Alberto Passerone

A diffusion-based approach to mixed adsorption kinetics

A new theoretical approach has been developed which allows mixed kinetics of non-ionic surfactants to be studied in the unitary framework of an extended diffusion-controlled problem. This treatment not only allows one to consider a large class of surface models, but also provides a better understanding of the relation existing between the presence of a potential barrier and the diffusion-controlled adsorption. In particular it is shown that the assumption of the presence of adsorption barriers does not depend on the local equilibrium hypothesis. In fact the theoretical treatment takes into account the possibility that mixed kinetics also occur in the absence of potential barriers. By this treatment a general equation is obtained which allows the adsorption as a function of time to be evaluated by numerical or analyical procedures for a given particular surface model.

A new experimental method for the measurement of the interfacial tension between immiscible fluids at zero bond number

Abstract A new methodology (Pressure Derivative Method) for measuring the interfacial tension σ between immiscible fluid phases at Δρg = 0 (zero Bond number) is presented. This method, which is particularly suitable for measurements under microgravity conditions, is also applicable on earth to liquid pairs with a small density difference. The method consists, essentially, of calculating σ by a linear fitting of experimental capillary pressure and curvature data, collected during the formation of a drop of one of the two liquids inside the other one. The curvature data are derived from the injected drops volume; thus, any pair of fluids can be used, including opaque liquids, for which optical curvature measurements are not possible. By a specific mathematical treatment of the experimental data, an overall accuracy on the order of less than 1% has been obtained. During the drop growth, a maximum in the pressure is reached, which can also be used to calculate σ by the Maximum Bubble Pressure technique (MBP). This value can be utilized as an internal check on the interfacial tension data obtained by the method proposed.

Drop formation instabilities induced by entrapped gas bubbles

Abstract During the formation of a liquid drop out of a capillary tube, instabilities can appear which cause an “instantaneous” variation in the drop volume. An analytical treatment of this phenomenon, which takes into account the presence of entrapped gas bubbles to explain the experimental observations is presented. A nondimensional number (called the Bubble Stability Number) has been derived, describing the influence of different experimental parameters (surface tension, radius of the capillary tube, external pressure, and volume of the entrapped gas) in determining the region of stable growth. Applications of this theory to systems which can present this type of instability are also discussed.

Surface tension and viscosity of industrial alloys from parabolic flight experiments — Results of theThermoLab project

The surface tension and the viscosity of a series of industrial alloys have been measured by the oscillating drop technique with an electromagnetic levitation device under reduced gravity conditions in several parabolic flights. It was demonstrated that the 20 seconds of reduced gravity available in a parabola were sufficient for melting, heating into the liquid phase, and cooling to solidification of typically 7 mm diameter metallic specimen. The surface tension and the viscosity were obtained from the frequency and the damping time constant of the oscillation which were evaluated from the temperature signal of a highresolution pyrometer. Alloys processed included steels, Ni-based superalloys, and Ti-alloys which were supplied by industrial partners to the project. Three to four parabolas were sufficient to obtain the surface tension and the viscosity over a large range in temperature.

Preliminary results of the facility for adsorption and surface tension (FAST) experiments onboard STS-107, in the framework of the project FASES

The Facility for Adsorption and Surface Tension studies (FAST) has recently flown on-board the Shuttle Columbia, in the SPACEHAB module, during the STS-107 mission. In this paper a first evaluation of the experimental results obtained is presented, together with the basic working principle of the Capillary Pressure Tensiometer, which is the core of the facility, and the main lines of the experiments performed during the flight.

Adsorption properties of C10E8 at water/ hexane interface investigated onboard STS-107, by the FAST facility

The dynamic adsorption properties of a non-ionic surfactant C10E8 at the water-hexane interface have been investigated in microgravity using FAST (Facility for Adsorption and Surface Tension studies) onboard the Shuttle STS-107. To obtain a complete characterisation of such model system, the Capillary Pressure tensiometer implemented in the facility has been used with different experimental methodologies allowing different aspects of adsorption dynamics to be investigated. Theoretical models specifically developed have been used for the data interpretation allowing the characterisation of the adsorption mechanisms.

Measurement of thermophysical properties of liquid metallic alloys in a ground- and microgravity based research program. The Thermolab Project

The ThermoLab project is concerned with the measurement of the thermophysical properties of industrial alloys in the liquid phase. The project combines long and short duration microgravity measurements based on containerless processing with an electromagnetic levitation device and a ground based experimental programme using conventional and containerless processing techniques. An overview of the project and representative results from the ground based experimental programme are given. Alloys investigated included Ni-based, Fe-based, a Cu-Sn-Mg alloy and a γ-TiAl alloy.

Results of microgravity investigation on adsorption and interfacial rheology of soluble surfactants from the experiment fast

Microgravity offers ideal diffusive conditions for the investigation of the dynamic processes involving surfactant adsorption. Here a summary of the results obtained during the FAST (Facility for Adsorption and Surface Tension studies) experiments on board the NASA Shuttle STS-107 is given. The measured data pertinent to interfacial rheology — i.e., the response of dynamic interfacial/surface tension to imposed perturbation of the interfacial area — for water-hexane and water-air interfaces, in the presence of different non-ionic surfactants are reported and utilised to test available models and to check fundamental aspects of the modelling of dilational rheology.

Facility for adsorption and surface tension studies (FAST) on board of shuttle STS-107 mission: Determination of the surface dilational modulus as a function of concentration and temperature for aqueous solutions of dodecyl-dimethyl-phosphine-oxide, in the 0.01–0.32 Hz frequency range

Measurements of dynamic-surface-tension responses to harmonic surface perturbations, in the low-frequency range, are reported for aqueous solutions of n-dodecyl-dimethyl-phosphine-oxide. From the primary observed results of the steady-state surface oscillation behaviour, the inherent surface dilational modulus is determined at different frequencies, at various concentrations up to the critical micellar concentration and at three temperatures (i.e., at T=15, 25 and 35 °C). The obtained experimental values are successfully interpreted in terms of the diffusion-controlled model. The experiments grant reliable information about the constitutive dynamic properties of air-liquid single interfaces.

An Auger Investigation of Oxygen-Enhanced Tin Segregation on a Liquid Pb-Sn Alloy.

SummaryAuger measurements of the surface composition have been performed on solid and molten Pb-5at%Sn alloys. They confirm the theoretical predictions that, in the absence of oxygen, liquid or solid alloys exhibit no significant surface segregation. On the contrary, surface composition measurements and simultaneous surface tension measurements clearly show that oxygen adsorption strongly affects the segregation of tin at the liquid-vapour interface. This surface enrichment is driven by the contribution of the reaction free energy and of the products surface tension.