Study Set Content:
Shearing Stress (F) Formula
F =
F′/A
Rate of Shear (G) or Velocity Gradient
o Is the difference of velocity (dv) between Formula
G =
dv/dr
Rate of shear should be directly proportional to shearing stress,
or:
F′/A= n dv/dr
coefficient of viscosity; simply viscosity
n
n Unit
poise (cgs unit dyne sec/cm-2 or g/cm sec)
Useful Conversion/Equation
o A more convenient unit for most work is the
centipoise
(cp) – 1 cp =0.01 poise
(cp) – 1 cp =0.01 poise
Reciprocal of viscosity
Fluidity(Ø)
(Ø)
Ø = 1/n
Is the absolute viscosity divided by the density of liquid at a
specific temperature;
Kinematic Viscosity
Kinematic Viscosity Units:
Units:stoke (s), centistoke (cs)
Shear Dependent Viscosity
o Plastic
o Pseudoplastic
o Dilatant
Time Dependent Viscosity
o Thixotropy
o Rheopexy
The majority of fluid pharmaceutical products are not simple
liquids and do not follow Newton’s law of flow.
Non-Newtonian Systems
Generally exhibited by liquid and solid heterogeneous
dispersions such as:
o Colloidal solutions,
o Emulsions,
o Liquid suspensions, and
o Ointments
When non-Newtonian materials are analyzed in a (blank) and results are plotted, various consistency curves,
representing three classes of flow (shear dependent) are
recognized:
rotational
viscometer
Bingham bodies
Plastic
Shear thinning systems
Pseudoplastic
Shear thickening system
Dilatant
A yield value must be overcome before the system begins to
flow
Plastic
