Nonequilibrium interfacial diffusivity resolves anomalies in monolayer hydrodynamics.

Hydrodynamics of surfactant monolayers are governed by surface tension, surface shear viscosity, surface dilatational viscosity, and surface diffusivity. While the first two have been studied comprehensively with consistent results, surface dilatational viscosity is plagued with anomalies, including apparent negative values. Here, full-field interfacial velocity measurements together with the Navier-Stokes equations reveal that a source of these anomalies is the use of equilibrium surface diffusivity for systems far from equilibrium. Specifically, surface diffisivity was found to be more than seven orders of magnitude (ten million times) larger in a nonsolenoidal interfacial flow field. Micron-sized phase domains in the monolayer are the likely culprit for the enhanced diffusivity.