Lets see what is thixotrophy
In case Newtoninan systems, when the rate of shear is increases, in the rheogram a straight line will be obtained passing through origin and when the rate of shear decreases the downward curve falls on the upward curve. In case of plastic & pseudoplastic flow the downward curve will fall left to the upward curve as the rate of shear is decreased.This shows that the whenever the stress is removed the structure won't regain its original state immediately and this is what known as thixotrophy, which is applicable to plastic and pseudoplastic systems only.
So what happens exactly in the thixotrophic systems is
The particles in case of thixotrophic systems have irregular in shape and the points are attached three dimensionally. Here the system behaves like a gel. But as the stress is applied, there the particles will come out of their shape and will get aligned
themselves. So this makes them to get converted from gel to sol form. So whenever the stress is removed these particles again try to regain their original position. But this will not occur immediately. The particles will undergo Brownian movement and there by they come in contact and by this way the consistency is regained. So the rheograms obtained in case of thixotrophic systems are highly dependent on the rate at which the shear is applied and the time interval at which the constant shear is given to the system.
Measuring this is so simple.
So there are two methods for the measurement of the thixotrphy.
In the first method the rate of shear is maintained constant whereas in the second case the rate of shear is increased.
In the first case the shear will be increased from point to b, then shear is kept constant for some time say t1seconds and then decreased. This results in the loop from which the downward curve gives the value of 1/U1. In the next time the constant shear is applied for longer period of time say t2 seconds and from the downward curve 1/U2 is obtained. From these values we can calculate thixotrophic co-efficient.
In the second case the stress is applied until some rate of shear say v1, and from the downward curve 1/U1 is obtained. Again the shear is increased to some larger value say v2 and then the shear is decreased resulting in the downward curve from which the 1/U2 value is obtained. From here the thixotrophic coefficient is obtained by the formula
M = U1 -U2/ln(v2/v1)
here U represents plastic viscocity. v rpresents max shearing rate
The opposite condition is antithixotrophy.
Here in this condition, there will be an elevation in resistance to flow by the increase and decrease of the rate of shear. This is generally observed in case of magnesia magma. When the system is subjected to the elevation and then the reduction of the shear rate, the system will get thickened in each cycle, the rate at which the thickening occurs between the cycles decreases and a point will reach at which there will be no change with the elevation and reduction of the shear rate.
Here what we can conclude is thixotrophy is an required property in case of liquid systems.
For example take suspension. Thixotrophy is an ideal condition because there will be no settling of the particles in the system and the system will get converted to fluid when shaken. So what we can conclude is if the thixotrophy value is more the rate of settling will be less. Even thixotrophy is the required property in case of emulsions lotions and ointments etc.
Martin's Physical Pharmacy and Pharmaceutical sciences by Patrick J. Sinko, 5thedition,Lippincott Williams and wilkins publishers, page no 566 to 569
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