Shear-Rate Dependent Surface Tension of Glass-Forming Fluids

We investigate the interface of a glass-forming fluid showing non-Newtonian rheology. By applying shear flow in the interface, we observe that the surface tension depends on the shear rate. Importantly, the standard way of determining surface tension from the pressure anisotropy caused by the interface can give rise to an effective surface tension in the non-Newtonian fluid that mixes bulk and interface properties. We show how the pressure anisotropy can be used to clearly define the bulk and interface regions and extract a genuine shear-rate dependent surface tension. The results have implications for measurement techniques related to the interfacial rheology of complex fluids.