Volumetric laser heating of nanosuspension microdroplets: Slow evaporation to mid-air explosion

Abstract

Opto-thermal interaction between a laser beam and a nanosuspension droplet is studied in this paper to analyze the thermal response of the droplet at different laser intensities depending on the optical properties of the nanosuspensions. A pulsed Nd:YAG laser beam is used to irradiate four types of droplet, 20wt% silver nanoparticle suspension, 10wt% and 5wt% germanium nanoparticle suspension and 8mM SDS solution, in an electrospray laser deposition experiment to observe the geometrical changes in their shape. A high-speed camera captured the images of the droplets, showing the decrease in their diameter and inception time for droplet slow evaporation as well as the pre-explosion time and an explosion regime map of four different explosion patterns: shallow, deep, directional and isotropic explosions depending on the laser peak intensity for 20wt% silver nanoparticle suspension microdroplet. The other three types of suspension, however, do not undergo slow evaporation or experience mid-air explosion. A theoretical conduction model has been developed to predict the explosion of the droplet by finding its bubble formation region inside the droplet. These results are corroborated by experiments that the explosion characteristics depend on the optical properties of the suspension such as reflectance, absorption coefficient and refractive index. The temperature distribution and the predicted bubble formation region for the four types of suspension are investigated, and only the silver nanoparticle suspension of 20wt% with low reflectance and high absorption coefficient is predicted to have bubble formation patterns within the droplet, similar to the four explosion patterns in the experiment. The conduction model agrees well with the experimental results and shows that it can predict the threshold laser peak intensity and explosion patterns of a droplet a priori at which explosion occurs.