Dipartimento di Ingegneria Chimica, Chimica
Industriale e Scienza dei Materiali (DICCISM)
Università degli Studi di Pisa
Via Dotisalvi 2 - 56126 Pisa - Italy
Tel. ++39-050-2217848; Fax: ++39-050-2217866;
See the Publication List.
In the first part of this research effort, the transport of heat, mass and momentum in two phase, macroscopically homogeneous systems is studied, both experimentally and theoretically. Familiar examples include the flow of suspensions through pipes and the heat and mass conduction through composite materials. In general, the systems that we study consist typically of two phases, in which one is finely divided; thus, on the macroscale, the composite can be viewed as an effective continuum, having a set of well-defined effective properties, such as an effective diffusivity for the transport of solutes through porous media, or an effective viscosity, for the flow of suspensions. Since, in principle, the theoretical determination of a given effective parameter requires the solution of a corresponding problem on a microscale, which from a practical standpoint is very difficult to implement, our approach is instead to determine these effective parameters using some sort of statistical averaging. In this way, we have determined the effective viscosity of neutrally buoyant suspensions, the effective reactivity, velocity and diffusivity of solutes in porous media, the mean thermocapillary velocity of polydisperse suspensions of bubbles, and the shear-induced diffusivity of suspensions of rigid spheres.
In another research effort, the phase separation of deeply quenched liquid mixtures into two phases is studied. Since the phase transition process can be triggered by changing either the temperature or the composition of the system, separation can be achieved either by heating and cooling the solvent mixtures across their miscibility curve, or by adding a solubility modifier. Two main topics are being currently investigated. First, the morphology of phase separation of liquid mixtures is studied from a fundamental point of view, both experimentally and theoretically, showing that it does not correspond to that of traditional nucleation-and-growth processes. Second, from a more technological viewpoint, we have developed a new process of solvent extraction, which a) is much faster than conventional approaches at extracting solutes from porous materials, and b) is rapid and emulsion-free even when surface active compounds are added to the system. The first result is due to the fact that in our process the pores are more effectively wetted, while the explanation of the second effect is that phase separation of critical liquid mixtures occurs without the appearance of moving interfaces. The new process has been applied in bioengineering to extract antibiotics from fermentation broths, and in remediation, to extract heavy metals from contaminated soils and sludge.
Finally, we have developed a general approach to simulate multiphase flow processes in confined geometries using the diffuse interface method. Multiphase flows are generally modeled assuming that the different phases are separated by an interface, that is a surface of zero thickness; imposing that the condition of local equilibrium is satisfied, all physical properties are allowed to change discontinuously across the interface. Naturally, that results in a free boundary problem, which means that one of the main problems of this approach is to determine the position of the interface. In microdevices, an additional difficulty is encountered, as most lengthscales of the systems are comparable to the real interface thickness; this problem arises also in modeling drop coalescence and break up and moving contact angles. In these cases, it is more reasonable to use a different approach, proposed at the end of the 19th century by Rayleigh and Van der Waals, where interfaces have a non-zero thickness, i.e. they are "diffuse", so that all quantities, such as density or composition, vary continuously. This approach was later generalized by Landau and Ginzburg, who developed the mean field theory of phase transition. In particular, when the equations of motion are derived, an additional stress, so called Korteweg stress, arise naturally as reversible body forces, that tends to minimize the free energy of the system. This approach has been applied to simulate several processes, such as a) mixing, spinodal decomposition and nucleation of macroscopically quiescent regular mixtures; b) deformation, coalescence and break-up of fluid volumes under shear flows; c) heat transfer enhancement due to phase change.