|Abstract:||Hydrogels are used to mimic the slippery nature of biological interfaces. They provide a controlled material property, which helps to connect the properties to lubricating ability. While most hydrogel lubrication studies assume a steady state of sliding, a recent study discovered that under constant sliding, the friction of a hydrogel interface was influenced strongly by the history of sliding to that point. That transient lubrication led to hysteretic friction when increasing or decreasing sliding speed. In this dissertation, I propose a macroscopic model which accurately captures the lubrication behavior in simulations, and then systematically determine whether three possible transient mechanisms can cause the behavior, which are viscoelasticity, poroelasticity, and transient adhesion.
Since the lubrication hysteresis is related to transient mechanics, I established a matrix of reference data by conducting lubrication experiments at various sliding durations under continuous contact by using a tribo-rheometry setup. The sliding durations in each step used in the studies are 5 values with logarithmic increments from 9 to 90 seconds. The shape of lubrication hysteresis changed with the duration, and the relationships between the shape and duration were identified.
The hysteresis shapes are computationally simulated under the various sliding durations used in the experiments by accurately modeling the transient friction using a rheological mechanics framework. The simulation reproduced the relationships found in the experiments. Furthermore, the steady-state responses were also simulated, which agrees with results of the previous hydrogel friction studies.
To challenge the first proposed mechanism driving the lubrication behavior, viscoelasticity, surface deformation of the transparent hydrogel was visualized using Confocal Laster Scanning Microscopy. Viscoelasticity is expected to change the deformation different to the profile of frictional input. However, the visualized deformation showed a transient behavior that closely followed the friction responses, which indicates that viscoelastic effect is negligible.
The second mechanism assessed was the ability to flow within the hydrogel, and thus contribute to the lubrication behavior. Polyacrylamide hydrogels with different monomer percentage of 5, 7.5, and 10 % and the same monomer to crosslinker ratio were investigated under indentation and sliding experiments to figure out the effect of diffusion properties on the transient lubrication. I characterized the diffusion properties using indentations. The diffusion coefficients of 8, 6, and 5 (10^-10 m^2/s) are found for the 5, 7.5, and 10 % gels, respectively. However, the lubrication hysteresis shapes were the same, which showed that the diffusion property is not related to the transient lubrication.
Since the frictional behavior is not related to material characteristics including viscoelasticity and poroelasticity, I attributed the transient frictional behavior to surface interactions. The rate-and-state friction model can describe the same transient frictional behavior in terms of the changing contact populations. Based on the above results and the rate-and-state friction model, I proposed a mechanical explanation of the transient lubrication; the degree of intervention by water at the interface determines the real contact area that changes the macroscopic surface adhesion and friction. Overall, this dissertation provides new knowledge on the mechanisms that drive the complex interface mechanics of a hydrogel sliding against an impermeable surface.