Drainage and stability of protein stabilized standing foam

Zebin Wang, Purdue University

Abstract

Evolution of liquid holdup profile in standing foams formed by whipping and stabilized by proteins in the presence of xanthan gum, was measured using magnetic resonance imaging at different pH, ionic strength, protein and gum concentration, centrifugal force, and protein type (Na-caseinate and β-lactoglobulin). Variation of foam bubble size was measured by microscopy. Foam was found to be most stable at pH5.l near the isoelectric point of protein, lower ionic strength, higher protein and gum concentration. A model for velocity of drainage of power law fluid in a Plateau border for simplified geometry was incorporated in a foam drainage model to predict the liquid holdup profile evolution. The predicted profiles agreed well with experimental results at small times. Shear surface elasticity and viscosity of proteins were measured by an interfacial rheometer. Dilatational elasticity and viscosity were inferred from the measurement of surface tension of a pulsating bubble exposed to protein solution using a model accounting for adsorption, desorption and relaxation. The surface rheological properties were taken into account in a finite element analysis of flow of power law fluid through an actual Plateau border geometry with mobile interface. These results and bubble size variation were then incorporated into the foam drainge model which gave an improved prediction. A linear stability analysis of a thin film on a solid surface indicated that the growth coefficient of perturbation decreased with an increase in surface elasticity and viscosity thereby establishing the role of interfacial rheological properties on film stability. Disjoining pressure of foam films stabilized by proteins in the presence of macromolecules due to electrostatic double layer repulsion, van der Waal's attraction, steric repulsion and depletion force due to nonadsorbed macromolecules were calculated for different system parameters. Formalism for the rupture of an equilibrium foam film with an immobile interface due to imposed mechanical perturbations modeled as Gaussian white noise was employed to predict the rupture time as a function of perturbation amplitude. The film rupture model was then combined with the foam drainage model to predict the variation of average bubble size, which was then compared with data for different perturbation amplitudes.

Degree

Ph.D.

Advisors

Narsimhan, Purdue University.

Subject Area

Agricultural engineering

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