Flow visualization and modeling of mixing in an interfacially-driven flow

The knife-edge viscometer (KEV) is a cm-scale cylindrical dish of fluid with a liquid-gas interface contacted by a thin rotating annular ring, the "knife-edge" that induces flow through interfacial shear. Coupling interfacial and bulk fluid dynamics, the KEV is a promising tool for studying mixing processes in physiology, biology, biochemistry, environmental science, and interfacial fluid physics. The rapidity and extent of mixing can be enhanced by periodically alternating the rotation direction of the knife edge and through modifying surface rheology, enabling more precise engineering of mixing performance. Here, the effects of shear oscillation period and surface shear viscosity on hydrodynamic response were investigated using planar laser-induced fluorescence (PLIF) of fluorescein dye, visualizing secondary flow in the meridional plane with quantification of dye mixing via fluorescence intensity. Experimental results are compared with axisymmetric COMSOL simulations, including a Boussinesq-Scriven interface model. This work provides the foundation for ongoing computational investigation of a microgravity analog to the KEV, the ring-sheared drop (RSD).