(314g) Fluid Mechanics of Rinsing Flows

The fluid mechanics of ?rinsing flows?, describing the use of a jet of one liquid or air to push away a second, coating liquid on a rigid flat surface, has been investigated. This phenomenon is common to everyday life but despite this, few studies have considered the rinsing flow process in detail. One experimental challenge is that the timescale of the process is short and does not reach a steady state. The present study has employed high speed imaging devices allowing the examination of the initial contact between the two fluids.

It is found that these flows present interesting variations to the classic hydraulic jump with a variety of new phenomena due to the presence of the second, coating layer. For example, a systematic study varying the viscosity ratio between the two fluids reveals a Taylor-Saffman instability at the moving interface between fluid contacts when the viscosity ratio between the coating fluid and the jet fluid is higher than ~25. The hydraulic jump circle size is also a strong function of the viscosity ratio and exhibits a power-law relationship.

In addition, the effect of the viscoelasticity of the coating fluid has been investigated in response to either water or air jets. Three classes of fluids with the same zero-shear viscosity have been investigated: glycerin-water Newtonian liquids, polymeric solutions, and worm-like micelle surfactant solutions. Analysis of the high speed videos reveal that the evolution of the hydraulic jump circle sizes have very different characteristics. For example, for water jet experiments, Newtonian liquids show an inverse exponential-like growth in circle size while polymeric solutions show a linear growth and for surfactant solutions, the circle size first increases then recoils due to the high elasticity of the fluid. In addition, it is observed that the Non-Newtonian liquids are able to suppress surface roughness in the hydraulic jump region due to a coupling of the extensional viscosities of these fluids and stagnation point flows in the vicinity of the hydraulic jump. Both the degree of roughness suppression and hydraulic jump circle size increase with the extensional viscosity of the solution.

The effect of altering the hydrophobicity of the flat surfaces on the efficiency of the rinsing process has also been investigated using scanning electron micrographs and Auger spectroscopy. Under the same conditions, the efficiency is affected by the wettability of the rinsing fluid on the surface and is a function of the radial position from the jet.