On the applicability of Drop Profile Analysis Tensiometry at high flow rates using an interface tracking method

Abstract

In this work studies of growing water droplets in air are presented, with the aim to assess the applicability limit of the Droplet Profile Analysis Tensiometry (PAT). High inflow rates are applied for systems containing surfactants with high adsorption rates. However, under dynamic formation conditions, the measured surface tension values deviate from the theoretical values even for pure systems. Therefore, Computational Fluid Dynamics (CFD) is applied to gain detailed insight into the hydrodynamics of the growing drop. Since the flow is dominated by surface tension forces, an interface tracking approach is applied, which is able to capture the flow in a physically correct way. At high flow rates the inflow jet is not fully dissipated before approaching the free surface. Therefore, the pressure profile inside the drop is not uniform as is required in the derivation of the Gauss Laplace equation. The shape of the drop no longer represents the Gauss Laplace profile corresponding to the theoretical surface tension coefficient. As result we can confirm that the evaluation of surface tension by fitting to the Gauss Laplace Equation is not valid for dynamic droplet formations. Indications for future improvements of the evaluation procedure are provided.