Critical laser frequency for nanofluid pendant droplet evaporation

Abstract

Droplet evaporation is a complex and fundamental topic that holds great scientific interest due to its relevance in numerous physical and biological processes. We systematically study laser-induced nanofluid droplet evaporation under varying light frequencies. Our findings indicate the existence of two spectral regimes where droplet evaporation is either enhanced or inhibited, which is in stark contrast to the constant regime observed under fixed laser power. The enhanced regime is attributed to the rapid heat transfer initiated by the formation of vapor microbubbles inside the droplet, causing an increase in the overall temperature of the droplet. Conversely, the inhibited regime is associated with reduced heat conduction inside the droplet resulting from localized cooling effects brought about by droplet evaporation. Correlations between heat transfer mechanisms and thermal responses at the droplet surface further support these observations. We also demonstrate that both convective and conductive heat transfers determine the critical light frequency to enhance droplet evaporation. Three light-driven flow patterns are additionally identified inside the droplet. These are photophobic, phototropic, and rolling flows, which are driven by the explosive bubble growth, surface tension gradients, and mass shifts in the droplet center, respectively. Understanding these properties is important for developing miniature evaporators, nanoparticle self-assembly, and various biomedicine applications requiring precise temperature and kinetic control.