Tensiometry and rheology of complex interfaces

Complex fluid-fluid interfaces play an important role in a variety of application domains, from emulsion and foam stability, to thin films in biomedical applications, to coating flow phenomena. The current work reviews progress made in particular for interfaces where rheological stresses, peculiar to the interface, play an important role. The developments made in the area of constitutive modeling are briefly reviewed to clarify which material functions can be measured. For shear rheometry, progress in analyzing the flow field in the measurement device has been key, combined with advances in control over surface concentration and microstructural evaluation. For dilation/compressional rheometry much work has been done on separating changes in the surface tension from the extra rheological stresses. Finally, we discuss how simple complex flows, such as thin film and drainage flows, offer a first step up in complexity and seem to present a good benchmark problem for testing constitutive equations and the interplay between transport phenomena, interfacial rheology and the changes in state variables.