THIXOTROPY, IRREVERSIBLE THIXOTROPY, ANTITHIXOTROPY

RHEOLOGY: Thixotropic behaviour of Pseudoplastic Plastic and Dialant fluid system, Irreversible thixotropy & Antithixotropy

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THIXOTROPY

Non-Newtonian systems such as plastic, pseudoplastic and dilatant systems at a given temperature show time dependent changes in the viscosity at varying shearing stresses. This behaviour is known as thixotropy and may be explained in the following manner:

1. Thixotropy in Plastic and Pseudoplastic Systems

In plastic and pseudoplastic systems, the viscosity gradually decreases on increases the shearing stress, at any given temperature. On removing the shearing stress, the viscosity is regained but not immediately but after some time lag. The term thixotropy is given to this phenomenon. It means "to change by touch" and may be described as a reversible isothermal transformation from gel to sol.

If a rheogram is obtained for such a system by plotting the rate of shear at various shearing stresses, a hysteresis loop is obtained. As the shearing stress is increased an up-curve is obtained. On reducing the shearing stress gradually, a down-curve is obtained. However, unlike Newtonian systems, the up-curve and the down-curve are not super-imposable. The down curve is shifted to the left side meaning that the viscosities of the down-curve are lower than that of the up-curve. This implies that the gel structure of the system is not reformed immediately but only after a lag time.

Fig: a) Thixotropy of Newtonian

b) Thixotropy of Pseudoplastic and plastic

Thixotropic systems contain asymmetric particles which set up a loose three dimensional structure. This structure confers a certain rigidity on the system and it resembles a gel. As shear is applied to the system, the structure breaks down and the material changes from a gel to a sol structure with decreasing viscosity. Upon removal of the stress, the structure begins to reform slowly and the initial structure is reformed with increasing viscosity after a lag time.

Examples of plastic systems showing thixotropy include bentonite gel and petrolatum and pseudoplastic systems showing thixotropy include dispersions of synthetic suspending agents. 

Bentonite gel is made up of a random network of hydrated elongated particles. On application of shearing stress, the elongated particles align with their long axes parallel to the direction of flow and the interparticle links are broken. The network therefore disintegrates and the apparent viscosity decreases. On removal of the shearing stress, the arrangement of dispersed particles gradually becomes less orderly under the influence of brownian motion and the gel network is reformed after a time lag.

2. Thixotropy in Dilatant Systems

In dilatant systems, an increase in the shearing stress causes an apparent increase in viscosity at a given temperature. On removal of the shearing stress, the viscosity decreases but after a lag time. This phenomenon is known as thixotropy in dilatant systems and may be described as a reversible isothermal transformation from sol to gel.

Example of dilatant systems include quicksands which exhibit thixotropic behaviour.

Fig:  Thixotropy of Dialant

Irreversible Thixotropy

The effect is similar to thixotropy. Application of shear stress causes breakdown of structure within the system but the structure does not reform on removal of the shear stress, or the time lag is so long that from a practical point of view the effect is irreversible. Examples of such systems include gels produced from high molecular weight polysaccharides which are stabilized by large number of secondary bonds. On application of high shear, the three dimensional structure of the polysaccharides is reduced to a two dimensional one and the original gel-like structure is never recovered.

Negative Thixotropy

At rest magnesia magma shows sol-like properties. On shaking, the system behaves like a gel and imparts greater suspendability. However, at equilibrium, it is readily pourable.

Antithixotropy or negative thixotropy represents an increase in consistency on the down curve. 

Fig: Antithixotropy

For example, magnesia magma exhibits an enhanced resistance to flow with increased time of shear compared to resting state. In the rheogram, the down-curve shifts to the right of the up-curve. When magnesia magma is sheared alternatively with increasing and then decreasing rates of shear, the magma thickens. As these cycles continue, the extent of increase in the thickening reduces gradually and finally reaches equilibrium state. There will be no change in the consistency curves on further cycles of shear rate. This type of behaviour is recorded.

Molecular interactions possible is : In the resting state, the system consists of a large number of individual particles and small floccules. When the product is sheared, the polymer molecular collisions are increased at a greater frequency. As a result, interparticle bonding increases. At equilibrium, large floccules are available in small numbers. However, the system exhibits sol form at equilibrium. When the product is allowed to rest, the large floccules breakup and gradually return to the original state of small floccules and individual particles.

Negative thixotropy should not be confused with the term 'dilatancy'. Dilatant systems are deflocculated and the volume of solids is high (more than 50%), whereas negative thixotropy is seen in a flocculated system containing low solid content (1 to 10%).