In Situ High Shear Perturbation of Surface Molecular Layers with Neutron Reflectivity.

We present the effect of in situ imposed shear on two adsorbed molecular layers using a flow cell that enables access to shear rates of up to 2 × 105 s-1 (shear stress: 2 × 102 Pa). The response of these molecular layers to shear is determined by the relative strength of the surface binding and the imposed shear. An adsorbed bilayer of palmitic acid, weakly adsorbed on a solid silicon surface, is found to be readily removed. In contrast an adsorbed bilayer of sodium bis(2-ethylhexyl) sulfosuccinate is essentially unchanged with the highest shear we could impose, despite being completely removed by rinsing with pure water (i.e., "dissolving" the layer). The surface binding strength of each species is independently captured through adsorption measurements. These results show, by direct in situ measurement, that whether a surface adsorbed additive is perturbed by an imposed high shear field depends on the nature of the adsorbed layer and the strength of its binding to the surface relative to the applied shear stress rather than the magnitude of the shear stress alone. This is an important finding with direct implications for the design and selection of additives in lubricant and anticorrosion formulations, where retention of a protective molecular layer under high-shear boundary conditions governs the performance.