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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. [ABSTRACT FROM AUTHOR]

Film drainage, essential in droplet and bubble coalescence and surface wetting, is influenced strongly by the stress boundary condition, in particular, when interfacial stresses are present. These stresses, caused by ubiquitous surface-active components, significantly impact the dynamics of liquid films. Through dynamic thin film balance experiments, we compare the effects of Marangoni stresses, interfacial viscosity, and interfacial viscoelasticity on the drainage of free-standing thin liquid films. These data serve to demonstrate that film deformation intricately depends on the interplay between these stresses and capillarity, resulting in widely varied drainage times. Seemingly subtle changes, especially in the local stress-carrying capacity of the interface, can lead to significant differences in film dynamics. This makes it a promising area for research into interfacial-rheologically active materials for stabilizing potentially more sustainable multiphase materials. [ABSTRACT FROM AUTHOR]

When surface-active molecules or particles assemble at fluid–fluid interfaces, these interfaces acquire complex rheological properties that are of importance in processes that involve flow and deformation of interfaces. Although much progress has been made, interfacial rheology measurements and, in particular, the measurement of interfacial rheological properties of polymers at the air-water interface remain challenging. These are due to weak interactions with the water subphase, the polymer backbone conformation, the glass transition of the interfacial layer, and memory effects. In the present work, we describe systematic rheological measurements of polymer-laden interfaces. The measurements were performed with four different interfacial shear rheometers that can be classified into two types: rheometers in which the surface pressure can be controlled independently, and devices based on fixtures mounted on standard rotational rheometers and lacking control of the surface pressure. We use poly(tert-butyl methacrylate) and poly(methyl methacrylate), two high glass transition temperature, hydrophobic polymers anchored to the water subphase by means of the acrylate group. Using a Langmuir–Pockels (LP) trough, we identify the transition of the polymer monolayer from a viscous to a solid elastic or soft-glassy interface as the polymer surface concentration increases by compression. Then, we compare the linear viscoelastic properties of the interface as obtained by each rheometer. Our results show poor reproducibility and comparability of the rheological data as obtained by different rheometers for the same polymer. This is mainly due to differences in the method used to prepare the layers. For LP-based devices, spreading under dilute conditions and subsequent compression yields layers of compressed glassy blobs with reproducible results. On the other hand, for devices without surface pressure control, deposition of the amount needed to reach a desired concentration may lead to the formation of ill-defined layers resulting in irreproducible data. Furthermore, we find that only when spreading the polymer to form a dilute layer and then controlling the surface pressure by compression, we can clearly distinguish the fluidlike from solidlike interfaces, and a clear correlation is observed between the surface pressure (or interfacial polymer concentration) and the rheological properties of the interface. [ABSTRACT FROM AUTHOR]

When surface-active molecules or particles assemble at fluid–fluid interfaces, these interfaces acquire complex rheological properties that are of importance in processes that involve flow and deformation of interfaces. Although much progress has been made, interfacial rheology measurements and, in particular, the measurement of interfacial rheological properties of polymers at the air-water interface remain challenging. These are due to weak interactions with the water subphase, the polymer backbone conformation, the glass transition of the interfacial layer, and memory effects. In the present work, we describe systematic rheological measurements of polymer-laden interfaces. The measurements were performed with four different interfacial shear rheometers that can be classified into two types: rheometers in which the surface pressure can be controlled independently, and devices based on fixtures mounted on standard rotational rheometers and lacking control of the surface pressure. We use poly(tert-butyl methacrylate) and poly(methyl methacrylate), two high glass transition temperature, hydrophobic polymers anchored to the water subphase by means of the acrylate group. Using a Langmuir–Pockels (LP) trough, we identify the transition of the polymer monolayer from a viscous to a solid elastic or soft-glassy interface as the polymer surface concentration increases by compression. Then, we compare the linear viscoelastic properties of the interface as obtained by each rheometer. Our results show poor reproducibility and comparability of the rheological data as obtained by different rheometers for the same polymer. This is mainly due to differences in the method used to prepare the layers. For LP-based devices, spreading under dilute conditions and subsequent compression yields layers of compressed glassy blobs with reproducible results. On the other hand, for devices without surface pressure control, deposition of the amount needed to reach a desired concentration may lead to the formation of ill-defined layers resulting in irreproducible data. Furthermore, we find that only when spreading the polymer to form a dilute layer and then controlling the surface pressure by compression, we can clearly distinguish the fluidlike from solidlike interfaces, and a clear correlation is observed between the surface pressure (or interfacial polymer concentration) and the rheological properties of the interface. [ABSTRACT FROM AUTHOR]

In inertialess suspensions of rigid particles, the rotational motion of each particle is governed by the so-called freely rotating condition, whereby the total torque acting on the particle must be zero. In this work, we study the effect of viscoelasticity of the suspending liquid on the rotation period of a sphere by means of three-dimensional finite element simulations, for conditions corresponding to a macroscopic shear flow. The simulation results capture the slowing down of the rotation, relative to the Newtonian case, which was recently observed in experiments. It is shown that such a phenomenon depends on the specific constitutive equation adopted for the viscoelastic liquid. Analysis of transients shows a clear correlation between rotation rate and the development of first normal stress difference. [ABSTRACT FROM AUTHOR]