Non –
Newtonian systems
The majority
of fluid pharmaceutical products are not simple liquids and do not follow Newton’s
law of flow. These systems are referred to as non-Newtonian. Non-Newtonian
behavior is generally exhibited by liquid and solid heterogeneous dispersions
such as colloidal solutions, emulsions, liquid suspensions, and ointments. When
non- Newtonian materials are analyzed in a rotational viscometer and results
are plotted, various consistency curves, representing three classes of flow,
are recognized, plastic, pseudoplastic and dilatant.
Plastic Flow
The curve
for the plastic flow represents a body that exhibits plastic flow, such
materials are known as Bingham bodies. Plastic flow curves do not pass through
the origin but rather intersect the shearing stress axis at a particular point
referred to as the yield value. (obtain after the straight part of the curve is
extrapolated to the axis).
A Bingham
body does not begin to flow until a shearing stress corresponding to the yield value
is exceeded. At stress below the yield value, the substance acts as an elastic
material. Yield value is an important property of certain dispersions. The slope
of the rheogram in plastic flow is termed the mobility, analogous to fluidity
in Newtonian systems, and its reciprocal is known as the plastic viscosity. U,
the equation describing plastic flow is,
U = F – f/G
Where f is
the yield value, or intercept, on the shear stress axis in dynes/cm2 ,
and F is shearing stress, G is rate of shear.
Plastic flow
is associated with the presence of flocculated particles in concentrated suspensions.
As a result, a continuous structure is set up throughout the system. A yield value
exists because of the contacts between adjacent particles (brought about by van
der Waals forces), which must be broken down before flow can occur. Consequently,
the yield value is an indication of force of flocculation. The more flocculated
the suspension, the higher will be the yield value. Frictional forces between
moving particles can also contribute to yield value. Any further increase in
shearing stress brings about a directly proportional increase in G, rate of
shear. In effect, a plastic system resembles a Newtonian system at shear
stresses above the yield value.
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