Interfacial shear rheology study of wet foams

The texture and the persistence of the wet foam of tiny sphere-shaped gas bubbles that tops the espresso coffee liquid phase are of the utmost importance for the quality of this brew. The prevailing liquid phase of the wet foam solubilizes biopolymers and natural surfactants which will participate to the formation of the liquid films framing the gas phase. The rheology of the interfacial layer can potentially contribute to stability and functionality of foam. This work follows a previous published paper (Piazza, Gigli & Bulbarello, 2007) in which the main macromolecules responsible for the espresso coffee foaming fraction, i.e. proteins/melanoidins complexes fraction and polysaccharides fraction were extracted from defatted and non-defatted commercial coffee powders and analyzed for their foaming attitude and interfacial viscoelastic properties. The development of interfacial viscoelastic dynamic moduli at the liquid/gas interface was monitored by means of a commercial interfacial rheometer. In this paper the rheology data are completed with tensiometric measurements (pendant drop technique) and a mathematical evaluation of the phenomena underlying the viscoelastic properties is suggested. The interfacial elastic modulus increases with time in a sigmoid-like mode and contains the particle coagulation kinetic, governed by Smoluchowsky equation, and the migration of particles from the bulk phase to the interface. Due to the complexity of the system, an appropriate mathematical description is here presented that overcomes some limitations of the existing models and describes the development of interfacial viscoelastic properties at the liquid/ gas interface on long observation scales. The proposed model contains a linear term of transport and the kernel function (considering the simplest case of hard rigid spherical particles in brownian motion), integrated in one differential equation to describe the coagulation and migration of particles to the interface.