Dynamics of Coulombic Fissions on Evaporating Droplets of Water and Nanofluids*

Electrically charged droplets are susceptible to Coulombic fissions (CF), a mechanism through which charges (and a small fraction of mass) are released upon reaching a charge limit (known as Rayleigh limit).  A stable evaporating droplet with a fixed amount of charge reaches the Rayleigh limit as its size decreases. After CF, the droplet returns to a stable state while evaporation continues, and the process repeats itself several times over the droplet lifetime.

CF in evaporating water droplets and water/alumina nanofluid droplets (NFD) were studied. Particle concentration is expected to vary with time and change the fluid properties, affecting the dynamics of its CF process. High-speed imagery was used to measure the droplet diameter and its deformations during CF.

Water droplets sustained successive CF at similar normalized diameters in various experiments regardless of initial droplet size and evaporation rate. Densely concentrated NFD deviated from this behavior and their deformations were damped compared to those of pure water due to higher viscosity. Dense NFD expelled larger amounts of mass during CF, with the droplet splitting in half in extreme cases. This study showed the differences between CF of pure and nanofluid droplets.