Tunable viscosity modification with diluted particles: When particles decrease the viscosity of complex fluids

While spherical particles are the most studied viscosity modifiers, they are well known only to increase viscosities, in particular at low concentrations. Extended studies and theories on non-spherical particles find a more complicated behavior, but still a steady increase. Involving platelets in combination with complex fluids displays an even more complex scenario that we analyze experimentally and theoretically as a function of platelet diameter, to find the underlying concepts. Using a broad toolbox of different techniques we were able to decrease the viscosity of crude oils although solid particles were added. This apparent contradiction could lead to a wider range of applications.Comment: 13+7 pages, 6+7 figure

Flow of Microemulsions Adjacent to Planar Surfaces

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