Entanglement between thermodynamIcs and iNterFacial INstabIliTies in binary fluids close to the critical point – INFINIT_2025

The aim of the present project is to understand the physics of interfacial instabilities induced in quasi-critical liquid-liquid systems for which the physical properties may diverge or diminish, and have a universal thermodynamic character. We will focus on the evolution of interfacial instabilities such as Rayleigh-Plateau (liquid jet instability), Kelvin-Helmoltz (instability due to a tangential stress jump) or Bénard-Marangoni/thermocapillary (capillary instability in the presence of a thermal gradient) by varying the temperature distance to the critical point from immiscible fluids system to a miscible one and vice versa. This study will be carried out at the same time experimentally in binary mixtures, theoretically with a linear stability analysis and numerically using the phase field approach. This is a highly ambitious project, as it tackles the still poorly resolved problem of the entanglement of thermodynamics and fluidics in a universal framework, challenging stability laws and diagrams under critical non-equilibrium conditions. The effect of the interfacial tension between immiscible phases is in fact well-known, manifesting itself in everyday life, from the shape of droplets and soap bubbles to the capillary rise and adhesion or the locomotion of insects on the surface of water. Despite its importance in fields as diverse as geosciences, polymer physics and multiphase flows, experiments and theory on the interfacial tension of miscible systems are still rare. The transition from the immiscible to the miscible state has not yet been addressed in the literature, and this is the aim of the present project. It is fundamental in nature, but understanding the effects of a phase transition is highly relevant to many industrial applications.