Nanotribology, standard friction, and bulk rheology properties compared for a Brij microemulsion

A microemulsion consisting of Brij 96, glycerol (co-surfactant), oil, and water was compared as concerns deformations in a surface forces apparatus whose surface where rendered hydrophobic by coating with a monolayer of condensed OTE (octadecyltriethoxysilane), as concerns tribology of the conventional kind during sliding between hydrophobic PDMS surfaces, and as concerns bulk rheology. In the bulk, light scattering characterization showed swollen spherical micelles with a 13 nm diameter. When squeezed to form thinner films than this, the effective viscosity measured rose by orders of magnitude. It appears that thin films in the range of thickness 13 to 7 nm are comprised of deformed micelles and that confinement to thinner films expels micelles with concomitant even more drastic structural deformation of the remaining micelles, until the thinnest films retain only adsorbed surfactant. Tentatively, this may explain why the friction response then became similar to that of surfactant itself [M. Graca, J.H.H. Bongaerts, J.R. Stokes, S. Granick, J. Colloid Interface Sci. 315 (2007) 662]. These measurements are considered to be the first comparison of microemulsion rheology in the bulk and in nanometer-thick films.