Periodic Thermal Oscillation Induces Mesoscale Soft Aggregates in a Fully Miscible Ethanol–Water Liquid

Inducing and stabilizing mesoscale structures in fully miscible molecular liquids remains challenging because local heterogeneity is usually transient and rapidly averaged out. Here, we show that periodic thermal oscillation drives a 50% (v/v) ethanol–water mixture along a traceable nonequilibrium ordering pathway from transient ethanol–water clusters to kinetically stabilized soft aggregates. The system evolves from initial clusters to water-containing droplets as liquid–liquid microphase-separated intermediates, then to ethanol-rich acicular flakes, and ultimately to micron-scale flake-stacked aggregates. Fluorescence spectroscopy, two-dimensional correlation analysis, and nuclear magnetic resonance reveal that this process begins with perturbation and relaxation of the water-dominated hydrogen-bond network, proceeds through a water–ethanol coassociated intermediate, and culminates in ordered ethanol aggregates. Thermal analysis and in situ Raman spectroscopy further show progressive stabilization across the structural hierarchy. These findings demonstrate that periodic thermal forcing can amplify transient molecular heterogeneity into mesoscale soft structures in a fully miscible liquid, providing a kinetic perspective on nonequilibrium cluster amplification and microphase-separated aggregate formation in hydrogen-bonded molecular systems.