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

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.