(211i) Extensional Rheological Measurements and Sprayability

In this paper we explore the influence of extensional deformations on spray droplet formation. Common to various atomisation mechanisms, the final droplet forms from the stretching of a ligament, typically at very large strains and strain rates. Hence it is anticipated that uniaxial extensional viscosity will dominate sprayability and final droplet size. We present results from an experimental study with flat fan sprays of aqueous colloidal dispersions together with, variously, high molecular weight polymeric thickeners or associative thickeners. We show that shear viscosity measurements fail to predict sprayability for these fluids. However, with some technique development in both experimental methodology and data analysis, we have been able to demonstrate that even with the highly non-Newtonian rheology exhibited by these fluids, Capillary Breakup Extensional Rheometer (CaBER) measurements at modest strain can be used to estimate a transient apparent extensional viscosity coefficient that is relevant to behaviour at the high strain encountered in a spray. Furthermore, these estimates of transient apparent extensional viscosity coefficient are found to quantitatively predict sprayability in the flat fan sprays studied.