Hydrodynamical pathways in the phase change of real fluids.

Identifying the incipient conditions of liquid-vapour transformation, the number of bubbles formed, their spatiotemporal scales, and the role of inertia remains a major challenge, reflecting how elusive the early stages of phase change are. Here, we present a theoretical framework that combines large deviation theory, multiphase fluctuating hydrodynamics and real fluid thermodynamics to compute the most probable nucleation pathways in metastable liquids. We identify the optimal trajectories connecting metastable and stable states and determine the full spatiotemporal structure of the nucleation process. Our results reveal that nucleation is not solely governed by thermodynamic forces, but is also shaped by hydrodynamic phenomena such as wave propagation and inertial effects. The approach predicts boiling thresholds for water, nitrogen, and helium, in agreement with experiments. It provides a unified, predictive description of phase-change kinetics linking microscopic fluctuations to macroscopic hydrodynamic observables, opening routes to prediction and control of phase change.