact area and friction. Some groups are also beginning to consider this fluid drainage during sliding from the near-surface region in a 'hydrodynamic' sense, in which the fluid motion controls the deformation, and thus the friction response [42,[73][74][75]. Finally, the combination of speed-dependent effects and surface adhesions has been considered in detail at the nanoscale [47].…”

“…Lubricity of hydrogels has been correlated with water content [42,46]. However, when the polymer concentration [27] and/or crosslink concentration is very low, the character of the hydrogel may deviate from an elastic solid, and not have sufficient structure to hold together as a solid material.…”