Temperature-Dependent Friction of Gemini Hydrogels

Discussion and hypotheses regarding the dominant dissipation mechanisms in Gemini hydrogel sliding have often implicated viscous dissipation through shear rate and mesh size-essentially, if the dissipation is simply a result of viscous shear through a single mesh dimension at the surface, increasing viscosity, increasing sliding speed and decreasing mesh size will all increase the frictional shear. Based on the measurements of gel shrinking coupled with increasing viscosity with decreasing temperature, the expectation is that the friction will increase dramatically with decreasing temperature. Potential frictional dissipation mechanisms in hydrogel sliding were investigated by measuring friction coefficients over a range of temperature from - 20 degrees C (253 K) to + 20 degrees C (293 K). The gels were run in the Gemini configuration and were made from 7.5 wt% polyacrylamide (PAAm) formulations swollen in water. After casting, the hydrogel solvent (water) was exchanged with a 40:60 dimethyl sulfoxide (DMSO)-water solution by volume, which permitted experiments to be conducted below the freezing point of water alone (0 degrees C). The hydrogels were found to shrink as the temperature decreased, and measurements suggest that the mesh size changed from roughly xi = 16 nm at + 20 degrees C to xi = 10 nm at - 20 degrees C. The viscosity of the 40:60 DMSO-water solution was measured on a parallel plate rheometer over the range in temperature and found to increase from eta = 1 mPa-s at + 20 degrees C to eta = 12 mPa-s at - 20 degrees C. Experiments were conducted with 2-mm-radius spherical probes on flat sheets under reciprocating contact at a normal load of 2 mN, sliding speeds of either V = 10 mu m/s or V = 1 mm/s and over a temperature range of - 20 degrees C to + 20 degrees C, at 5 degrees C increments. The entire system was allowed to equilibrate at each temperature, and the friction coefficients and associated uncertainties are reported. The friction behavior at V = 10 mu m/s increased in proportion to the changes in viscosity and mesh size following the models, and had friction coefficients of mu = 0.01 at + 20 degrees C that increased to mu = 0.16 at - 20 degrees C. The high-speed sliding experiments at V = 1 mm/s were in the soft elastohydrodynamic lubrication regime, and the friction coefficient increased in proportion to the ratio of the viscosity changes, mu = 0.01 at + 20 degrees C to mu = 0.09 at - 20 degrees C. In the high-speed experiments, there was a dramatic shift in behavior from low to high friction around T = - 3 degrees C (270 K). Following the hypotheses of thermally activated friction, plots of the natural log of normalized friction (ln(mu/mu(o))) versus 1/RT were analyzed for both data sets and give activation energies of E-a = 16 +/- 2 kJ/mol at V = 1 mm/s and E-a = 20 +/- 2.5 kJ/mol at V = 10 mu m/s. These activation energies are within the range of the activation energy of the viscosity for the DMSO-water solution, E-a = 15.9 +/- 0.3 kJ/mol. Overall, these experiments clearly demonstrate that Gemini hydrogel friction increases with decreasing temperature and suggest that both viscosity and mesh size contribute to friction under direct contact sliding.