# Resonance-Driven Discrete Growth and Chemical Reactivity of Optically Levitated Droplets

## TL;DR

This study shows how light-induced thermal locking affects droplet growth and chemical reactions, revealing new insights into environmental sulfate formation.

## Contribution

The paper introduces thermal locking caused by chemical reactions and its impact on reaction mechanisms and rates in optically levitated droplets.

## Key findings

- Thermal locking leads to (semi-)discrete droplet growth via competing condensation and evaporation processes.
- Thermal locking can accelerate chemical reactions and shift dominant mechanisms toward photoinduced pathways.
- Light-induced surface reactions produce droplet growth patterns similar to those from surface-driven reactions.

## Abstract

Optical levitation provides a powerful platform for probing the physicochemical properties of nano- and microparticles. In optical levitation experiments involving nonreacting droplets, metastable states, or so-called “thermally locked” states, can emerge. However, there has been no report on thermal locking induced by chemical reactions or the impact of thermal locking on the reaction mechanisms or rates. Herein, we investigate the growth of optically levitated aqueous droplets in which sulfate forms through the SO2-NO2 and the SO2-Mn2+-O2 heterogeneous reactions—2 environmentally important sulfate formation pathways. We observe (semi-)discrete droplet growth occurring via consecutive thermally locked states, which result from the competition between water vapor condensation driven by sulfate formation and evaporation driven by droplet heating through resonant absorption of the trapping laser. By combining Mie theory-based analysis of the stimulated Raman scattering and droplet thermodynamics, we develop an approach to retrieve the key properties (e.g., temperature, pH, and molality) of thermally locked droplets and demonstrate that chemistry-driven thermal locking results in a signature particle growth pattern. Comparison of sulfate formation rates in locked versus unlocked droplets further reveals that thermal locking can accelerate chemical reactions or even change the dominant mechanism by promoting photoinduced reaction pathways. As light intensity enhancement within the droplet is localized near the droplet surface, the photoinduced reactions lead to droplet growth patterns similar to those driven by surface reactions. This work uncovers a novel phenomenon emerging from light–droplet interactions, offering a mechanistic framework for leveraging thermal locking to probe droplet properties and study chemical reactions under resonant conditions.