Evaluation of a novel multimode interfacial rheometer

Determination of the rheological properties and the interfacial structure–property relationships for complex fluid–fluid interfaces can be of crucial importance for the understanding of physiological and biomedical systems, designing and engineering industrial processes, and developing environmental remediation technologies. For the measurement of interfacial shear rheological material functions, it has been determined that the control of the surface pressure during the application of deformation is essential for obtaining reproducible data especially when measuring complex interfaces, such as particle- and polymer-laden interfaces. Moreover, the study of complex fluid interfaces is complicated by kinematically mixed interfacial flow fields, which include both shear and dilatation (shape and area changes), leading to a possible complex flow history. To address this, specialized rheometers have been developed to provide clear kinematic conditions. For instance, a radial trough has been introduced to enhance the study of dilatational interfacial rheology, effectively solving the challenges posed by the mixed flow fields typical in standard rectangular Langmuir–Pockels (LP) troughs or pendant drops. In the present work, we utilize a new trough instrument, the Quadrotrough (QT), capable of performing on the same device (and sample) independent dilatational and shear deformations at the air/liquid interface under strain and surface pressure control. Brewster angle microscopy (BAM) imaging is carried out in situ simultaneously with rheological measurements. Thus, the QT embodies the combined advantages of the circular trough and the controlled surface pressure shear interfacial rheometer. The interfacial rheology of poly(tert-butyl methacrylate) at the air/water interface was measured for both pure dilatation and pure shear in steady and small amplitude oscillatory (SAO) dilatation (D) and shear (S) modes on the same interface. BAM images were obtained during shear and compression. The results obtained by the QT were highly reproducible and in good agreement with measurements performed previously using the LP trough-mounted double wall ring rheometer and the radial trough. The Hencky strain model was employed to derive steady shear and dilatational interfacial moduli. Very good agreement was observed between the steady and complex shear moduli. However, the dilatational moduli measured under steady compression were markedly smaller than those measured by small amplitude oscillatory dilatational at fixed molecular areas, further highlighting the complicating factor of deformation history on material properties for complex interfaces. In summary, the QT has been shown to be a valuable tool for exploring interfacial rheology and providing insights into complex interfacial systems.