The response of interfacial layers to deformations in size and shape depends on their composition. The corresponding main mechanical quantities are elasticity and viscosity of dilation and shear, respectively. Hence, the interfacial rheology represents a kind of two-dimensional equivalent to the traditional bulk rheology. Due to growing interest in the quantitative understanding of foams and emulsions, more works are dedicated to studies on interfacial rheology. This overview presents the theoretical basis for traditional and recently developed experimental tools and discusses their application to different interfacial systems. While dilational rheology provides information on the composition of mixed interfacial layers, the shear rheology gives answers essentially on structures formed at an interface. The most frequently used methods at present are the oscillating drop and bubble tensiometry methods for dilational deformations and oscillating ring/bicone rheometers for shear deformations.

Rheology of interfacial layers

Emulsions, i.e., the dispersion of liquid droplets in a nonmiscible liquid phase, are overwhelmingly present in food products. In such systems, both liquid phases (generally, oil and water) are separated by a narrow region, the oil-water interface. Despite the fact that this interface is very thin (in the nanometer range), it represents a large surface area and controls to a great extent the physicochemical stability of emulsions. This review provides an overview of the aspects that govern the composition, structure, and mechanical properties of interfaces in food emulsions, taking into account the complexity of such systems (presence of numerous surface-active molecules, influence of processing steps, and dynamic evolution due to chemical changes). We also review methods that have conventionally, or recently, been used to study liquid-liquid interfaces at various scales. Finally, we focus on the link between interfacial properties and the physical, chemical, and digestive stability of emulsions at different levels and point out trends to control stability via interfacial engineering.

Formation, Structure, and Functionality of Interfacial Layers in Food Emulsions.

We review the state of the art in foam and highly concentrated emulsion rheology, with an emphasis on progress made over the last five years. Since the structures and physico-chemical processes relevant for foams and emulsions are closely analogous, comparing the knowledge recently gained in these two neighboring fields brings fresh insight. In this spirit, we review how the macroscopic mechanical response arises from a coupling between interfacial energy and long range molecular interactions, entropic effects, interfacial rheology, and dynamics at the droplet or bubble scale. We present experiments and models concerning elasticity, osmotic pressure, yielding and flow behavior.

Rheology of foams and highly concentrated emulsions

Role of pH-induced structural change in protein aggregation in foam fractionation of bovine serum albumin

Water contamination by organic pollutants is ubiquitous and hence a global concern due to detrimental effects on the environment and human health. Here, it is demonstrated that amyloid fibrils aerogels are ideal adsorbers for removing organic pollutants from water. To this end, amyloid fibrils prepared from β-lactoglobulin, the major constituent of milk whey protein, are used as building blocks for the fabrication of the aerogels. The adsorption of Bentazone, Bisphenol A, and Ibuprofen, as model pollutants, is evaluated under quasi-static conditions, without use of energy or pressure. Through adsorption by amyloid fibrils aerogel, excellent removal efficiencies of 92%, 78%, and 98% are demonstrated for Bentazone, Bisphenol A, and Ibuprofen, respectively. Furthermore, the maximum adsorption capacity of amyloid fibrils aerogel for Bentazone, Bisphenol A, and Ibuprofen is 54.2, 50.6, and 69.6 mg g-1 , respectively. To shed light on the adsorption equilibrium process, adsorption isotherms, binding constants, saturation limits, and the effect of pH are evaluated. Finally, the regeneration of the aerogel over three consecutive cycles is studied, exhibiting high reusability with no significant changes in its removal performance. These results point at amyloid fibrils aerogels as a sustainable, efficient, and inexpensive technology for alleviating the ubiquitous water contamination by organic pollutants.

Amyloid Fibrils Aerogel for Sustainable Removal of Organic Contaminants from Water

Macroscopic properties of aqueous β-lactoglobulin (BLG) foams and the molecular properties of BLG modified air-water interfaces as their major structural element were investigated with a unique combination of foam rheology measurements and interfacial sensitive methods such as sum-frequency generation and interfacial dilatational rheology. The molecular structure and protein-protein interactions at the air-water interface can be changed substantially with the solution pH and result in major changes in interfacial dilational and foam rheology. At a pH near the interfacial isoelectric point BLG molecules carry zero net charge and disordered multilayers with the highest interfacial dilatational elasticity are formed at the air-water interface. Increasing or decreasing the pH with respect to the isoelectric point leads to the formation of a BLG monolayer with repulsive electrostatic interactions among the adsorbed molecules which decrease the interfacial dilational elasticity. The latter molecular information does explain the behavior of BLG foams in our rheological studies, where in fact the highest apparent yield stresses and storage moduli are established with foams from electrolyte solutions with a pH close to the isoelectric point of BLG. At this pH the gas bubbles of the foam are stabilized by BLG multilayers with attractive intermolecular interactions at the ubiquitous air-water interfaces, while BLG layers with repulsive interactions decrease the apparent yield stress and storage moduli as stabilization of gas bubbles with a monolayer of BLG is less effective.

/pdf/ph-effects-on-the-molecular-structure-of-b-lactoglobulin-447ah6jfd6.pdf

pH effects on the molecular structure of β-lactoglobulin modified air-water interfaces and its impact on foam rheology.

/pdf/yield-stress-and-elasticity-of-aqueous-foams-from-protein-2esc1pqkw0.pdf

Yield stress and elasticity of aqueous foams from protein and surfactant solutions – The role of continuous phase viscosity and interfacial properties

Die rheologischen Eigenschaften von Schaumen sind fur die Qualitat vieler Produkte von entscheidender Bedeutung. Um die charakteristischen Parameter scheinbare Fliesgrenze und Speichermodul bei Kenntnis der physikalischen Eigenschaften Blasengrose, Oberflachenspannung und Gasvolumenanteil voraussagen zu konnen, existieren bereits Modellgleichungen. In dieser Arbeit wird gezeigt, dass diese nicht alle Einflussgrosen berucksichtigen. Prasentiert wird eine empirische Modellgleichung, die den Einfluss der Viskositat der kontinuierlichen Phase berucksichtigt und die Verteilung der Blasengrosen auf physikalisch sinnvolle Weise einbezieht. Daruber hinaus wird verdeutlicht, dass Grenzflacheneigenschaften eine entscheidende Rolle fur die Schaumrheologie spielen.



The rheological properties of foams are of particular importance especially in the food sector. Already existing models allow for the prediction of the rheological parameters apparent yield stress and storage modulus when the physical properties bubble size, surface tension and gas volume fraction are known. This work demonstrates that these are not all influencing parameters. Here an extended empirical model is presented that takes the continuous phase viscosity into account and additionally includes the distribution of the bubble sizes in a physically meaningful way. Furthermore, it is shown that interfacial layer properties also play a crucial role in foam rheology.

Einfluss der Flüssigkeitsviskosität auf das rheologische Verhalten von Schäumen†Effect of the Liquid Viscosity on the Rheological Behavior of Foams

Die rheologischen Eigenschaften von Schäumen sind für die Qualität vieler Produkte von entscheidender Bedeutung. Um die charakteristischen Parameter scheinbare Fließgrenze und Speichermodul bei Kenntnis der physikalischen Eigenschaften Blasengröße, Oberflächenspannung und Gasvolumenanteil voraussagen zu können, existieren bereits Modellgleichungen. In dieser Arbeit wird gezeigt, dass diese nicht alle Einflussgrößen berücksichtigen. Präsentiert wird eine empirische Modellgleichung, die den Einfluss der Viskosität der kontinuierlichen Phase berücksichtigt und die Verteilung der Blasengrößen auf physikalisch sinnvolle Weise einbezieht. Darüber hinaus wird verdeutlicht, dass Grenzflächeneigenschaften eine entscheidende Rolle für die Schaumrheologie spielen.

Einfluss der Flüssigkeitsviskosität auf das rheologische Verhalten von Schäumen

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pH effects on the molecular structure of β-lactoglobulin modified air-water interfaces and its impact on foam rheology.

Yield stress and elasticity of aqueous foams from protein and surfactant solutions – The role of continuous phase viscosity and interfacial properties

Einfluss der Flüssigkeitsviskosität auf das rheologische Verhalten von Schäumen†Effect of the Liquid Viscosity on the Rheological Behavior of Foams

Einfluss der Flüssigkeitsviskosität auf das rheologische Verhalten von Schäumen