Surface dilatational viscosity: Fact or fiction?

The free surface of liquids, water in particular, can exhibit significant excess shear viscosity in the presence of surfactants, including monomolecular films of DPPC, the main constituent of lung surfactant. Detailed velocity measurements have been reconciled with numerical simulations for many different monolayers exhibiting either Newtonian or non-Newtonian responses to shear, where the interfacial velocity field is solenoidal. The situation is altogether different for surface dilatational viscosity, which is the more relevant quantity for non-solenoidal interfacial velocity fields. To avoid complications with interfacial accelerations, this work uses an oscillating floor and fluid inertia to produce a non-solenoidal velocity field at a flat interface with a fixed area, studying the competing effects of Marangoni stress associated with surface tension gradients and any intrinsic combination of surface dilatational and shear viscosity. The flow geometry is rectangular, with a width twice the depth and a span more than six times the depth. Interfacial velocity measurements verify the expected half-period-flip symmetry. These measurements are compared with 2D simulations utilizing the measured equation-of-state and diffusivity of DPPC.