Measuring Molecular Transport in Levitated, Viscous Microdroplets Using Confocal Fluorescence Microscopy

FARHAD IZADI, Michael Jacobs, Texas State University

     Abstract Number: 366
     Working Group: Instrumentation and Methods

Abstract
The viscosity and phase state of aerosol particles impact climate and air quality by influencing reactivity, hygroscopicity and ultimately human health. Aerosol particles can exist in glassy or gel-like phase states with significant diffusion limitations. In these diffusion-restrictive phase states, strong chemical gradients can form in particles, which ultimately affects their reactivity and atmospheric fate. Single particle levitation techniques have been used to measure the phase states of suspended microdroplets and particles. These techniques typically report on either the viscosity of the particle (e.g., by measuring relaxation timescales of coalesced particles) or diffusion of water, Dw (e.g., via isotopic exchange measurements). However, diffusion of molecular species other than water cannot necessarily be predicted from a particle’s viscosity and Dw. As a result, new techniques are required to measure the transport of molecules within levitated particles.

In this work, we use our recently developed quadrupole electrodynamic trap confocal microscopy (QET-CM) technique to investigate the phase and mixing states of coalesced microdroplets. By coalescing two microdroplets with different compositions, we create droplets with strong chemical gradients. Using a low-cost spinning disk confocal microscope, we are able to measure the evolving 3D spatial distribution of fluorophores the levitated microdroplet as the chemical gradient relaxes. In initial measurements, we demonstrate the capabilities of the QET-CM technique by measuring the molecular diffusion of various fluorescent dyes (including as rhodamine 6G and fluorescein) in glucose microdroplets exposed to different relative humidities. In addition, we demonstrate that our technique can also measure charge distribution within highly viscous systems by using a pH-sensitive fluorescent dye to measure the relaxation of pH gradients within merged droplets. Ultimately, the QET-CM technique has the capability of reporting on a wide variety of different transport phenomena in highly viscous microdroplets that were previously inaccessible using single-droplet levitation techniques.