Polymer fluctuation lubrication in hydrogel gemini interfaces

Abstract

Interfacial sliding speed and contact pressure between the sub-units of particulate soft matter assemblies can vary dramatically across systems and with dynamic conditions. By extension, frictional interactions between particles may play a key role in their assembly, global configuration, collective motion, and bulk material properties. For example, in tightly packed assemblies of microgels - colloidal microspheres made of hydrogel - particle stiffness controls the fragility of the glassy state formed by the particles. The interplay between particle stiffness and shear stress is likely mediated by particle-particle normal forces, highlighting the potential role of hydrogel-hydrogel friction. Here we study friction at a twinned "Gemini" interface between hydrogels. We construct a lubrication curve that spans four orders of magnitude in sliding speed, and find qualitatively different behaviour from traditional lubrication of engineering material surfaces; fundamentally different types of lubrication occur at the hydrogel Gemini interface. We also explore the role played by polymer solubility and hydrogel-hydrogel adhesion in hydrogel friction. We find that polymer network elasticity, mesh size, and single-chain relaxation times can describe friction at the gel-gel interface, including a transition between lubrication regimes with varying sliding speed.

Mesh Size Control of Polymer Fluctuation Lubrication in Gemini Hydrogels

“…Gemini hydrogel interfaces can provide exceptionally low friction coefficients under conditions traditionally not thought to promote lubrication, namely, low contact pressure and low sliding speed [16,37]. In the series of experiments described here, the samples with the largest mesh size (ξ = 9.4 ± 1.1 nm) exhibited the lowest measured friction coefficients (µ ~ 0.005), and maintained this behavior over a range of sliding speeds from V s = 30-1,000 µm s -1 .…”

“…In previous work on Gemini hydrogel friction, we found that the lubrication curve differed dramatically from the classical engineering Stribeck curve in several major ways [16,37]. First, at slow sliding speeds, where the effects of hydrodynamic lubrication are negligible, Gemini hydrogel friction is actually lowest.…”

“…By this analysis, the Gemini hydrogel interface had a contact pressure of ~3 kPa. Effective contact modulus for each of the five hydrogel samples was calculated for Gemini interfaces from force-displacement curves using the Johnson-Kendall-Roberts (JKR) theory as described in Pitenis et al The moduli ranged between 1.5-120 kPa, comparable to values found in literature [37,42].…”

“…Friction measurements were performed on a high-speed, unidirectional, pin-on-disk microtribometer illustrated in Figure 3a and described in Pitenis et al [37]. The PAAm hydrogel probe was molded onto a 4-40 stainless steel set screw and fastened onto a titanium double flexure cantilever assembly with a normal stiffness of 161 µN/µm and a lateral stiffness of 75 µN/µm.…”

“…Hydrogels are crosslinked networks of polymer chains that are swollen with water; a representative flexible polymeric hydrogel network is illustrated in Figure 1. Hydrogels are water-permeable materials, which can be easily created with varying mesh size (), water content, permeability, and elastic properties [15][16][17][18].…”