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