A strategy to halt dissolution of particle-coated air bubbles in water based on interfacial rheology design is presented. Whereas previously a dense monolayer was believed to be required for such an "armored bubble" to resist dissolution, in fact engineering a 2D yield stress interface suffices to achieve such performance at submonolayer particle coverages. We use a suite of interfacial rheology techniques to characterize spherical and ellipsoidal particles at an air-water interface as a function of surface coverage. Bubbles with varying particle coverages are made and their resistance to dissolution evaluated using a microfluidic technique. Whereas a bare bubble only has a single pressure at which a given radius is stable, we find a range of pressures over which bubble dissolution is arrested for armored bubbles. The link between interfacial rheology and macroscopic dissolution of ∼ 100 µm bubbles coated with ∼ 1 µm particles is presented and discussed. The generic design rationale is confirmed by using nonspherical particles, which develop significant yield stress at even lower surface coverages. Hence, it can be applied to successfully inhibit Ostwald ripening in a multitude of foam and emulsion applications.interfacial rheology | foams | yield stress | Ostwald ripening | emulsions T uning the interparticle interaction potential in bulk suspensions has long been a strategy to engineer the properties of colloidal suspensions. In this work, we apply this paradigm to interfacial materials, specifically particle-stabilized drops and bubbles. These systems with high interfacial area have broad applicability from food formulation and processing (1, 2), encapsulation (3, 4), ultrasound medical technologies (5), to lowweight/high-strength materials (6). One of the key challenges in using solid stabilized emulsions and foams in applications is curtailing Ostwald ripening, which causes the growth/shrinkage of large/small bubbles and increased size heterogeneity (7).Ripening occurs due to differences in the Laplace pressure in bubbles of different radii; large bubbles grow, while small bubbles shrink. This suggests that strategies to impart a resistance to dilation or compression of the interface would retard or entirely stop Ostwald ripening. Previously, fully covered, "jammed," particle coated bubbles were shown to fully resist dissolution of this nature (8)(9)(10)(11)(12). When the ratio of particle size to bubble size is large (a/R > 0.1), specific faceted shapes may moreover reduce the mean curvature to zero, thereby reducing the driving force to zero (10). However, stability is also observed at much smaller a/R ratios, suggesting other factors come into play. Previous work supposed the particles do not interact with each other, but since such interactions have a major role in interfacial rheology, they can potentially contribute to bulk bubble and emulsion stability as well.Here, we design and characterize model viscoplastic interfacial systems consisting of spherical and nonspherical particles at an air-water int...

In this paper, we describe an experimental protocol designed to ensure well-defined thermal and shear histories of a waxy crude oil sample to be submitted to rheological measurements. We discuss the criteria for choosing an appropriate pretreatment, the geometry, and the temperature range, among other parameters. This protocol includes the so-called thermal cycle test, which is shown to play a key role in determining the appropriate test conditions. Possible sources of measurement error are discussed in detail.

Interfacial rheology becomes important when surface active species such as surfactants, particles, or proteins are present in sufficient quantities at liquid-liquid interfaces and interact between them. Interfacial rheometry measurements are challenging for various reasons. The mechanical response of the thin interface is often weaker compared to that of bulk materials and so one is often measuring close to the lower force and torque limits of rheometers, hence signal-to-noise ratios merit closer attention. In addition, the role of both instrument and sample inertia is more important for interfacial rheometry compared to bulk rheometry. Effects of misalignment and imperfections of the measurement geometries lead to effects of surface and line tension. Finally, peculiar for interfacial rheometry is the need to deconvolute the contributions of flow and deformation in the surrounding phases from that at the interface. Whereas some of these aspects have received attention in previous works, a clear and unambiguous view on the operating limits of interfacial rheometers has been missing. In the present work, we investigate the different experimental challenges and develop a generic methodology, which provides a clear definition of the operating limits of various interfacial rheometers including the interfacial needle shear rheometer, the double wall ring, and the bicone geometries. We validate this methodology by investigating the limitations defined intrinsically by the instrument as well as the ones emerging from the properties of the interface of interest for an interface composed of fatty alcohols which represents a challenging test case. The results provide cautionary examples and clear guidelines for anyone measuring interfacial rheology with these direct rheological techniques.

Asphaltenes have been suggested to play an important role in the remarkable stability of some water-in-crude oil emulsions, although the precise mechanisms by which they act are not yet fully understood. Being one of the more polar fractions in crude oils, asphaltenes are surface active and strongly adsorb at the oil/water interface, and as the interface becomes densely packed, solid-like mechanical properties emerge, which influence many typical interfacial experiments. The present work focuses on purposefully measuring the rheology in the limit of an insoluble, spread Langmuir monolayer in the absence of adsorption/desorption phenomena. Moreover, the changes in surface tension are deconvoluted from the purely mechanical contribution to the surface stress by experiments with precise interfacial kinematics. Compression “isotherms” are combined with the measurement of both shear and dilatational rheological properties to evaluate the relative contributions of mechanical versus thermodynamic aspects, i.e., to evaluate the “interfacial rheological” versus the standard interfacial activity. The experimental results suggest that asphaltene nanoaggregates are not very efficient in lowering interfacial tension but rather impart significant mechanical stresses. Interestingly, physical aging effects are not observed in the spread layers, contrary to results for adsorbed layers. By further studying asphaltene fractions of different polarity, we investigate whether mere packing effects or strong interactions determine the mechanical response of the dense asphaltene systems as either soft glassy or gel-like responses have been reported. The compressional and rheological data reflect the dense packing, and the behavior is captured well by the soft glassy rheology model, but a more complicated multilayer structure may develop as coverage is increased. Potential implications of the experimental observations on these model and insoluble interfaces for water-in-crude oil emulsion stability are briefly discussed.

In the present work a polymeric transient viscoelastic network is used as a model system to investigate several fundamentals of interfacial viscoelasticity and non-linear behavior, in simple shear, compression and for simple mixed deformations. A supramolecular polymer bilayer, characterized by long but finite relaxation times, is created at the water-air interface using a layer-by-layer assembly method. The possibility of studying the individual layers starting from an unstrained reference state enabled the independent quantification of the equilibrium thermodynamic properties, and the viscoelastic response of the bilayer could be studied separately for shear and compressional deformations. Time-and frequency-dependent material functions of the layer were determined in simple shear and uniform compression. Moreover, a quasi linear neo-Hookean model for elastic interfaces was adapted to describe step strain experiments on a viscoelastic system by allowing the material properties to be time-dependent. The use of this model made it possible to calculate the response of the system to step deformations. Within the linear response regime, both stress-strain proportionality and the superposition principle were investigated. Furthermore, the onset of non-linear behavior of the extra stresses was characterized in shear and for the first time in pure compression. We conclude by investigating the multilayer system in a rising bubble setup and show that the neo-Hookean model is able to predict the extra and deviatoric surface stresses well, up to moderate deformations.