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 Oil-in-water emulsion Water-in-oil emulsion Water Oil mm

Multiple Emulsions Water-in-oil-in-water emulsionOil-in-water-in-oil emulsion Water Oil mm

Emulsion Size < 0.5  m  m  m >3  m

Number Distributions < 0.5  m  m  m >3  m Number Very few large droplets contain most of the oil

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

Volume Fraction  =Total volume of the dispersed phase  Total volume of the system Close packing,  max Monodisperse Ideal ~0.69 Random ~0.5 Polydisperse Much greater

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

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

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)  Continuous phase viscosity  density difference g Acceleration due to gravity d  droplet diameter v droplet terminal velocity v s Stokes velocity

Flocculation and Coalescence Film rupture Rehomogenization Collision and sticking (reaction) Stir or change chemical conditions 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 rgrg

Gelled Emulsions Thin liquid Viscous liquidGelled 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 non- spherical 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