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
A Liquid-Film Tensiometer (LIFT) for Microgravity Experiments
Liggieri L;Ravera F;Santini E;Ferrari M;Loglio G
2011
Abstract
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) 498I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
