Quantized friction across ionic liquid thin films

Abstract

Ionic liquids - salts in the liquid state under ambient conditions - are of great interest as precision lubricants. Ionic liquids form layered structures at surfaces, yet it is not clear how this nano-structure relates to their lubrication properties. We measured the friction force between atomically smooth solid surfaces across ionic liquid films of controlled thickness in terms of the number of ion layers. Multiple friction-load regimes emerge, each corresponding to a different number of ion layers in the film. In contrast to molecular liquids, the friction coefficients differ for each layer due to their varying composition.

The Lubrication Ability of Ionic Liquids as Additives for Wind Turbine Gearboxes Oils

“…(Figure 5b). It has been reported [12][13][14][15][16][17][18][19][20][21] that ILs have the ability to form highly ordered absorbed layers on metal surfaces. This layer prevents direct contact between mating surfaces, reducing friction and wear.…”

“…Their unique properties, including high thermal stability, non-volatility, non-flammability, high ionic conductivity, wide electrochemical window, and miscibility with organic compounds make them ideal candidates for many engineering applications. In addition, ILs have the ability to form stable ordered layers and protective tribo-films [12][13][14] in the area between the two materials in contact, reducing friction and wear.…”

Controlling turbulent drag across electrolytes using electric fields

“…For ionic liquids, seminal works by Perkin et al show that nanoconfined ionic liquids near a charged surface arrange themselves in a layered structure of alternating cation-rich and anion-rich layers [13,14]. The number of ion layers confined between the surfaces is an integer dependent on the surface separation, and thus the friction coefficient in the low velocity regime also shows a "quantized" behaviour as a function of surfaces separation [15]. Applying a potential difference between the surfaces and the bulk switches the composition of the ion layers between the surfaces from cationenriched at negative potentials to anion-enriched at positive potentials.…”

Unravelling nanoconfined films of ionic liquids

“…[4,31]). The normal force measured in these experiments is oscillatory, and the period of the oscillation suggests that an odd number of layers is confined between surfaces.…”

“…Room temperature ionic liquids are used in diverse fields ranging from solvents in chemical synthesis [1,2], to electrochemical supercapacitors [3] and lubricants [4]. The non-volatility and wide electrochemical window of stability of ionic liquids render them good candidate electrolytes for electrochemical and energy storage applications, especially in nanodevices where a large voltage is applied over small length-scales (for a recent review of ionic liquids at electrified interfaces, see [5]).…”