Ultrafast Heating Unveils Hidden Liquid–Liquid Phase Separation

In supercooled liquids, phase selection is governed by competing transformation timescales, yet crystallization typically pre-empts access to metastable liquid states. Liquid–liquid phase separation (LLPS) has long been proposed as a fundamental source of mesoscale heterogeneity, yet direct experimental access to heating-induced LLPS has remained elusive. Here we demonstrate that ultrafast heating opens a kinetic window in which crystallization and liquid–liquid demixing become temporally separable. Using a model supercooled metallic liquid, we directly resolve the emergence and growth of coexisting liquid populations on millisecond timescales by correlating ultrafast calorimetry with nanoscale real-space imaging and reciprocal-space structural probes. We show that subtle endothermic signatures in the supercooled regime—previously attributed to relaxation phenomena—constitute thermodynamic fingerprints of LLPS. The resulting chemical partitioning preconfigures subsequent crystallization pathways, revealing how competing timescales govern access to hidden regions of the liquid free-energy landscape. Our results establish kinetic control as a general route to reveal metastable liquid–liquid coexistence that is otherwise masked by crystallization, providing a framework for understanding non-equilibrium phase selection in supercooled liquids.