Continuous Single-Droplet Tracking Reveals Scaling Laws of Surface-Confined Mn(II)-Catalyzed SO₂ Oxidation

Here, we continuously tracked the radius of one optically levitated droplet to determine reaction location directly from its growth trajectory of the same evolving microdroplet. Benchmark SO₂ autoxidation experiments recovered distinct scaling behaviors for bulk reaction at pH 5.0 and interfacial reaction at pH 3.8.

Applying the same analysis to Mn(II)-catalyzed SO₂ oxidation showed that the catalytically effective Mn inventory is diluted with droplet surface area rather than volume, providing direct dynamic evidence for surface-specific Mn catalysis during sulfate-driven hygroscopic growth. Broadband light-scattering aerosol optical tweezers enabled week-long tracking with nanometer-scale radius precision over a broad size range.