Session V02: Polymers and Soft Solids at Interfaces: Tribology, Wear, Rheology and Interactions

Live

Small scale contact between a soft, liquid-coated layer and a stiff surface is found in many situations, from synovial fluid on articular cartilage to adhesives in humid environments. Moreover, many model studies on soft adhesive contacts are conducted with soft silicone elastomers, which usually possess uncrosslinked molecules (i.e. liquid silicone oil), especially for soft commercially available kits. The presence of liquid near the contact line can cause capillary forces on the particle. We consider a similar situation in where silicone oil is already coated onto the surface prior to making contact with a microparticle. More specifically, we investigate how the thickness of the oil layer relates to the indentation depth of a glass microsphere on a soft PDMS network. A simple model that balances the capillary force of the oil layer with the elastic and surface forces from the substrate is proposed to predict the position of the particle.   

Live

Soft and porous materials are present in a variety of contexts, from physiological or pharmaceutical applications to fundamental physics of interfacial soft matter, while including the example of nanoscale polymer layers. An intermediate layer of a soft material, present between a liquid phase and a solid boundary, can induce remarkable changes in the lubrication mechanics, for example for the motion of a colloidal particle submitted to a given force near this very particular interface . Describing the motility of a microscopic particle in such a context, and adding the porosity of the nearby surface to the elastohydrodynamic coupling, represents a relatively unexplored field. Here, we establish theoretical models and perform numerical simulations for the particle motion. Preliminary experiments with Surface Force Apparatus (SFA) on PNIPAM hydrogel samples are also presented, and applications to Atomic Force Microscopy (AFM) are discussed.   

Live

Hydrogels are cross-linked polymer networks that can absorb and retain a large fraction of water, often up to 90 percent by weight. They are widely used in many engineering applications as well as agriculture industries due to their ultralow friction, biocompatibility, and chemical transport capabilities. Previous research in our lab has identified and characterized three distinct regimes of friction in polyacrylamide (PAAm), polyacrylic acid (PAA), and agarose hydrogels on smooth surfaces. Most striking, near a critical velocity, the friction coefficient decreases by an order of magnitude and displays relaxation over multiple timescales. Here we examine this regime in closer detail for PAA hydrogels. We find that the frictional relaxation time decreases exponentially with sliding velocity. Additionally, any pre-shearing of the hydrogel will induce changes in the relaxation time prior to experiments, and if left to rest, the hydrogel friction will return to its original state. This behavior is qualitatively consistent with model of two superimposed relaxation processes: a velocity-dependent shearing and extension of entangled polymer chains, and a constant relaxation timescale from thermal fluctuations. However, we also show that the sliding interface can retain a “memory” of its initial sliding direction, a memory that persists over 24 hours of continuous experiments.   

Live

The motion of a particle in the vicinity of a soft interface gives rise to elastohydrodynamic forces, due to the coupling between the flow induced by the motion and the deformation of the soft interface. One example is the emergent non-inertial lift force that repels on a particle sliding near a soft surface from the latter. We perform asymptotic calculations of the elastohydrodynamic interactions between a spherical particle and a soft boundary (in the small-deformation limit), generalizing the expressions known for an infinite cylinder. This allows for direct comparison with experiments involving spherical colloids. In particular, we present experimental measurements at the nanoscale of the elastohydrodynamic lift force, using an Atomic Force Microscope in quantitative agreement with the derived asymptotic theory.   

Live

High-water-content hydrogels are being increasingly explored for applications in biomedicine, water filtration, and hybrid materials. They are generally known to be lubricious, or very low friction, but their friction exhibits time dependence and history dependence related to their response under stress. The response to surface stress for a dilute or semi-dilute hydrogel arises in part from how water assists with supporting and distributing the load; this is inherently linked to the osmotic pressure holding the water in the bulk. Here I present two vignettes illustrating this connection. The first considers the competitive rates of surface slip and pressure-driven dehydration due to applied loads. Given initial measurements of friction at very low and very high speeds, the ratio of the timescales of these effects can predict friction along the intermediate spectrum. The second vignette describes the inherent generation of less-dense surface layers (~single microns) that arise from the bulk due to the discontinuity of osmotic pressure between the bulk and the open bath submerging a crosslinked hydrogel. These layers are confirmed with multiscale indentation techniques and material creep localized to the near-surface region.   

Live

Two sliding frictional surfaces can display a strong intermittent response known as stick-slip, characterized by a succession of loading phases, interrupted by sudden macroscopic slip events. The static friction coefficient describes the onset of these events; however, it is now known not to be a well-defined material constant [1]. A stochastic nature of this friction coefficient has recently been suggested, with macroscopic slips triggered by microscopic avalanches. Such small-scale events act as scars where inertia leads to local weakening; when a critical size is reached, a slip-event propagates along the interface [2]. We use experiments and simulations to investigate the role of inertial interactions through bulk elasticity of a frictional layered system. Specifically, we investigate a system of stacked plates with frictional interfaces forced to slide simultaneously. We observe that the statistical properties of the stick-slip events in the multilayered set-up deviate from that of a single layer, suggesting a nontrivial coupling across the stack. This work may bring a better understanding of the seismic behavior of layered geological formations.

[1] Ben-David, O., & Fineberg, J. (2011). PRL, 106(25), 254301.
[2] de Geus, T. W. J, et al. (2019). PNAS, 116(48), 23977-23983.   

Live

Frictional properties of materials in contact depend on an array of parameters including surface roughness, normal force, and sliding speed. For soft materials, bulk properties such as the elastic modulus influence the friction coefficient as well. The friction coefficient is therefore not fixed for a specific combination of materials but varies with each parameter, making the friction coefficient a system parameter. We use a 3D-printed tribometer and measure the friction coefficient of 4 hydrogels: polyacrylamide, physically and chemically cross-linked gelatin, and agar. For these hydrogels, we find two normal force-dependent frictional regimes caused by the flattening of surface asperities. Imaging of these asperities using cryo-SEM revealed that the asperities are around 1 µm in size. The nature of the asperities and the friction coefficient depend on the type of polymer network. We vary the asperity size using 3D-printed molds and find that the friction coefficient in the first regime changes with surface roughness. The friction coefficient in the second regime is constant for different asperity sizes and varies with polymer type. Our findings highlight the importance of surface properties and bulk properties in soft material friction.   

Live

Pickering emulsions are used over surfactant stabilized emulsions in a wide range of applications including personal care products, pharmaceuticals, and oil recovery because of their enhanced stability and lower toxicity. We examine here the use of nanodiamonds (ND), a relatively novel carbonaceous filler with high adsorption activity, small size, and large surface area to create solid stabilized emulsions. Using a system consisting of isopropyl palmitate and water, stabilized by hydroxylated NDs, we investigate the stability, rheology and frictional behavior of these emulsions as a function of ND concentration. Optical microscopy reveals increasing ND concentration results in smaller droplet sizes, due to the greater availability of particles that can be adsorbed on the oil-water interface. This behavior is consistent with our rheological results of higher G’ and yield stress with increased ND, as the presence of smaller droplets facilitate the formation of a densely packed network. Microstructure recovery after breakdown at different stress levels correlate with the ratio of applied stress to the yield stress for each ND concentration. Tribological behavior of ND emulsions was also investigated using a soft model contact and the data related to the rheological characteristics.   

Live

Soft tribology is of increasing importance given the usage of deformable materials in robotics, haptics, and biomechanics studies. The introduction of microscale patterns to soft surfaces in fully flooded conditions changes the elastohydrodynamic lubrication (EHL) friction. We previously showed that soft patterns introduce local friction peaks at the micro- to macro-EHL transition, μ_c. Despite these advances, the effect of pattern bending on frictional behavior is not well understood. To investigate the influence of bending on EHL friction, we fabricate elastomer patterns consisting of stripes of varying height-to-width aspect ratios c/a. The space between the stripes is held constant. We find that when c/a < 0.75, the aspect ratio has no influence on μ_c while c/a > 0.75 results in a power-law decrease in μ_c. To explain this phenomenon, we employ cantilever beam theory and empirical statistical regression. Both models show that the reduction in μ_c is a result of larger bending angles of the stripes as their aspect ratios increase. These results suggest that significant bending of tall and slender patterns leads to a reduction in the effective load-bearing surface area of the stripes in the micro-EHL regime.   

Live

Soft tribology is used as an analogue for mechanical movement of lubricated biological surfaces such as those encountered in oral processing and cartilage motion, which has been found to be useful in the design of food, cosmetics and biomedical devices. However, these applications often incorporate multiphase fluid with complex rheological behaviour, where the tribological response deviates from the typical Stribeck curve. It is hypothesised that oil ‘plating out’ effect at a critical speed is dependent on the rheology of emulsion. We studied soft lubrication of model emulsions with oil droplets dispersed in non-Newtonian hydrocolloid solutions designed to be iso-viscous at low- and high-shear rates respectively. The switchover from oil- to aqueous-dominated tribological response is observed at a critical speed for emulsions with an oil: aqueous (high-shear) viscosity ratio above 4. Above this limit, the critical speed for oil-dominated contacts is strongly dependent on both low- and high-shear viscosity. Below the limit, lubrication of emulsions follows the behaviour of non-Newtonian aqueous phase. This work enables development of a framework explaining the drivers for deviation from the Stribeck curve for complex multiphase fluids.   

On Demand

While much research has strived to understanding the dynamics of confined liquids by rigid solid surface, the behaviors of liquids confined between compliant surfaces remains unclear. In this work, we employed poly(N-isopropylacrylamide) (PNIPAM) microgels of varied crosslinking density as the confined liquids as well as confining surface coatings. We analyzed the packing structure and mobility of confined PNIPAM microgel particles between PNIPAM coated surfaces of varied particle-to-surface compliance ratio and gap spacings. We observed that confining surfaces of low compliance show much weaker effect on the dynamics of confined microgel particles of varied elasticity, where confined microgels at the gap spacing approximately smaller than 10 particle layers exhibit little deviation from their bulk behaviors. Intriguingly, we observed that no confinement effect is observed when the compliance of microgel particles is equal to or comparable to that of confining surfaces. Hence,commensurate compliance between confining liquid and surface could give new insight to design low-friction lubricants and lubricious coatings.