Some fundamental differences in the adhesion and friction of rough versus smooth surfaces

Augmented Incremental Potential Contact for Sticky Interactions

We introduce a variational formulation for simulating sticky interactions between elastoplastic solids. Our method brings a wider range of material behaviors into the reach of the Incremental Potential Contact (IPC) solver recently developed by (Li et al. 2020). Extending IPC requires several contributions. We first augment IPC with the classical Raous-Cangemi-Cocou (RCC) adhesion model. This allows us to robustly simulate the sticky interactions between arbitrary codimensional-0, 1, and 2 geometries. To enable user-friendly practical adoptions of our method, we further introduce a physically parametrized, easily controllable normal adhesion formulation based on the unsigned distance, which is fully compatible with IPC's barrier formulation. Furthermore, we propose a smoothly clamped tangential adhesion model that naturally models intricate behaviors including debonding. Lastly, we perform benchmark studies comparing our method with the classical models as well as real-world experimental results to demonstrate the efficacy of our method.

Nonmonotonic Friction due to Water Capillary Adhesion and Hydrogen Bonding at Multiasperity Interfaces

Capillary adhesion due to water adsorption from the air can contribute to friction, especially for smooth interfaces in humid environments. We show that for multiasperity (naturally oxidized) Si-on-Si interfaces, the friction coefficient goes through a maximum as a function of relative humidity. An adhesion model based on the boundary element method that takes the roughness of the interfaces into account reproduces this nonmonotonic behavior very well. Remarkably, we find the dry friction to be significantly lower than the lubricated friction with macroscopic amounts of water present. The difference is attributed to the hydrogen-bonding network across the interface. Accordingly, the lubricated friction increases significantly if the water is replaced by heavy water (D_{2}O) with stronger hydrogen bonding.

Progressive Molecular Rearrangement and Heat Generation of Amorphous Polyethene Under Sliding Friction: Insight from the United-Atom Molecular Dynamics Simulations

Frictional heat has been widely used in various polymer-based advanced manufacturing. The fundamental understanding of the thermodynamics of the interfacial friction of polymer bulk materials can help to avoid compromising the process controllability. In this work, we have performed united-atom molecular dynamics (MD) simulations to reveal the interfacial friction heating mechanism of amorphous polyethene (PE) in both the single sliding friction (SSF) and reciprocating sliding friction (RSF) modes. Different from the traditional view that the plastic deformation was the primary source of heat generation, the RSF process with no apparent plastic deformation in this work shows a better heat generation performance than SSF, where plastic deformation dominated the friction process. Our analysis uncovers that the mechanism of the interfacial friction heating enhancement in RSF can be attributed to the concentrated high-frequency chain motion related to molecular rearrangement, which is not clearly related to the deformation degree.

Effects of Imprinted 3D Surface Patterning on Localized Changes in the Tribology of Human Stratum Corneum

Natural surfaces may exhibit remarkable surface properties due to their structure. In the case of skin, its surface topography (microrelief) influences many of its perceived sensorial properties (shine, color, touch). Imprinted patterns can modify the original microrelief, inducing a completely new set of perceived properties. To explore the effects of superimposed biomimetic surface textures on the friction of skin, human stratum corneum was prepared with and without an imprinted regular, micrometer-sized, 3D grid pattern. Atomic Force Microscopy (AFM) and optical profilometry indicated that the inherent, smaller-scale roughness of the stratum corneum remained when lines with heights of 20-200 μm and spacings of 600-2000 μm were introduced, but it was somewhat reduced on the grid lines. Surface Forces Apparatus (SFA) friction experiments on stratum corneum were performed at low speed (μm/s, back-and-forth sliding) and at more realistic, high speed (cm/s, rotational sliding). Two stratum corneum surfaces in contact did not adhere to one another, and they had a friction coefficient μ of 0.1, or lower, at low sliding speed. An interesting loading-unloading hysteresis was observed, with lower friction force on unloading, in particular, when the contact was on a grid line of the patterned samples. This suggests that the patterning locally induced different mechanical properties of the stratum corneum and that its recovery was not immediate on unloading. When one stratum corneum surface slid against a rigid glass surface, the friction coefficient was always higher than that when two stratum corneum surfaces were in contact. At high sliding speed, much higher friction coefficients were found between one stratum corneum surface and a rigid, smooth surface, μ ≥ 1. The results demonstrate that topograpic patterning by imprinting clearly modifies the tribological response of stratum corneum. This approach provides a simple method for exploring the development of biomimetic modifications of skin texture.

Conformal adhesion enhancement on biomimetic microstructured surfaces

Inspired by the superior adhesive ability of the gecko foot pad, we report an experimental study of conformal adhesion of a soft elastomer thin film on biomimetic micropatterned surfaces (micropillars), showing a remarkable adhesion enhancement due to the surface patterning. The adhesion of a low-surface-energy poly(dimethylsiloxane) tape to a SU-8 micropatterned surface was found be able to increase by 550-fold as the aspect ratio increases from 0 to 6. The dependency of the adhesion enhancement on the aspect ratio is highly nonlinear. A series of peeling experiment coupled with optical interference imaging were performed to investigate the adhesion enhancement as a function of the height of the micropillars and the associated delamination mechanisms. Local elastic energy dissipation, side-wall friction, and plastic deformations were analyzed and discussed in terms of their contributions to the adhesion enhancement. We conclude that the local adhesion and friction events of pulling micropillars out of the embedded polymer film play a primary role in the observed adhesion enhancement. The technical implications of this local friction-based adhesion enhancement mechanism were discussed for the effective assembly of similar or dissimilar material components at small scales. The combined use of the micro/nanostructured surfaces with the van der Waals interactions seem to be a potentially more universal solution than the conventional adhesive bonding technology, which depends on the chemical and viscoelastic properties of the materials.

Molecular simulation of the frictional behavior of polymer-on-polymer sliding

Molecular simulations of the sliding processes of polymer-on-polymer systems were performed to investigate the surface and subsurface deformations and how these affect tribological characteristics of nanometer-scale polymer films. It is shown that a very severe deformation is localized to a band of material about 2.5 nm thick at the interface of the polymer surfaces. Outside of this band, the polymer films experience a uniform shear strain that reaches a finite steady-state value of close to 100%. Only after the polymer films have achieved this steady-state shear strain do the contacting surfaces of the films show significant relative slippage over each other. Because severe deformation is limited to a localized band much thinner than the polymeric films, the thickness of the deformation band is envisaged to be independent of the film thickness and hence frictional forces are expected to be independent of the thickness of the polymer films. A strong dependency of friction on interfacial adhesion, surface roughness, and the shear modulus of the sliding system was observed. Although the simulations showed that frictional forces increase linearly with contact pressure, adhesive forces contribute significantly to the overall friction and must therefore be accounted for in nanometer-scale friction. It is also shown that the coefficient of friction is lower for lower-density polymers as well as for polymers with higher molecular weights.

Effect of surface pattern on the adhesive friction of elastomers

We present experimental results for the friction of a flat surface against a hexagonally patterned surface, both being made of PolyDiMethylSiloxane. We simultaneously measure forces of range 10 mN and observe the contact under sliding velocities of about 100 μm/s. We observe adhesive friction on three different pattern heights (80, 310, and 2100 nm). Two kinds of contacts have been observed: the flat surface is in close contact with the patterned one (called intimate contact, observed for 80 nm) or only suspended on the tops on the asperities (called laid contact, observed for 2100 nm). In the range of velocities used, the contact during friction is similar to the static one. Furthermore, our experimental system presents a contact transition during friction for h=310 nm.

Properties of confined and sheared rhodamine B films studied by SFA-FECO spectroscopy

We have used a surface forces apparatus and multiple beam interferometry to measure the absorbance of thin films of rhodamine B in water/ethylene glycol solutions while applying and measuring normal and lateral (shear) forces. Both normal and shear forces induced changes in the absorption spectra indicating a change in molecular alignment, and rhodamine-rhodamine and rhodamine-surface interactions. We also measured differences in the absorbance spectra in different regions of the contact indicating, as expected, that the stresses are not uniform throughout the contact area. We also observed crystallization (solidification) parallel to the shearing direction.

Lubrication and wear properties of grafted polyelectrolytes, hyaluronan and hylan, measured in the surface forces apparatus

Hyaluronan is believed to have an important function in the boundary biolubrication of articular cartilage. Using a Surface Forces Apparatus, we tested the tribological properties of surface bound, rather than "free" hyaluronan. The grafting process of the polyelectrolyte included either a biological route via an HA-binding protein or a chemical reaction to covalently bind the polymer to a lipid bilayer coated surface. In another reaction, we constructed a surface with covalently grafted hylan (crosslinked hyaluronan). We studied the normal and shear forces between these surfaces. None of the systems demonstrated comparable lubrication to that found between cartilage surfaces except at very low loads. Both grafted hyaluronan and hylan generated coefficients of friction between 0.15 and 0.3. Thus, the polysaccharide, which is a constituent of the lamina splendens (outermost cartilage layer), is not expected to be the responsible molecule for the great lubricity of cartilage; however, it may contribute to the load bearing and wear protection of these surfaces. This was concluded from the results with hylan, where a thin gel layer was sufficient to shield the underlying surfaces from damage even at applied pressures of over 200 atmospheres during shear. Our study shows that a low coefficient of friction is not a requirement for, or necessarily a measure of, wear protection.