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Properties of Emulsions and Foams

Properties of Emulsions and Foams. FDSC400. Goals. Properties of emulsions Type Size Volume fraction Destabilization of emulsions Creaming Flocculation Coalescence Foams. Emulsion.

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Properties of Emulsions and Foams

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  1. Properties of Emulsions and Foams FDSC400

  2. Goals • Properties of emulsions • Type • Size • Volume fraction • Destabilization of emulsions • Creaming • Flocculation • Coalescence • Foams

  3. Emulsion A fine dispersion of one liquid in a second, largely immiscible liquid. In foods the liquids are inevitably oil and an aqueous solution.

  4. Types of Emulsion mm Water Oil Oil-in-water emulsion Water-in-oil emulsion

  5. Multiple Emulsions mm Water Oil Water-in-oil-in-water emulsion Oil-in-water-in-oil emulsion

  6. < 0.5 mm 0.5-1.5 mm 1.5-3 mm >3 mm Emulsion Size

  7. Very few large droplets contain most of the oil Number Distributions Number • < 0.5 mm • 0.5-1.5 mm • 1.5-3 mm • >3 mm

  8. Large droplets often contribute most to instability Median (Volume in class Total volume measured) Polydispersity Note log scale

  9. 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

  10. Viscosity Viscosity is the force required to achieve unit flow rate Maximum induced flow rate /ms-1 Force /N Distance/ m Slope viscosity /Nm-2s Force per unit area /Nm-2 No slip at the wall Shear rate /s-1

  11. Emulsion droplets disrupt streamlines and require more effort to get the same flow rate Emulsion Viscosity Dispersed phase volume fraction Viscosity of emulsion Continuous phase viscosity

  12. 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

  13. Emulsion Destabilization • Creaming • Flocculation • Coalescence • Combined methods

  14. Creaming Buoyancy (Archimedes) h Continuous phase viscosity Dr density difference g Acceleration due to gravity ddroplet diameter v droplet terminal velocity vs Stokes velocity Friction (Stokes-Einstein)

  15. Flocculation and Coalescence Collision and sticking (reaction) Stir or change chemical conditions FLOCCULATION Rehomogenization Film rupture COALESCENCE

  16. Flocculation leads to an increase in viscosity Water is trapped within the floc and must flow with the floc Effective volume fraction increased Rheology of Flocculated Emulsions rg

  17. Gelled Emulsions Thin liquid Viscous liquid Gelled solid

  18. Creaming & Slight Flocculation • Flocs have larger effective size • Smaller Dr • Tend to cream much faster

  19. Creaming & Extreme Flocculation • Heavily flocculated emulsions form a network • Solid-like properties (gel) • Do not cream (may collapse after lag period)

  20. Foams Concentrated Dilute

  21. Dilute Foams • Somewhat similar to emulsions • Various modes of formation • Large (~mm) spherical bubbles • Very fast creaming • Ostwald ripening

  22. Distorted non-spherical gas cells Very high volume fraction, often >99% Concentrated Foams

  23. Water drains from foam under gravity As water leaves, faces of film are brought closer together Foam Drainage

  24. Film Rupture • Film must thin then burst • Inhibited by surfactant repulsion/interfacial film • Self-repair by the Gibbs-Marangoni effect

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