Morphology of an aggregate of adhesive oil droplets in a spinning cylinder

The spinning drop method is a classic technique to measure the interfacial tension of two immiscible liquids of different densities, such as a drop of oil in water. If an oil droplet is placed within a rotating container filled with water, the droplet will move to the centre of rotation. As the rotation speed increases, the oil droplet will elongate and, conversely, as the rotation slows down, the droplet will become more spherical. In this technique, the surface area of the droplet is set by a balance between the centripetal force and the interfacial tension. Here, we present an experiment studying a granular analogue of this system to probe the effective interfacial tension of an aggregate of microscopic monodisperse oil droplets in an aqueous bath. The droplets are adhesive, buoyant, and friction is negligible. In the experiments, oil droplets are deposited in a cylinder filled with an aqueous solution of surfactant and will accumulate at a barrier under the influence of buoyancy. The droplets form a cluster along the centre of rotation in response to centripetal forces. The cluster undergoes changes in shape as the rotation speed is varied. This 'spinning-aggregate' experiment allows us to investigate connections between continuum liquids and granular systems.