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1. Meso-scale dislocations and friction of shape-complementary soft interfaces

   https://doi.org/10.1098/rsif.2020.0940  

   He, Zhenping; Liu, Zezhou; Li, Meng; Hui, Chung-Yuen; Jagota, Anand (February 2021, Journal of The Royal Society Interface)

   null (Ed.)

   The interface between two surfaces patterned with complementary shapes such as arrays of ridge–channel structures or pillars accommodates relative misorientation and lattice mismatch by spontaneous production of dislocation arrays. Here, we show that the relative sliding of such an interface is accomplished by dislocation glide on the interfacial plane. An exception is the singular case where the lattices are perfectly matched across the sample dimension, in which case sliding is accompanied by motion of edge-nucleated defects. These are meso-scale analogues of molecular sliding friction mechanisms between crystalline interfaces. The dislocations, in addition to the long-range elastic energy associated with their Burgers vectors, also cause significant out-of-plane dilation, which props open the interface locally. For this reason, the sliding friction is strongly pressure dependent; it also depends on the relative orientation of the patterns. Sliding friction can be strongly enhanced compared with a control, showing that shape-complementary interfaces can be engineered for strongly enhanced pressure- and orientation-dependent frictional properties in soft solids. 

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2. Sliding friction of a pillar array interface: part II, contact mechanics of single pillar pairs

   https://doi.org/10.1039/D3SM01324C  

   Xiao, Xuemei; Kaur, Jasreen; Zhu, Bangguo; Jagota, Anand; Hui, Chung-Yuen (February 2024, Soft Matter)

   Insects and small animals often utilize structured surfaces to create friction during their movements. These surfaces typically consist of pillar-like fibrils that interact with a counter surface. Understanding the mechanical interaction between such surfaces is crucial for designing structured surfaces for engineering applications. In the first part of our study, we examined friction between poly(dimethylsiloxane) (PDMS) samples with surfaces patterned with pillar-arrays. We observed that sliding between these surfaces occurs through the interfacial glide of dislocation structures. The frictional force that resists this dislocation glide is a result of periodic single pillar-pillar contact and sliding. Hence, comprehending the intricate interaction between individual pillar contacts is a fundamental prerequisite for accurately modeling the friction behavior of the pillar array. In this second part of the study, we thoroughly investigated the contact interaction between two pillars located on opposite sides of an interface, with different lateral and vertical offsets. We conducted experiments using PDMS pillars to measure both the reaction shear and normal forces. Contact interaction between pillars was then studied using finite element (FE) simulations with the Coulomb friction model, which yielded results that aligned well with the experimental data. Our result offers a fundamental solution for comprehending how fibrillar surfaces contact and interact during sliding, which has broad applications in both natural and artificial surfaces. 

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3. The evolution of rock friction is more sensitive to slip than elapsed time, even at near-zero slip rates

   https://doi.org/10.1073/pnas.2119462119  

   Bhattacharya, Pathikrit; Rubin, Allan M.; Tullis, Terry E.; Beeler, Nicholas M.; Okazaki, Keishi (July 2022, Proceedings of the National Academy of Sciences)

   Nearly all frictional interfaces strengthen as the logarithm of time when sliding at ultra-low speeds. Observations of also logarithmic-in-time growth of interfacial contact area under such conditions have led to constitutive models that assume that this frictional strengthening results from purely time-dependent, and slip-insensitive, contact-area growth. The main laboratory support for such strengthening has traditionally been derived from increases in friction during “load-point hold” experiments, wherein a sliding interface is allowed to gradually self-relax down to subnanometric slip rates. In contrast, following step decreases in the shear loading rate, friction is widely reported to increase over a characteristic slip scale, independent of the magnitude of the slip-rate decrease—a signature of slip-dependent strengthening. To investigate this apparent contradiction, we subjected granite samples to a series of step decreases in shear rate of up to 3.5 orders of magnitude and load-point holds of up to 10,000 s, such that both protocols accessed the phenomenological regime traditionally inferred to demonstrate time-dependent frictional strengthening. When modeling the resultant data, which probe interfacial slip rates ranging from 3 . μ m · s − 1 . to less than 10 − 5 μ m · s − 1 , we found that constitutive models where low slip-rate friction evolution mimics log-time contact-area growth require parameters that differ by orders of magnitude across the different experiments. In contrast, an alternative constitutive model, in which friction evolves only with interfacial slip, fits most of the data well with nearly identical parameters. This leads to the surprising conclusion that frictional strengthening is dominantly slip-dependent, even at subnanometric slip rates. 

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4. Dynamic Fluid‐Like Graphene with Ultralow Frictional Molecular Bearing

   https://doi.org/10.1002/adma.201903195  

   Jeon, Intak; Park, Gee_Hoon; Wang, Pan; Li, Ju; Hunter, Ian_W; Swager, Timothy_M (September 2019, Advanced Materials)

   Abstract Fluid‐like sliding graphenes but with solid‐like out‐of‐plane compressive rigidity offer unique opportunities for achieving unusual physical and chemical properties for next‐generation interfacial technologies. Of particular interest in the present study are graphenes with specific chemical functionalization that can predictably promote adhesion and wetting to substrate and ultralow frictional sliding structures. Lubricity between stainless steel (SS) and diamond‐like carbon (DLC) is experimentally demonstrated with densely functionalized graphenes displaying dynamic intersheet bonds that mechanically transform into stable tribolayers. The macroscopic lubricity evolves through the formation of a thin film of an interconnected graphene matrix that provides a coefficient of friction (COF) of 0.01. Mechanical sliding generates complex folded graphene structures wherein equilibrated covalent chemical linkages impart rigidity and stability to the films examined in macroscopic friction tests. This new approach to frictional reduction has broad implications for manufacturing, transportation, and aerospace. 

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5. Effect of Atomic Corrugation on Adhesion and Friction: A Model Study with Graphene Step Edges

   https://doi.org/10.1021/acs.jpclett.9b02501  

   Chen, Zhe Vazirisereshk (January 2020, The journal of physical chemistry letters)

   This Letter reports that the atomic corrugation of the surface can affect nanoscale interfacial adhesion and friction differently. Both atomic force microscopy (AFM) and molecular dynamics (MD) simulations showed that the adhesion force needed to separate a silica tip from a graphene step edge increases as the side wall of the tip approaches the step edge when the tip is on the lower terrace and decreases as the tip ascends or descends the step edge. However, the friction force measured with the same AFM tip moving across the step edge does not positively correlate with the measured adhesion, which implies that the conventional contact mechanics approach of correlating interfacial adhesion and friction could be invalid for surfaces with atomic-scale features. The chemical and physical origins for the observed discrepancy between adhesion and friction at the atomic step edge are discussed. 

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