Time Effects during Flow - Thixotropy

Many suspensions (particularly those that are weaMy flocculated or structured to reduce sedimentation) show time effects during flow. At any given shear rate, the viscosity of the suspension continues to decrease with increasing the time of shear on stopping the shear, the viscosity recovers to its initial value. This reversible decrease of viscosity is referred to as thixotropy. 

The most common procedure of studying thixotropy is to apply a sequence of shear stress - shear rate regimes within controlled periods. If the flow curve is carried out within a very short time (say increasing the rate from 0 to say 500 s in 30 s and then reducing it again from 500 to 0 s within the same period), one finds that the descending curve is below the ascending one. 

The above behaviour can be explained from consideration of the structure of the system. If, for example, the suspension is weakly flocculated, then on applying a shear force on the system this flocculated structure is broken down (and this is the cause of the shear thinning behaviour). On reducing the shear rate back to zero the structure builds up only in part within the duration of the experiment (30 s). 

The ascending and descending flow curves show hysteresis that is usually referred to as thixotropic loop . If the same experiment is now repeated over a longer time (say 120 s for the ascending and 120 s for the descending curves), the hysteresis decreases, i.e. the thixotropic loop becomes smaller. 

The above study may be used to investigate the state of flocculation of a suspension. Weakly flocculated suspensions usually show thixotropy and the change of thixotropy with applied time may be used as an indication of the strength of this weak flocculation. 

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Time Effects during Flow Thixotropy and Negative (or Anti-) Thixotropy... 

The basic principles of rheology and the various experimental methods that can be applied to investigate these complex systems of food colloids have been discussed in detail in Chapter 7. Only a brief summary is given here. Two main types of measurements are required (1) Steady-state measurements of the shear stress versus shear rate relationship, to distinguish between the various responses Newtonian, plastic, pseudo-plastic and dilatant. Particular attention should be given to time effects during flow (thixotropy and negative thixotropy). (2) Viscoelastic behaviour, stress relaxation, constant stress (creep) and oscillatory measurements. 

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