Advances in large amplitude oscillatory dilatational surface rheology - A review

Large amplitude oscillatory dilatational (LAOD) surface rheology is an emerging tool for characterising the nonlinear mechanical behaviour of air-liquid and liquid-liquid interfaces in multiphase systems, such as in foams and emulsions. These interfaces, stabilised by surfactants or emulsifiers, exhibit complex viscoelastic properties that significantly influence bubble and droplet stability under large deformation conditions. While conventional methods mainly focus on linear viscoelastic behaviour, LAOD allows us to explore the nonlinear deformation behaviour of interfaces, offering deeper insights into the mechanical properties of interfaces upon large deformation. This review highlights key advances in the dilatational rheology of interfacial films, providing a comprehensive background on LAOD experimental design, data collection, and analysis techniques. Recent developments in data analysis have revolutionised the LAOD technique. While Lissajous plots gave qualitative insights in the past, the new general stress decomposition (GSD) method allows for more quantitative analysis of the nonlinearities. GSD can quantitatively separate density-driven and actual rheological (i.e. network) contributions in the stress response. This separation reveals previously hidden rheological responses, allowing more accurate quantification of interfacial mechanics and deeper mechanistic insights into interfacial stabilisation phenomena. We will discuss how these LAOD methodologies have significantly enhanced our understanding of interfaces stabilised by surfactants, proteins, particles, and mixed systems. The main take-home-message: The emergence of advanced LAOD methodologies has redefined how we characterise nonlinear interfacial mechanics. Future research should focus on expanding GSD applications and revisiting previously studied interfaces with GSD, which will quantify the previously qualitatively described behaviour of interfacial films.