Recent highlights in nanoscale and mesoscale friction

Abstract

Friction is the oldest branch of non-equilibrium condensed matter physics and, at the same time, the least established at the fundamental level. A full understanding and control of friction is increasingly recognized to involve all relevant size and time scales. We review here some recent advances on the research focusing of nano- and mesoscale tribology phenomena. These advances are currently pursued in a multifaceted approach starting from the fundamental atomic-scale friction and mechanical control of specific single-asperity combinations, e.g., nanoclusters on layered materials, then scaling up to the meso/microscale of extended, occasionally lubricated, interfaces and driven trapped optical systems, and eventually up to the macroscale. Currently, this "hot" research field is leading to new technological advances in the area of engineering and materials science.

Keywords: atomic force microscopy; dissipation; friction; mesoscale; nanomanipulation; nanoscale; scale bridging; structural lubricity; superlubricity.

Figure 1

a) Scheme of a MoO₃ nanocrystal on MoS₂. The AFM tip is firmly positioned on top on the nanocrystal and can facilitate continuous manipulation of the structure. b) Friction of the MoO₃ nanostructure as a function of the time obtained by continuous recording of friction loops. The initial friction decreases with time (as described by the time constants) until a stationary friction level is reached. This effect can be attributed to thermolubricity related to the friction-driven temperature increase at the interface. Reprinted with permission from [65], copyright 2017 American Chemical Society.

Figure 2

a) Example of a nanomanipulation during which half of a nanoparticle is scan-imaged, before the tip pushes it out of the image frame along the fast scan axis (yellow arrow). b) Friction trace observed during the manipulation. The AFM tip makes contact with the particle at x ≈ 30 nm. The stable lateral force level observed in region II has then been used as a measure for the interfacial friction between particle and substrate. c) Dependence of friction on the contact area obtained for an ensemble of Au nanoparticles. The absolute values fall well into the range anticipated by application of scaling laws for the specific material combination. Reprinted with permission from [47], copyright 2016 Springer Nature.

Figure 3

Dependence of nanoparticle shear stress on the contact area. a) Relative shear stress obtained from MD simulations as a function of the particle radii normalized by the lattice constant d. Calculations have been performed for different shear moduli G of the particles: low values of G result in a saturation of the shear stress. Reprinted with permission from [101], copyright 2016 American Physical Society. b) Experimental data obtained for Sb nanoparticles sliding on HOPG (gray) and MoS₂ (red). While a constant decrease of shear stress with particle size is observed for the HOPG substrate, a saturating shear stress is found on MoS₂. Reprinted with permission from [100], copyright 2017 American Chemical Society.

Figure 4

Front perspective: a snapshot of a MD-simulated frictional interface between a colloidal monolayer and an optical periodic substrate potential representing the surface corrugation. Background: the overlayer/substrate lattice mismatch (an experimentally tunable parameter) generates a network of localized solitonic structures (highlighted by the particle colors), the mobility of which rules the tribological response of the monolayer.

Figure 5

a) A graphene nanoribbon manipulated along a Au(111) surface. A probing tip lifts the GNR vertically, detaches it partially, and subsequently moves it along the horizontal direction. A simultaneous measurement of the lateral forces shows that the incommensurability of the GNR–Au contact grants superlubric sliding [58]. b) The simulated static force as function of the GNR length [71]. This force is not growing with the length, but is oscillating with the periodicity of the moiré pattern. This mild dependence of friction on the contact size is characteristic of superlubric conditions. The length and orientation of a GNR are under direct experimental control: Experiments are also consistent with friction not systematically increasing with the GNR length. c) The moiré pattern of GNR in the orientation [1−21] (R0) and [−101] (R30) over the Au(111) surface. Experimentally, R30 is preferred and exhibits the smallest lateral forces. Panels b) and c) are adapted from [71].

Figure 6

Single-molecule tribology. a) Schematic drawing of the experiment: A single porphyrin molecule is attached to the AFM apex and dragged over a Cu(111) surface. b) By recording the mechanical response of the sliding molecule, the AFM scan maps the atomic lattice of Cu(111). c) Tip–sample stiffness trace extracted from the image showing a stick–slip modulation. Reprinted with permission from [183], copyright 2016 American Chemical Society.

Figure 7

Non-contact friction experiments of NbSe₂. At certain voltages and distances, one finds dramatically increased non-contact friction. This is related to the local disturbance of the charge-density wave, which leads to phase slips. a) Schematics of the probing tip above the charge-density wave system. b) STM image of the NbSe₂-surface revealing the CDW. c) Non-contact friction dissipation as a function of distance and voltage. Reprinted with permission from [188], copyright 2013 Springer Nature.

Figure 8

The “Swiss Nanodragster” (SND), a 4’-(p-tolyl)-2,2’:6’,2”-terpyridine molecule, was moved across an Au(111) surface on the occasion of the first nanocar race held in Toulouse in April 2017. The required distance of 100 nm of controlled motion was covered through the application of voltage pulses. a) Schematics of the manipulation of the molecule. Left inset: Structure of the SND molecule. Right inset: High-resolution AFM image of the SND molecule. b) A sequence of manipulation steps, as observed by STM imaging between the manipulation steps. Reprinted with permission from [59], copyright 2017 American Chemical Society.

Figure 9

Example of a change of conformation potentially triggered at surfaces. a) Trans (1) and cis (2) isomers. By depositing and/or annealing, the molecule can be turned from trans to cis and vice versa. b) After the deposition of Fe atoms, the molecules can be switched from trans into cis conformation.