The Coalescence of Drying Droplets

Abstract:
The Weber number (We), a dimensionless number used to describe the ratio of a fluid's inertia to its surface tension, is often plotted as a function of the impact parameter (B), a measure of how head-on a collision is, as a way of mapping collisional outcomes of two fluid droplets. These regime maps, as they are known, have been studied experimentally and models have been suggested for predicting the collisional outcome. However, the focus of previous work has been the collisions of submillimetre droplets. The initial experiment proposed here will focus on the collisions of water-water droplets of ~ 30-80 um radius. This novel size makes the proposed work relevant to the research of aerosols and their microphysical properties. A comparison can be made with existing collisional regime maps of water to determine whether droplet size, and the shift in the kinetic regime, influences the collisional outcome. This will be investigated using two microdroplet dispensers to collide droplets, imaged by a stroboscopic imaging technique. Following this, the effect of viscosity on the regime map can be determined using the same method, with varying concentrations of aqueous sucrose droplets. This is likely to be limited by the ability of the dispensers to eject high viscosity droplets (> 20 mPa.s), which may lead to the use of an electrodynamic trap in order to have control over droplet drying time prior to coalescence. Both instrumentations have the potential to be combined with cavity enhanced Raman spectroscopy to determine droplet composition and heterogeneity.