Irradiation-driven Evaporation of Micro Droplets in an Optical Trap

TL;DR

This study investigates how intense IR laser irradiation affects the evaporation of optically levitated water droplets, revealing a transition from heat-driven to diffusion-driven evaporation as droplets shrink.

Contribution

It demonstrates the limitations of the D$^2$-Law under strong IR irradiation and uncovers a transition in evaporation mechanisms for small droplets.

Findings

Evaporation initially driven by laser heating following a linear law.

A transition occurs at 2-3 μm radius from heat-driven to diffusion-driven evaporation.

Strong IR irradiation significantly alters droplet evaporation dynamics.

Abstract

Small droplets are irradiated with visible and infrared light in many natural and industrial environments. One of the simplest ways to describe their evaporation is the D$^2$-Law. It states that the evaporation rate is proportional to $t^{-1/2}$, and $R^{-1}$. However, models like the D$^2$-Law do not account for the volumetric heating of light and the effect of strong irradiation on individual droplets is not fully understood. Here we show the effects of IR irradiation on optically levitated water droplets. We find that, under strong irradiation of up to $10^8 W/m^2$, the droplet evaporation is initially driven by the heat from the laser following the power law $dR / dt \sim R$, i.e. the inverse of the D$^2$-Law. Then, when the droplets shrink to 2 - 3 $\mu$m in radius a turnover occurs from irradiation-driven back to diffusion-driven evaporation. Our findings support the understanding of droplet evaporation in cases such as rocket engines or internal combustion, where the radiation from the flame will heat water and fuel droplets.