Flow of Microemulsions Adjacent to Planar Surfaces

Abstract

Microemulsions consist of water, oil and surfactant. Although thermodynamically stable, domains of pure water and oil are formed on nanometer length scales and a surfactant film in between that are ideally observable by small angle scattering experiments. The bicontinuous microemulsion displays a sponge structure that forms when equal volumes of water and oil are mixed. Being exposed to hydrophilic planar surfaces, a lamellar order is found in the vicinity to the solid-liquid interface. The typical depth of the lamellae is 40 to 60nm, i.e. 4 to 6 perfect domains [1,2], before the perforations describe the decay to the bicontinuous phase. The membrane modes observed by neutron spin echo spectroscopy under grazing incidence are faster at the interface than in bulk [3]. This is an evidence for the lubrication effect, a facilitated flow of the lamellae along the interface. Employing clay platelets, the same effect could be observed in a bulk sample [4]. Furthermore, at smaller platelet diameters, the favorable modes of the lamellae were cut, and the overall dynamics slowed down similar to the bulk. Thus, the perfection of modes at the interface is connected to the platelet diameter. At rather high flow rates, the perforated transition region was reduced in size, while the perfect lamellae were persistent [2]. In macroscopic rheology experiments (Fig.1 left), the microemulsion with rather large clay platelets showed evidence for the lubrication effect on macroscopic scales, while at lower clay dimensions the viscosity was extraordinarily high [5] (Fig.1 right). Motivated by this effect, the rheology of crude oils with large clay platelets showed decreased viscosities at low temperatures (below 0°C). The dynamic asymmetry of the aromatic and aliphatic portions and the lamellar alignment of the domains may explain these findings