Properties of Emulsions and Foams FDSC400
Dec 22, 2015
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
Multiple Emulsions
Water-in-oil-in-water emulsion Oil-in-water-in-oil emulsion
Water
Oil
m
Emulsion Size
• < 0.5 m
• 0.5-1.5 m
• 1.5-3 m
• >3 m
Number Distributions
•<
0.5
m
•0.
5-1.
5 m
•1.
5-3
m•
>3
m
Num
ber Very few large
droplets contain most of the oil
Median
Pol
ydis
pers
ity
Large droplets often contribute most to instability
(Vol
ume
in c
lass
T
otal
vol
ume
mea
sure
d)
Note log scale
Volume Fraction=Total volume of the dispersed phase Total volume of the system
Close packing, max
MonodisperseIdeal ~0.69
Random ~0.5
PolydisperseMuch greater
ViscosityViscosityViscosity is the force required to achieve unit flow rate
Force /N
Dis
tanc
e/ m
No slip at the wall
Maximum induced flow rate /ms-1
Shear rate /s-1Fo
rce
per
unit
are
a /N
m-2
Slope
visc
osity
/Nm
-2 s
Emulsion Viscosity
Emulsion droplets Emulsion droplets disrupt streamlines disrupt streamlines and require more and require more effort to get the effort to get the same flow ratesame flow rate
5.20 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
CreamingBuoyancy(Archimedes)
Friction(Stokes-Einstein)
dv3
cs
gdv
18
2
Continuous phase viscosity density differenceg Acceleration due to gravityddroplet diameterv droplet terminal velocityvs Stokes velocity
6
3gdFb
Flocculation and Coalescence
Film ru
pture
Rehom
ogenization
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
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 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