Zero discharge fluid dynamic gauging for studying the thickness of soft solid layers

Abstract

Fluid dynamic gauging (FDG) is an experimental technique for measuring the thickness and strength of solid layers on solid or porous substrates by relating the flow rate of liquid through a nozzle positioned near to, but not touching, the layer. This paper shows that such measurements can be made with the liquid both being ejected from and sucked into the nozzle, so that there is no net flow of liquid into or out of the system. This zero discharge (ZFDG) mode has considerable potential for sterile and aseptic operation. Discharge coefficient profiles are reported for a range of flow rates and show very good agreement with computational fluid dynamics simulations, indicating that the shear stress imposed on the surface can be estimated with confidence. The use of ZFDG to study the thickness and removal of soft layers was demonstrated with layers of a commercial petroleum jelly (Vaseline®) layer. The Vaseline® layers exhibited cohesive breakdown on 316 stainless steel, indium tin oxide coated polyethylene terephthalate (PET), and glass under suction mode testing. The cohesive strength agreed with the yield stress measured in separate rheological tests. When exposed to ejecting liquid on glass, however, the layers switched from cohesive breakdown to an adhesive rupture mechanism which is attributed to an instability in the layer and the observed tendency to slip on smooth substrates.