Lubrication Mechanisms of Lamellar Fatty Acid Fluids

The lubrication mechanisms of different lamellar fluids are investigated as they are introduced in the thin contact zone between two macroscopic surfaces in motion in a friction measurement set-up. We simultaneously measure the film thickness and its lubricative properties under controlled contact kinematics. The lamellar phases consist of nanometric flat bilayers of fatty acid surfactant molecules organized in periodic stacks separated by a water/ethylene diamine solution. First, we examine the film forming capability of these phases when the two surfaces are moving at the same velocity, i.e. in “pure rolling” conditions. We observe the growth of a thick film in the contact which eventually reaches a stable value. The relatively high viscosity of the film leads to a situation of so-called “starved lubrication”. By modelling the film build-up process, we determine the viscosity of the lubricant and its piezoviscosity. As shear is applied between the surfaces, the lubricant film exhibits a constant thickness and a rather low frictional response. We correlate this behaviour to the combination of a relatively high viscosity value together with a low piezoviscosity. Through the addition of a hydrophobic liquid (naphthenic oil) to the initial system, we increase the bilayer thickness whilst keeping the lamellar characteristic packing distance constant. This changes both the film forming capability and frictional behaviour of the lamellar fluid. We propose a model to account for the observed friction responses of both lamellar phases and discuss the shear localization in the lubricant film.