Thin-Film Lubrication in the Water/Octyl β-d-Glucopyranoside System: Macroscopic and Nanoscopic Tribological Behavior

Abstract

The friction properties of the water-based surfactant system C8 (octyl ß-D-glucopyranoside) are investigated both at the macro and nanoscale in ring-on-plate and AFM friction experiments, respectively. Surface characterization and measurement of the friction gap during sliding, together with the tribological behavior show a strong shear rate dependence of the friction behavior. High shear rates of approximately 10 6 s -1 in the macroscopic friction experiments induce a molecular alignment of the surfactants in the friction gap. This generates an anisotropic viscosity which allows to carry a high load, but exhibits low viscosity in shear direction. When the nanoscale and macroscale friction experiments are normalized to the same shear rate, almost identical frictional behavior is observed in the two regimes.

Macroscopic Friction Studies of Alkylglucopyranosides as Additives for Water-Based Lubricants

“…The strong surfactant-surface interaction ensures the formation of a stable lubricating film. This demonstrates that an anisotropic viscosity can be achieved under a high shear rate due to the strong orientation of C8 molecules along shear direction and is conducive to low shear stress and friction [50].…”

“…Our measurement depicts a synergic effect of surface-surfactant interaction and molecule orientation under a high shear rate on friction. Previous work has confirmed that the friction of 40% C8 aqueous solution is low and its tribological mechanism can be described by the "thin film lubrication" model, whose features are a surface-bound lubricant layer and a fluid film with anisotropic viscosity [50,[56][57][58][59]. The strong surfactant-surface interaction ensures the formation of a stable lubricating film.…”

“…During the measurement, the friction partner oscillates periodically and sinusoidally on the steel surface, which results in a cyclic change of sliding speed from 0 to 0.16 m/s with a mean value of 0.1 m/s. Taking 1.4 and 29.7 mPa•s as the viscosity for the 10% and 40% C8 solution [50] and assuming the surface roughness R a1 = R a2 after the friction test, which equals 0.48 and 0.48 µm lubricated by 10% and 40% solution, the lambda ratio is calculated to be 0.3-41 and 1.5-189 at low and high concentrations, respectively. At a low concentration, the lubricant layer acts as a boundary lubricant, where the steel surface asperities are occasionally in contact, as the minimum film thickness is lower than the average film thickness or the average surface asperities.…”

“…At higher concentrations (above 20%), the higher viscosity induced a stronger hydrodynamic uplift effect to avoid asperities contact. The lubrication system shifts towards the "thin film lubrication" [50], thus it reaches a lower friction under high pressure, as the lubricant molecules become oriented in the friction gap. This view is supported by the value calculated for the lambda ratio, which is much higher than 1 in this regime.…”

“…In this work, structurally well-defined alkyl glucopyranosides were used for rheological and tribological analyses. In a previous study, low COF in a binary octyl-ß-D-glucopyranoside (C8)/H 2 O system with a concentration of 40% was already demonstrated [50]. It was assumed that the interaction between the surface and the shear-induced molecular orientation is responsible for the improvement in tribological properties.…”