The ring-sheared drop is a containerless system where shear is imparted by two contact rings, one rotating and the other stationary. In microgravity, aqueous drops can be studied in the air at the centimeter scale. Drops of this scale can also be studied experimentally on Earth, but the effects of gravity need to be mitigated by density matching the drop liquid and its surrounding fluid. The use of silicone oil drops surrounded by an aqueous solution allows density matching while retaining the viscosity ratio of the aqueous-air system in microgravity. The imposed shear drives a meridional flow in the drop which leads to a pear-shaped drop. A perturbation analysis with the capillary number as the small parameter is used to account for this mean drop deformation. The theory and time-averaged experiments agree, particularly at smaller ring rotation rates where the capillary number in the experiments is smaller. On top of the mean deformation, there is a smaller amplitude nonaxisymmetric deformation, which for slower ring rotation rates consists of a rotating wave with azimuthal wavenumber m = 1, that is, synchronous with the rotating ring. This is traced back to imperfections in the wetting and contact between the drop and the rotating ring in the experiment. At larger ring rotations, the experiments detect further unsteadiness with a broad frequency peak at about one third the ring rotation rate. Nonlinear simulations of the outer flow, assuming a nondeforming drop, find that at these ring rotations, the outer flow is unsteady with a similar frequency peak.
The ring-sheared drop is a module for the International Space Station to study sheared fluid interfaces and their influence on amyloid fibril formation. A 2.54-cm diameter drop is constrained by a stationary sharp-edged ring at some latitude and sheared by the rotation of another ring in the other hemisphere. Shearing motion is conveyed primarily by the action of surface shear viscosity. Here, we simulate microgravity in the laboratory using a density-matched liquid surrounding the drop. Upon shearing, the drop’s deformation away from spherical is found to be a result of viscous and inertial forces balanced against the capillary force. We also present evidence that the deformation increases with increasing surface shear viscosity.
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