Scientists discovered that liquids can act like they have 'surface tension' even when they’re perfectly mixed, just because of the way they move.
Normally, surface tension (like the 'skin' on water) only happens at the boundary between two different things, like oil and water. This paper shows that 'Korteweg stresses'—a type of internal tension—can emerge in fluids that are totally mixed together simply through the physics of how they flow. It means that even if you have two liquids that want to blend, they can still push against each other like they’re separated by a physical membrane. This discovery rewrites the rules for how pollutants move in groundwater or how chemicals mix in industrial vats. It turns out 'flow' can create structure and boundaries where we previously thought there were absolutely none.
A hydrodynamic origin of Korteweg stresses from shear-induced horizontal buoyancy
arXiv · 2604.12058
Recent study \cite{rajamanickam2025shear} of non-Boussinesq fluids in narrow channels identified a novel shear-induced horizontal buoyancy force that emerges upon depth-averaging the Navier--Stokes equations. This note demonstrates that this force is formally equivalent to the divergence of a Korteweg stress tensor. Unlike classical Korteweg stresses, which are typically attributed to molecular-scale cohesive potentials or implemented through assumed constitutive relations, we show that this eme