Instabilities and elastic recoil of the two-fluid circular hydraulic jump

The two-fluid circular hydraulic jump, also called “rinsing flow,” is a common process where a jet of one liquid impinges upon a layer of a second liquid. We present an experimental analysis of rinsing flows using a high-speed camera and model fluids to decouple the effect of shear-thinning and elasticity. Varying the rheology of the coating fluid produced several types of instabilities at both the air–liquid interface and liquid–liquid interface. Layered “stepped jumps” and “crowning” on the rim of the jumps were both suppressed by fluid elasticity, while Saffman–Taylor fingering patterns showed strong dependence on both shear-thinning and normal stresses. In addition, the hydraulic jump evolution was quantitatively determined using a laser triangulation technique, and “recoil” of the jump front resulting from fluid elasticity was observed. Our work shows that the non-Newtonian two-fluid circular hydraulic jump is very complex, and the instabilities that arise also introduce additional complications when developing theoretical models.