Microscale Flow Characterization of Solutal Marangoni Instabilities in Evaporating Binary Droplets Using STAR-APTV

The Marangoni effect, driven by surface tension gradients, governs a wide range of interfacial transport phenomena across scales—from the classical “tears of wine” to the complex fluid dynamics observed in evaporating multicomponent droplets. Although traditionally described as a macroscopic interfacial phenomenon, its fundamental origin lies in molecular-scale interactions and compositional asymmetries at the interface—an aspect often overlooked in conventional descriptions. In this talk, we investigate evaporating binary-mixture droplets, where selective evaporation induces highly unstable interfacial conditions that lead to spontaneous, chaotic mixing flows and sudden interfacial instabilities. Despite their ubiquity, the onset mechanisms, temporal evolution, and structural dynamics of such flows remain poorly understood. To address this gap, we introduce STAR-APTV (Segmentation and Tracking Assisted by AI for Robust - Astigmatic Particle Tracking Velocimetry), a framework that enables time-resolved 3D3C (three-dimensional, three-component) velocity field measurements for capturing complex internal flow dynamics. Our system, optimized for sparse particle conditions with a particle number density of N_s≈0.5, achieves over 70% particle detection accuracy—marking a significant advancement in microscale velocimetry. Combined with micro-PIV, STAR-APTV allows us to resolve both chaotic internal circulations and sudden bursting flows at the droplet interface. These observations provide a richer and more complete picture of interfacial instabilities by bridging the macroscopic manifestations of the Marangoni effect with its microscopic origins. This presentation aims to elucidate the small-scale features of Marangoni-driven flows and to offer a new experimental framework for probing interfacial transport phenomena in multicomponent systems.