Stoke's law

Stoke’s Law
Photo
courtesy
of
Patrick
Willems
on
Flickr

Prof. Abd Karim Alias
Universiti Sains Malaysia

Stoke's Law calculates rate of destabilization of an
emulsion by equating gravitational force with the opposing
hydrodynamic force:
where V = velocity of separation (or rate of creaming), cm/sec;
g = acceleration of gravity (980 cm/sec);
r = droplet radius (cm);
d1 = density of disperse phase (g/cm3);
d2 = density of continuous phase (g/cm3);
µ = viscosity of the continuous phase (g/cm.sec) (µ = 0.01 at 20 °C)
Stoke’s Law
2gr2 (d1 – d2)
9µ

You are making orange soda from
an orange flavor emulsion. The density of the
orange oil is 0.85 g/cm3, the density of a 10%
sugar solution is 1.04 g/cm3, the average
particle radius is 3.0 micron, and µ = 0.01 g/
cm·sec. What is V? Will the emulsion remain
stable?
Stoke’s Law
PROBLEM 1
2gr2 (d1 – d2)
9µ

Photo
courtesy
of
Brent
Moore
on
Flickr

You homogenize the beverage mix so that
the particle radius is decreased to 0.3 micron.
What is V? Will the emulsion remain stable?
Stoke’s Law
PROBLEM 2
2gr2 (d1 – d2)
9µ

Photo
courtesy
of
Brent
Moore
on
Flickr

You add brominated vegetable oil to the
orange oil so the overall density is 0.95 g/
cm3 . What is V for particle radius 3.0 micron?
For 0.3 micron? Will the emulsion remain
stable?
Stoke’s Law
PROBLEM 3
2gr2 (d1 – d2)
9µ

Photo
courtesy
of
Brent
Moore
on
Flickr

For the above problems, which approach
stabilized the emulsion more -- decreasing
particle size or increasing density of the
dispersed phase?
Stoke’s Law
PROBLEM 4
2gr2 (d1 – d2)
9µ

Photo
courtesy
of
Brent
Moore
on
Flickr

Stoke's law

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Stoke's law