Controllable Friction of Green Ionic Liquids via Environmental Humidity

Abstract

Intelligent control of friction is an attractive but challenging topic. In this work, it is investigated if it would be possible to adjust friction in a lubricated contact by controlling environmental humidity. By exploiting the ability to adjust the environmental humidity by various saturated salt solutions, friction behavior of contacts lubricated with Choline l‐Proline ([Cho][Pro]) is modulated in a wide range of relative humidity (RH). The friction increases when the environmental humidity is increased and decreases when water is partially evaporated to a lower RH. It is thus possible to control friction by environmental humidity. The addition of water in ionic liquids (ILs) causes a decrease in viscosity, but as the tests are calculated to be performed in boundary lubrication the viscosity change is not the main factor for the change in friction. The friction sensitivity of RH can be explained by the effect of adhesion on the water uptake from humid air by [Cho][Pro]. Furthermore, the reversible changes of H‐bond types determined by the water content could be another explanation to the altered friction.

Design and applications of biocompatible choline amino acid ionic liquids

“…Higher friction coefficients are found at higher water content, and evaporation of water reduces friction. 100…”

“…In contrast to lignin, which provides extra mechanical support, the addition of water has been found to increase the coefficient of friction of ChPro. 100 Studies have shown that water forms short, linear hydrogen bonds with amino acid anions. 29,101 Therefore, water is likely to compete with hydrogen bonds among ions and weaken the lubricant film at the interface, ultimately leading to the dissolution of amino acid, and cleavage of the lubricant film.…”

Cholinium amino acid-based ionic liquids

“…The results show that their toxicities are significantly lower than that of traditional ILs such as 1-butyl-3-methyl imidazolium tetrafluoroborate and that ChAAILs can be used as high-performance and environmentally friendly lubricants. Hua et al ( 2020 ) have shown that the frictional properties of [Cho][Pro] can be controlled by altering the environmental humidity. The viscosity of the ChAAIL increases at higher humidity, while it decreases when water is evaporated.…”

Nanolubricant additives: A review

“…Liquid lubricants can form a hydrodynamic or elastohydrodynamic lubrication film on friction interfaces during sliding friction, therefore widely used in the mechanical industry. Apart from lubricating oils, ionic liquids can sometimes be used as liquid lubricants [12]. When mechanical equipment is under start-up and shutdown stages and/or under extreme friction conditions, liquid lubricants cannot form a stable lubricating film between the friction pairs.…”