Dispersed Systems. FDSC400 2004 Version. Goals. Scales and Types of Structure in Food Surface Tension Curved Surfaces Surface Active Materials Charged Surfaces. COLLOIDAL SCALE. Dispersed Systems.
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A kinetically stable mixture of one phase in another largely immiscible phase. Usually at least one length scale is in the colloidal range.
The same oil is split into 0.1 cm radius droplets, each has a volume of 0.004 cm3 and a surface area 0.125 cm2.
As we need about 5000 droplets we would have a total area of 625 cm2
We have 20 cm3 of oil in 1 cm radius droplets. Each has a volume of (4/3.p.r3) 5.5 cm3 and a surface area of (4.p.r2) 12.5 cm2.
As we need about 3.6 droplets we would have a total area of 45.5 cm2
Weak interfacial tension
Little to be gained by breaking
Strong interfacial tension
Strong energetic pressure to reduce area
e.g., emulsionsTendency to break
Hydrophilic head group (charged or polar)
Hydrophobic tail (non-polar)
Sequence of more water soluble subunits
Sequence of less water soluble subunits
Surface concentration /mg m-2
No binding below a certain concentration
ln Bulk concentration
Interface partly “hidden”
Surface pressure – the ability of a surfactant to lower surface tension
p = g-g0
A fine dispersion of one liquid in a second, largely immiscible liquid. In foods the liquids are inevitably oil and an aqueous solution.
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
< 0.5 mm
>3 mmEmulsion Size
(Volume in class Total volume measured)
Note log scale
f=Total volume of the dispersed phase
Total volume of the system
Close packing, fmax
Emulsion droplets disrupt streamlines and require more effort to get the same flow rateEmulsion Viscosity
Viscosity of emulsion
Continuous phase viscosity
h Continuous phase viscosity
Dr density difference
g Acceleration due to gravity
v droplet terminal velocity
vs Stokes velocity
Stir or change chemical conditions
Function of energy barrier
Droplets approach each other
Protein layers overlap
Proteins repel each other mechanically & by osmotic dehydration
What happens when protein molecules on different droplets are reactive?
Flocculation leads to an increase in viscosity effort to get the same flow rate
Water is trapped within the floc and must flow with the floc
Effective volume fraction increasedRheology of Flocculated Emulsions