Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale, they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance. Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments. We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories of friction can be applied at the nanoscale.

When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, “softness,” designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively.

We report here the first measurements of the complex modulus of the isolated red blood cell (RBC). Because the RBC is often larger than capillary diameter, important determinants of microcirculatory function are RBC deformability and its changes with pathologies, such as sickle cell disease and malaria. A functionalized ferrimagnetic microbead was attached to the membrane of healthy RBC and then subjected to an oscillatory magnetic field. The resulting torque caused cell deformation. From the oscillatory forcing and resulting bead motions, which were tracked optically, we computed elastic and frictional moduli, g' and g", respectively, from 0.1 to 100 Hz. The g' was nearly frequency independent and dominated the response at all but the highest frequencies measured. Over three frequency decades, g" increased as a power law with an exponent of 0.64, a result not predicted by any simple model. These data suggest that RBC relaxation times that have been reported previously, and any models that rest upon them, are artifactual; the artifact, we suggest, arises from forcing to an exponential fit data of limited temporal duration. A linear range of response was observed, but, as forcing amplitude increased, nonlinearities became clearly apparent. A finite element model suggests that membrane bending was localized to the vicinity of the bead and dominated membrane shear. While the mechanisms accounting for these RBC dynamics remain unclear, methods described here establish new avenues for the exploration of connections among the mechanical, chemical, and biological characteristics of the RBC in health and disease.

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Polymer nanocomposite films (PNCFs) with extremely high concentrations of nanoparticles are important components in energy storage and conversion devices and also find use as protective coatings in various applications. PNCFs with high loadings of nanoparticles, however, are difficult to prepare because of the poor processability of polymer-nanoparticle mixtures with high concentrations of nanoparticles even at an elevated temperature. This problem is exacerbated when anisotropic nanoparticles are the desired filler materials. Here we report a straightforward method for generating PNCFs with extremely high loadings of nanoparticles. Our method is based on what we call capillary rise infiltration (CaRI) of polymer into a dense packing of nanoparticles. CaRI consists of two simple steps: (1) the preparation of a two-layer film, consisting of a porous layer of nanoparticles and a layer of polymer and (2) annealing of the bilayer structure above the temperature that imparts mobility to the polymer (e.g., glass transition of the polymer). The second step leads to polymer infiltration into the interstices of the nanoparticle layer, reminiscent of the capillary rise of simple fluid into a narrow capillary or a packing of granules. We use in situ spectroscopic ellipsometry and a three-layer Cauchy model to follow the capillary rise of polystyrene into the random network of nanoparticles. The infiltration of polystyrene into a densely packed TiO2 nanoparticle layer is shown to follow the classical Lucas-Washburn type of behaviour. We also demonstrate that PNCFs with densely packed anisotropic TiO2 nanoparticles can be readily generated by spin coating anisotropic TiO2 nanoparticles atop a polystyrene film and subsequently thermally annealing the bilayer film. We show that CaRI leads to PNCFs with modulus, hardness and scratch resistance that are far superior to the properties of films of the component materials. In addition, CaRI fills in cracks that may exist in the nanoparticle layer, leading to the healing of nanoparticle films and the formation of defect-free PNCFs. We believe this approach is widely applicable for the preparation of PNCFs with extremely high loading of nanoparticles and potentially provides a unique approach to study capillarity-induced transport of polymers under extreme confinement.

Atomic force microscopy (AFM) is a powerful tool for studying tribology (adhesion, friction, and lubrication) at the nanoscale and is emerging as a critical tool for nanomanufacturing. However, nanoscale wear is a key limitation of conventional AFM probes that are made of silicon and silicon nitride (SiNx). Here we present a method for systematically quantifying tip wear, which consists of sequential contact-mode AFM scans on ultrananocrystalline diamond surfaces with intermittent measurements of the tip properties using blind reconstruction, adhesion force measurements, and transmission electron microscopy (TEM). We demonstrate direct measurement of volume loss over the wear test and agreement between blind reconstruction and TEM imaging. The geometries of various types of tips were monitored over a scanning distance of approximately 100 mm. The results show multiple failure mechanisms for different materials, including nanoscale fracture of a monolithic Si tip upon initial engagement with the surface, film failure of a SiNx-coated Si tip, and gradual, progressive wear of monolithic SiNx tips consistent with atom-by-atom attrition. Overall, the method provides a quantitative and systematic process for examining tip degradation and nanoscale wear, and the experimental results illustrate the multiple mechanisms that may lead to tip failure.

Biological materials achieve directional reinforcement with oriented assemblies of anisotropic building blocks. One such example is the nanocomposite structure of keratinized epithelium on the toe pad of tree frogs, in which hexagonal arrays of (soft) epithelial cells are crossed by densely packed and oriented (hard) keratin nanofibrils. Here, a method is established to fabricate arrays of tree-frog-inspired composite micropatterns composed of polydimethylsiloxane (PDMS) micropillars embedded with polystyrene (PS) nanopillars. Adhesive and frictional studies of these synthetic materials reveal a benefit of the hierarchical and anisotropic design for both adhesion and friction, in particular, at high matrix–fiber interfacial strengths. The presence of PS nanopillars alters the stress distribution at the contact interface of micropillars and therefore enhances the adhesion and friction of the composite micropattern. The results suggest a design principle for bioinspired structural adhesives, especially for wet environments.

This letter describes the physics and application of an approach to transfer printing that utilizes targeted shear loading to modulate stamp adhesion in a controlled and repeatable fashion. Experimental measurements of pull-off forces as functions of shear and stamp dimension reveal key scaling properties and provide a means for comparison to theory and modeling. Examples of printed structures in suspended and multilayer configurations demonstrate some capabilities in micro/nanoscale materials assembly.