Luminescent nanoparticles in a shrinking spherical cavity -- probing the evaporating microdroplets of colloidal suspension -- optical lattices and structural transitions

TL;DR

This study uses charged luminescent nanoparticles as probes within evaporating microdroplets to observe optical resonances and structural transitions, revealing lattice formation and collapse at the droplet surface.

Contribution

It introduces a novel method employing luminescent nanoparticles to monitor internal structural dynamics of evaporating colloidal microdroplets via optical resonance effects.

Findings

Significant modulation depth increase in cavity resonances.

Observation of bi-stability in luminescence signals.

Detection of lattice formation and collapse at droplet surface.

Abstract

We investigated the possibility of using charged luminescent nanoparticles as nanoprobes for studying the evolution scenarios of surface and internal structure of slowly evaporating free (light-absorbing) microdroplets of suspension. Three concentrations (1, 10 and 50 mg/ml) of luminescent nanoparticles were used. Single microdroplets were kept in a linear electrodynamic quadrupole trap and the luminescence was excited with a CW IR laser with an irradiance of ~50 W/mm2. Since the microdroplet acted as an optical spherical resonance cavity, the interaction of nanoparticles with light both reflected and modified the internal light field mode structure. Depending on the nanoparticle concentration used, it led, among others, to a very significant increase in modulation depth and narrowing of spherical cavity resonance maxima (morphology dependent resonances - MDRs) observed both in luminescence and scattering, the abrupt changes in the ratio between the luminescence and the scattering and the bi-stability in luminescence signal. The observed phenomena could be attributed to the interaction of optical MDRs with nanoparticle lattice shells forming and changing their structure at the microdroplet surface. In this way, the formation and collapse of such lattices could be detected.