Search

Purpose : The formation and stability of emulsions in vitrectomized eyes is linked to the properties of the silicone oil-aqueous humor interface, in particular the surface tension. In the presence of natural surfactants, such as serum and plasma, the value of the surface tension is likely to change, but little quantitative information is presently available. To this end we perform accurate experiments measuring the interfacial properties of silicone oil (Siluron 1000) with anaqueous solution in the presence of endogenous-like proteins that can act as surfactants....

Liquid-films are constituted by a thin layer of a liquid in air or between two liquid phases. Such configuration, practically impossible to obtain with pure liquids, can be stabilised for limited times by the utilisation of surfactants and other surface-active species, including nanoparticles. The physico-chemical properties of the liquid films reflect quite directly on the properties of the corresponding emulsions and foams. In particular, the stability in time and against mechanical perturbations of the liquid film between approaching droplets is one of the factors determining the stability of an emulsion. Studying the properties of liquid films is then important to establish quantitative models useful to setup products and processes utilising foams and emulsions or to develop effective methodologies for a controlled breakdown of an emulsion into their liquid components. Liquid films are characterized by a very small thickness, as compared to transversal dimensions, which strongly limits the exchange of surface active molecules between the internal part of the liquid film and the adjacent volume. In addition, such thickness, in the limit case brings the surfactant molecules adsorbed at the two sides of the film to interact, which manifests as a pressure which must be overcome to produce further thinning of the film: the so-called "disjoining pressure". These circumstances have an important effect on the film dynamic behaviour, and are the main fundamental differences with the common liquid interface behaviour. Microgravity can be particularly useful in the investigation of the physico-chemical characteristics of liquid films which are mostly related to its destabilisation. In fact the thinning and destabilisation of the liquid-film is driven by two major drainage processes, driven respectively by capillary and gravitational forces. The latter can be strongly attenuated by weightlessness, allowing the possibility to investigate thinning as subject to capillarity alone. The experiment LIFT (Liquid-Film Tensiometer), selected and supported by the Italian Space Agency, is conceived with this aim. The dynamic properties of the film will be investigated by means of a specific tensiometric technique, based on a modification of Capillary Pressure Tensiometry. Such technique, already implemented in the ESA facility FASTER for Columbus EDR, was already shown to be very suitable to study single liquid interfaces under microgravity. LIFT is in fact being designed to be housed inside the facility FASTER. LIFT will allow measurements of the film dynamic tension(the equivalent of liquid interfacial tension) and the investigation of the film dilational rheology, that is the response of the film tension to perturbations of its interfacial area. The thickness of the film will be as well measured by means of an advanced interferometric technique. The present contribution is aimed at providing an overview of the LIFT programme, giving a description of the techniques to be utilised and of the planned experiments and reporting some results obtained with the laboratory prototypes. References: 1) Y.H. Kim, K Koczo, D. T. Wasan, J. Colloid Interface Sci.,187 (1997) 29 2) V.I. Kovalchuk; J. Krägel, A.V. Makievski, F. Ravera, L. Liggieri, G. Loglio, V.B. Fainerman, R. Miller, J. Colloid Interface Sci., 280 (2004) 498

Emulsions are fine dispersions of immiscible liquids (for example, oil and water) widespread in nature and in industrial products. Therefore mastering methodologies for formulating stable emulsions with tailored properties (for example, in food technologies or for cosmetics and pharmaceuticals), or to destabilise on purpose very stable emulsions (for example, for crude oil upstream) is an important technological target. Emulsions are stabilised against separation by means of surface-active molecules, generically known as surfactants, which segregating at the interface prevents the coalescence of the disperse droplets. MAP-FASES (Fundamental and Applied Studies in Emulsion Stability) is a research project supported by the European Space Agency, aimed at investigating the relationships between the interfacial properties of surfactant adsorption layers at liquid-liquid interfaces and the stability and features of the corresponding emulsions. In order to approach such important topic, the FASES project covers a wide spectrum of investigations and modellings: from the interaction of surfactants with particles, to droplet dynamics in emulsions, passing through the properties of single liquid-liquid interfaces and films.The research programme also includes the investigation of particle-stabilised emulsions. These emulsions are widely diffuse in nature (Pickering emulsions) and have an increasing importance for different products, technologies and applicative topics. Benchmark experiments are planned onboard the International Space Station (ISS), exploiting the purely diffusive conditions and the absence of buoyancy, which allows for the accurate investigation of interfacial properties and of the fundamental processes involved in emulsion destabilisation. In the framework of the project innovative diagnostics have been conceived and implemented into suitable experiment modules for the ISS. In particular, the experiment container FASES for the Fluid Science Laboratory, designed to investigate collective properties of the emulsion, such as, destabilisation process and droplets interaction and dynamics. The module allows for the serial investigation of emulsion samples of different nature (water-in-oil, oil-in-water, surfactant-stabilised, particle-stabilised) and composition (water/oil ratio, surfactant and particle concentrations) by means of two diagnostics. Optical Scan Microtomography is utilised for the investigation of transparent (dilute) emulsions, while a methodology utilising Differential Scan Calorimetry is used to investigate opaque (concentrate) emulsions. Properties of oil-water interfaces relevant for emulsion stability, such as dilational viscoelasticity of surfactant adsorbed layers and surfactant adsorption kinetics, are instead investigated by means of the facility FASTER (Facility for Adsorption and Surface Tension), based on the application of Capillary Pressure Tensiometry. Here we provide an overview of the FASES research programme and of the utilised experimental techniques and facilities.

In several natural or industrial processes liquid interfaces show a dynamic surface/interfacial tension (IT), as result of kinetic effects in the interfacial layers, involving the adsorption of surface-active species: from common surfactant molecules, to proteins and nanoparticles. These kinetics effects are triggered in response to perturbations of the system chemical composition (for example the surfactant concentration) and/or of the available interfacial area. The latter is the subject of the so-called surface dilational rheology and it is particularly relevant for the evolution and the stability of systems characterised by a large specific area, such as liquid films, emulsions, foams and ensembles of droplets and bubbles in general. For a deeper understanding of these phenomena, studying dilational rheology and adsorption kinetics under microgravity conditions is particularly significant because it provides simplified geometrical constraints and transport properties, due to the weightlessness and the consequent attenuation of convection. For investigating the topic and measuring the related physical quantities, specific methodologies have been developed. Among them Capillary Pressure Tensiometry is one of the most suitable method. Based on the direct application of the Laplace equation, CPT allows for the measurement of dynamic interfacial tension of a bubble/drop subject to different area variation stimuli. In particular measuring the response of IT to harmonic oscillation of the interfacial area (Oscillating Bubble/Drop method), allows for the calculation of the interfacial dilational viscoelasticity: the most significant quantity involved in surface dilational rheology. Results of previous experiments onboard the Space Shuttles References: 1) L. Liggieri, R. Miller, Current Opinions in Colloid and Interface Science 15 (2010), 256. 2) F. Ravera, G. Loglio, V. Kovalchuk, Colloid and Interface Science 15 (2010), 217-228. 3) V.I. Kovalchuk, F. Ravera , L. Liggieri , G. Loglio , P. Pandolfini, A.V. Makievski, S. Vincent-Bonnieu,nJ. Kraegel, A. Javadi, R. Miller, Advances in Colloid and Interface Science 161 (2010) 102-114

Introduzione a volume di una Serie: "Bubble and Drop Interfaces". Series "Progress in Interface and Colloid Science" (L. Liggieri, R. MIller, Eds.), vol. 2. Brill-Leiden, 2011