Deformation and instabilities of differentially rotating drops

The Ring-Sheared Drop (RSD) module is a containerless flow device aboard the International Space Station for studying the effects of shear flow on protein aggregation and solidification at fluid interfaces with minimal wall nucleation effects. In the RSD a 2.5cm drop is constrained by a thin, stationary contact ring in one hemisphere and is sheared by the steady rotation of another ring in the other hemisphere. An earth-based analogue experiment using silicone oil drops immersed in an aqueous solution allows density matching in order to study the flow and associated deformation of the drop. The imposed shear drives a meridional flow in the drop. This leads to a pear-shaped mean deformation. The mean drop deformation has been quantified with a perturbation analysis using the capillary number (Ca) as the small parameter. The results agree with time-averaged experimental measurements, particularly at smaller ring rotation rates where Ca in the experiments is smaller. The experiment reveals an unsteady nonaxisymmetric drop deformation, which is correlated to instability in the outer flow. These experiments reveal some of the limitations of simulating microgravity using earth-based analogues.