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Thixotropy (1) (1)

The document discusses thixotropy, which is defined as the reversible transformation of a semi-solid material from a solid state to a liquid state and back again due to applied stress over time. Examples of thixotropic materials include ketchup, paints, and yogurt. A key characteristic of thixotropic materials is their hysteresis loop, which is formed on a rheogram by the upcurve and downcurve of viscosity with increasing and decreasing shear rates. The area of the hysteresis loop can be used to measure the degree of thixotropic breakdown. Thixotropy is a desirable property for liquid pharmaceuticals as it allows the material to have a high consistency at rest but flow easily upon shaking before reg

Health & Medicine◦
Presented By: Muhammad Adnan Roll No:06331913004 Thixotropy
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.
Thixotropy Rheogram: The rheogram of thixotropic material depends on. (a)-Rate at which shear increased or decreased. (b)-Length of time during which sample is subjected to shear.
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.
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.
Approach 1: Structural breakdown with time at constant shear rate B=Thixotropic coefficient(rate of breakdown with time at constant rate of shear)
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
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.
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