Confocal fluorescence imaging of levitated particles and microdroplets

The phase and mixing states of aerosol particles play an important role in controlling their atmospheric aging processes. While numerous characterization techniques exist to detect the phase states and morphologies of individual deposited aerosol particles, there are fewer techniques available to study the phase and mixing states of levitated aerosol particles. In this work, a new method is presented that uses a low–cost, spinning disk confocal microscope to image the morphology and phase states of particles and microdroplets levitated within a quadrupole electrodynamic trap (QET). The QET confocal microscopy (QET-CM) technique maps the 3-dimensional distribution of fluorophores embedded in levitated particles with micron resolution. A series of benchmark experiments are presented to demonstrate the capabilities of the technique. First, confocal z-slice imaging is used to map the 3-dimensional distribution of two separate fluorophores in a levitated, effloresced NaCl particle. Then, the ability to measure temporal changes in phase state and morphology is demonstrated by imaging changes in the distributions of two fluorophores embedded in a NaCl particle as it deliquesces in the QET. Finally, the QET-CM technique is used to detect liquid-liquid phase separation in levitated droplets and easily distinguish between homogeneous, core-shell, or partially engulfed morphologies. Ultimately, the QET-CM technique presented here can be used to rigorously assess the phase state and morphology of levitated aerosol particles, which could aid our understanding of the impact of atmospheric aerosol on the environment and human health. Copyright © 2025 American Association for Aerosol Research