Hydrodynamical pathways in the phase change of real fluids.

At what conditions do liquids transform into vapour, how many bubbles form, on what spatiotemporal scales, and do inertial effects matter? These questions remain unanswered, reflecting how elusive the incipient 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 fluids. 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.