Dilational Rheology and Adsorption Kinetics Investigation of Liquid Interfaces in Microgravity by the FASTER Facility.

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