Development of methods for surface tension and charge measurements of picolitre aerosol droplets

Abstract

Aerosols have large surface area to volume ratios compared to bulk solutions, making interfacial composition extremely important when discussing chemical processes involving aerosols. The chemical composition of aerosol interfaces is relevant in many fields, like industrial printing processes, pharmaceutical drug development, and climate change. Mounting evidence indicates the droplet-air interface is key for describing observations of accelerated chemistry in aerosols. The presence of surface active compounds adds another layer of complexity, requiring considerations of molecular partitioning to fully understand aerosol interfacial chemistry. Surface charge is also known to impact interfacial composition and reactions of aerosols. Mechanisms that govern how aerosol are charged are still poorly understood.

Monitoring droplet surface oscillations is a contactless approach to investigate aerosol surface composition. Droplet surface oscillations are governed by cohesive forces such as surface tension and viscosity, and can be disrupted by repulsive Coulombic forces when droplets are charged. The stroboscopic imaging of droplet surface oscillation is a method capable of measuring dynamic surface tension changes in picolitre droplets. Monitoring dynamic surface tension of surfactant-laden droplets can inform on surfactant partitioning timescales relevant in interfacial chemistry. However, due to the timescales at which surface oscillations dampen, current applications of the method have experimental timescales that do not fully capture surfactant partitioning. Here, a new method was developed to expand the experimental timescales of the stroboscopic imaging technique through re-excitation of surface oscillations by droplet coalescence.

Methods by which aerosol surfaces are charged and impacts of charge on aerosol
chemistry are poorly constrained. Here, parameters affecting droplet charge were
investigated, and initial tests on applying the stroboscopic method to measure droplet charge were performed.

Overall, this work demonstrates the importance of in-situ measurements of droplet properties such as surface tension and charge, which can differ from those of macroscopic solutions.

Date of Award	17 Mar 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	Bryan R Bzdek (Supervisor)