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Properties of Emulsions and Foams FDSC400 Goals • Properties of emulsions – Type – Size – Volume fraction • Destabilization of emulsions – Creaming – Flocculation – Coalescence • Foams Emulsion A fine dispersion of one liquid in a second, largely immiscible liquid. In foods the liquids are inevitably oil and an aqueous solution. Types of Emulsion mm Water Oil Oil-in-water emulsion Water-in-oil emulsion Multiple Emulsions mm Water Oil Water-in-oil-in-water emulsion Oil-in-water-in-oil emulsion Emulsion Size A • • • • < 0.5 mm 0.5-1.5 mm 1.5-3 mm >3 mm Number Distributions • • • • < 0.5 mm 0.5-1.5 mm 1.5-3 mm >3 mm Very few large droplets contain most of the oil 30 Emulsion 3 Median 25 Frequency /% (Volume in class Total volume measured) 35 Emulsion 5 20 15 Large droplets often contribute most to instability Emulsion 1 10 5 0 0.1 1 Diameter /mm 10 Note log scale Volume Fraction f=Total volume of the dispersed phase Total volume of the system Close packing, fmax Monodisperse Ideal ~0.69 Random ~0.5 Polydisperse Much greater Viscosity Viscosity is the force required to achieve unit flow rate No slip at the wall Force /N Force per unit area /Nm-2 Distance/ m Maximum induced flow rate /ms-1 Shear rate /s-1 Emulsion Viscosity Dispersed phase volume fraction Viscosity of emulsion 0 2.5f Continuous0.8phase viscosity Slope=2.5 0.7 Emulsion droplets disrupt streamlines and require more effort to get the same flow rate Viscosity 0.6 0.5 0.4 0.3 0 0.2 0.1 0 0 0.05 0.1 0.15 Volume Fraction 0.2 0.25 Chemical Composition Interfacial layer. Essential to stabilizing the emulsion Oil Phase. Limited effects on the properties of the emulsion Aqueous Phase. Aqueous chemical reactions affect the interface and hence emulsion stability Emulsion Destabilization • • • • Creaming Flocculation Coalescence Combined methods Creaming Buoyancy (Archimedes) Friction (Stokes-Einstein) 3dv Fb d g 3 6 2 d g vs 18c Continuous phase viscosity density difference g Acceleration due to gravity d droplet diameter v droplet terminal velocity vs Stokes velocity Flocculation and Coalescence FLOCCULATION COALESCENCE Rheology of Flocculated Emulsions • Flocculation leads to an increase in viscosity • Water is trapped within the floc and must flow with the floc • Effective volume fraction increased rg Gelled Emulsions Thin liquid Viscous liquid Gelled solid Creaming & Slight Flocculation • Flocs have larger effective size • Smaller • Tend to cream much faster Creaming & Extreme Flocculation • Heavily flocculated emulsions form a network • Solid-like properties (gel) • Do not cream (may collapse after lag period) Foams Concentrated Dilute Dilute Foams • • • • • Somewhat similar to emulsions Various modes of formation Large (~mm) spherical bubbles Very fast creaming Ostwald ripening Concentrated Foams • Distorted nonspherical gas cells • Very high volume fraction, often >99% Foam Drainage • Water drains from foam under gravity • As water leaves, faces of film are brought closer together Film Rupture • Film must thin then burst • Inhibited by surfactant repulsion/interfacial film • Self-repair by the Gibbs-Marangoni effect
