Up-Converting Luminescent Nanoparticles as Probes of Surface Dynamics in Single Evaporating Microdroplets of Suspension

TL;DR

This study uses up-converting luminescent nanoparticles in levitated microdroplets to probe surface dynamics during evaporation, revealing nanoparticle organization, surface saturation, and resonance effects through optical measurements.

Contribution

It introduces a novel method combining luminescent nanoparticles and optical resonance to study surface dynamics in evaporating microdroplets.

Findings

Nanoparticle surface saturation occurs during evaporation.

Resonance signals oscillate due to nanoparticle migration.

Theoretical models agree with experimental observations.

Abstract

We have investigated the optically measurable properties of single evaporating microdroplets of suspensions containing up-converting luminescent nanoparticles (Gd2O3:Er3+), levitated in a linear electrodynamic trap. These microdroplets served as spherical optical resonators, with their resonance properties influenced by the distribution and interactions of nanoparticles (NPs) acting as nanoprobes. Using a combination of light scattering theories, we examined the evolution of the microdroplet radius, nanoparticle surface organization, and effective refractive index during evaporation. Up-conversion luminescence, driven by 805 nm excitation, enhanced the resolution of these observations. Key findings reveal that the increasing NP density in the surface layer leads to surface saturation, followed by transitions to a gel-like state through successive layer formation and collapse. Scattering and luminescence signals exhibit complex oscillatory behaviors linked to resonance phenomena and nanoparticle migration driven by photophoretic forces. Theoretical modeling closely aligns with experimental trends, confirming the interplay between NP distribution and optical resonance properties.