Lubricated friction around nanodefects

Abstract

The lubrication properties of nanoconfined liquids underpin countless natural and industrial processes. However, our current understanding of lubricated friction is still limited, especially for nonideal interfaces exhibiting nanoscale chemical and topographical defects. Here, we use atomic force microscopy to explore the equilibrium and dynamical behavior of a model lubricant, squalane, confined between a diamond tip and graphite in the vicinity of an atomic step. We combine high-resolution imaging of the interface with highly localized shear measurements at different velocities and temperatures to derive a quantitative picture of the lubricated friction around surface defects. We show that defects tend to promote local molecular order and increase friction forces by reducing the number of stable molecular configurations in their immediate vicinity. The effect is general, can propagate over hundreds of nanometers, and can be quantitatively described by a semiempirical model that bridges the molecular details and mesoscale observations.

Impact of water on the lubricating properties of hexadecane at the nanoscale

“…64,65 Additionally, water molecules create surface singularities where the AFM tip can be subject to pinning. 66 Here, our AFMbased experimental setup allows for precise quantification of the interface's lubrication properties with nanometre lateral resolution. This makes it possible to directly compare areas of uniform hexadecane-mica interface with regions where water nanodroplets are present within a same experiment.…”

“…12 To confirm this hypothesis, we conducted shear measurements at different temperatures. In pure liquids, the shear force usually decreases with increasing temperature 28,66 and the lubricant tends to become more viscous (shear phase closer to 90°). Here we expect more nanodroplets to nucleate at the interface when the temperature is raised, hence inducing an increase in shear force.…”

“…Further studies are needed to test the present findings on systems closer to real-life applications such as the metal oxides surfaces, 14,19 routinely used in engines and machinery. At the nanoscale, lubricated friction is likely to result from the combined effects of the molecular arrangement around hydrophilic nano-domains and at topographical defects 66 which may favour the specific pinning of certain molecules. The impact of tribological contacts resulting from the larger roughness inherent to most functioning systems will also need to be taken into account.…”

Real-time tracking of ionic nano-domains under shear flow

“…The topography is reconstructed from the feedback corrections. The phase lag between the driving oscillation and the cantilever oscillation varies freely and carries information on the energy interactions between the cantilever tip and the interface 19 , 55 , 75 , 78 – 80 .…”

“…Examples span from biological enzymes 13 to sliding surfaces 7 , nanotubes 8 and graphene oxide membranes 14 .The behaviour of ions at solid-liquid interfaces is often described using continuum models such as the Gouy-Chapman-Stern 15 or the Dynamic Stern Layer 16 models. While highly successful for describing macroscopic systems at equilibrium, these models can fall short at the nanoscale when atomistic details such as specific solvation effects and local defects or singularities become important 7,15,[17][18][19][20][21][22] . For example, the specific organisation of ions into domains is common in biomedical and technological processes such as the effects of lithium on mitochondrial membranes 23,24 and ion transport through nanofluidic channels 8,25 .…”