2. Thixotropy:
It is defined as:
The isothermal and slow recovery on standing of material which
consistency is lost through shearing is called Thixotropy.
It is also called Gel-Sol-Gel Transformation.
In shear thinning system when a fluid is agitated by applying shearing
stress and it is kept aside it is expected to return its original state of
fluidity but takes longer time to recover compared to the time taken for
agitation.
It is shown by Plastic and Pseudoplastic fluid.
Ketchup, Consumer paints and Yoghurts are examples.
5. From the graph the up curve ab is
obtained up to maximum point b
By reducing rate of shear down curve
bc is obtained. In Non Newtonian
system it is displaced to the left of
up curve.
In this graph the material has low
consistency at any rate of shear on
down curve compare to that shown
on up curve.
In newtonian system down curve
superimposed to up curve.
6. Measurement of Thixotropy:
The most important characteristic of thixotropic system is the Hysteresis Loop
formed by upcurve or down curve of rheogram.
The area of hysteresis loop is used to measure the thixotropic breakdown and be
obtained by means of Planimeter.
Two approaches are used to estimate degree of thixotropy of Bingham bodies.
(1)- To determine structural breakdown with time at constant rate of shear.
(2)-To determine structural breakdown due to increasing shear rate.
7. Approach 1:
Structural breakdown with time at constant
shear rate
B=Thixotropic coefficient(rate of breakdown
with time at constant rate of shear)
8. Approach 2:
Structural breakdown
due to increasing shear
rate.Two hysteresis loop
are obtained.
M=Loss in shearing
stress per unit increase
in shear rate
9. Applications Of Thixotropy:
Thixotropy is desirable property in liquid pharmaceutical systems.It has following
applications in pharmacy.
(I)-A high consistency in the container yet pour are spread easily.
(II)-A well formulated suspension will not settle out readily in the container.It
will become fluid on shaking and will remain so long enough for a dose to be
dispensed.It will regain consistency to maintain particles in a suspended state.
(III)-It is also desirable with emulsions ,lotions, creams ,ointments and parental
suspension to be used for Intramuscular Depot Therapy.
