No surface tension needed for stable phase interfaces and droplets

Droplets, bubbles, and their phase interfaces are often thought to imply the presence of surface tension. Analysis of droplet formation at Diesel conditions, i.e. at pressures exceeding the critical pressure of the injected fuel, exclusively focuses on mixing effects that may cause the formation of surface tension. Here, we introduce a physical mechanism that causes stable interfaces without any surface tension: thermal gradient induced interfaces (TGIIF) are based on distributions of density and thermal conductivity, and can occur at high subcritical and arbitrary supercritical pressures. TGIIF cause the formation of droplets even in pure supercritical fluids, in absence of phase equilibrium and surface tension. As if surface tension was acting, a diffuse droplet of a pure cool transcritical fluid in a hot environment stabilizes, and its interfacial density gradient sharpens rather than diffuses; a droplet with elliptical cross-section regresses towards a circular cross section. The existence of TGIIF thus challenges our concept of droplets and the interpretation of experimental data: the observation of a circular droplet shadowgraph does not necessarily imply the presence of phase equilibrium and surface tension, but could show a TGIIF.