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Abstract While many mechanistic studies have focused on the lubricious properties of ionic liquids (ILs) on ideally smooth surfaces, little is known about the mechanisms by which ILs lubricate contacts with nanoscale roughness. Here, substrates with controlled density of nanoparticles are prepared to examine the influence of nanoscale roughness on the lubrication by 1‐hexyl‐3‐methyl imidazolium bis(trifluoromethylsulfonyl)imide. Atomic force microscopy is employed to investigate adhesion, hydrodynamic slip, and friction at the lubricated contact as a function of surface topography for the first time. This study reveals that nanoscale roughness has a significant influence on the slip along the surface and leads to a maximum slip length on the substrates with intermediate nanoparticle density. This coincides with the minimum friction coefficient at sufficiently small contact stresses, likely due to the lower resistance of the IL film to shear. However, at the higher pressures applied with a sharp tip, friction increases with nanoparticle density, indicating that the IL is not able to alleviate the increased dissipation due to roughness. The results of this work point toward a complex influence of the surface topology on friction. This study can help design ILs and nanopatterned substrates for tribological applications and nano‐ and microfluidics. 

Abstract Ionic liquids (ILs) are proposed as potentially ideal electrolytes for use in electrical double layer capacitors. However, recent discoveries of long‐range electrostatic screening in ILs have revealed that this understanding of the electrical double layer in highly concentrated solutions is still incomplete. Through precise time‐dependent measurements of wide‐angle X‐ray scattering and surface forces, novel molecular insight into their electrical double layer is provided. An ultraslow evolution of the nanostructure of three imidazolium ILs is observed, which reflects the reorganization of the ions in confined and unconfined (bulk) states. The observed phase transformation in the bulk consists of the ILs ordering over at least 20 h, reflected in an expansion or contraction of the spacing between the ions organized in domains of ≈10 nm. This transformation justifies the evolution of the electrical double layer measured in force measurements. Subtle differences between the ILs arise from the intricate balance between electrostatic and non‐electrostatic interactions. This work reveals a new time scale of the evolution of the IL structure and offers a new perspective for understanding the electrical double layer in ILs, with implications on diverse areas of inquiry, such as energy storage, lubrication, as well as micro‐ and nanoelectronics devices.