Numerical Simulation of Ethanol-Water-NaCl Droplet Evaporation

A quantitative description of droplet evaporation is important to aerosol research for nanofabrication, spray drying, fuel combustion, pollution control, and respiratory medical treatments. Evaporation is a movingboundary problem with coupled mass and heat transport. An explicit finite-difference methodology and computer code has been developed for simulation of an evolving droplet, property data for size, and profiles for various compositions and temperature. The code accurately predicts the evaporation of pure water and pure ethanol droplets. To understand aerosol-assisted evaporation-induced self-assembly and the formation mechanism for single-crystal NaCl core/hexagonally ordered mesoporous silica shell particles, evaporation of ethanol-water-NaCl droplets in N2 has been investigated by numerical simulation. The extended universal quasichemical (UNIQUAC) model with a Debye-Hückel term is used to describe the vapor-liquid phase equilibrium. For 1-2-μm-radius droplets with a number density of 107∼108/cm3, it takes only tens of milliseconds to reach phase equilibrium after adiabatic or isothermal evaporation at 25 °C in the drying zone. The droplets entering a heating zone can be simply treated like a single-stage flash evaporation at 25 °C. For a 1-μm-radius droplet, after 0.18 ms of evaporation at 100 °C in N2, the NaCl saturation ratio reaches levels as high as 1.3, first at the droplet center, where the initial NaCl nucleation and crystallization happens as a result of relatively quick evaporation and a steep gradient in the concentration of ethanol, an antisolvent for NaCl. NaCl crystallization “consumes” NaCl molecules near the droplet center and quenches the formation of new stable NaCl nuclei, favoring the formation of only one single-crystal NaCl core per droplet. The code provides guidance for the custom engineering of aerosol nanoparticle architectures.