Properties of Emulsions and Foams

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Properties of Emulsions and Foams

• FDSC400

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Goals

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

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Emulsion

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

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Types of Emulsion
mm
Water
Oil
Oil-in-water emulsion
Water-in-oil emulsion
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Multiple Emulsions
mm
Water
Oil
Water-in-oil-in-water emulsion
Oil-in-water-in-oil emulsion
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Emulsion Size

• lt 0.5 mm
• 0.5-1.5 mm
• 1.5-3 mm
• gt3 mm

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Number Distributions
Number

• lt 0.5 mm
• 0.5-1.5 mm
• 1.5-3 mm
• gt3 mm

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Large droplets often contribute most to
instability
Median
(Volume in class ?Total volume measured)
Polydispersity
Note log scale
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Volume Fraction
fTotal volume of the dispersed phase ?
Total volume of the system
Close packing, fmax Monodisperse Ideal
0.69 Random 0.5 Polydisperse Much greater
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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
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Emulsion Viscosity
Dispersed phase volume fraction
Viscosity of emulsion
Continuous phase viscosity
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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
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Emulsion Destabilization

• Creaming
• Flocculation
• Coalescence
• Combined methods

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Creaming
Buoyancy (Archimedes)
h Continuous phase viscosity Dr density
difference g Acceleration due to gravity d
droplet diameter v droplet terminal velocity vs
Stokes velocity
Friction (Stokes-Einstein)
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Flocculation and Coalescence
Collision and sticking (reaction)
Stir or change chemical conditions
FLOCCULATION
Rehomogenization
Film rupture
COALESCENCE
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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
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Gelled Emulsions
Thin liquid
Viscous liquid
Gelled solid
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Creaming Slight Flocculation

• Flocs have larger effective size
• Smaller Dr
• Tend to cream much faster

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Creaming Extreme Flocculation

• Heavily flocculated emulsions form a network
• Solid-like properties (gel)
• Do not cream (may collapse after lag period)

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Foams
Concentrated
Dilute
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Dilute Foams

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

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Concentrated Foams

• Distorted non-spherical gas cells
• Very high volume fraction, often gt99

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Foam Drainage

• Water drains from foam under gravity
• As water leaves, faces of film are brought closer
  together

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Film Rupture

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