Thixotropy

Thixotropy

Thixotropy is the property of some non-Newtonian fluids to show a time-dependent change in viscosity. For dilatant (shear-thickening) thixotropic fluids, the longer the fluid undergoes shear, the more its viscosity. For pseudoplastic (shear-thinning) thixotropic fluids, the longer the fluid undergoes shear, the lower its viscosity.

Many gels and colloids are pseudoplastic thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated with a reversible gel–sol transformation phenomenon. When sheared by mixing, such as simple shaking, the matrix relaxes and forms a solution with the charac-teristics of a liquid dosage form for ease of use. Although pseudoplastic fluids show thinning with only increasing rate of shear, thixotropic fluids show increasing flow and thinning as the duration of mixing increases, even at the same shearing forces. On setting, the higher-viscosity plastic state resumes as a network gel forms and provides a rigid matrix that stabilizes suspensions and gels.

The molecular basis of thixotropic behavior of fluids lies in changes in intermolecular interactions of solute on persistence of shear. Thus, pseudo-plastic thixotropic systems show thinning behavior not only with increasing shear but also with increasing duration of shear, owing to changes in the alignment of solute molecules that reduce their intermolecular interactions. Conversely, dilatant thixotropic systems show thickening behavior not only with increasing shear but also with increasing duration of shear, owing to swelling and/or intermolecular entanglement or interparticle interactions, which cause increased interparticle bonding or intermolecular attractive interactions with time.

The term thixotropic fluid is sometimes used to refer to pseudoplastic (shear-thinning) thixotropic behavior. Conversely, the term negative thix-otropy or antithixotropy is sometimes used to refer to a dilatant (shear-thickening) thixotropic behavior, that is, a time-dependent increase rather than a decrease in apparent viscosity on application of a shearing stress.

The main advantage of pseudoplastic thixotropic preparations is that the particles remain in suspension during storage, but when required for use, the pastes are readily made fluid by tapping or shaking. This is true for both pseudoplastic and thixotropic fluids. For thixotropic fluids, the duration of shearing or shaking, even at the same shear stress, also impacts shear thin-ning to the fluid. For example, concentrated parenteral suspensions con-taining from 40% to 70% w/v of procaine penicillin G in water show high inherent pseudoplastic thixotropic behavior.

Hysteresis loop

Hysteresis loop represents different path of response (shear rate) to the experimental parameter (shear stress) when the experimental parameter is increased or decreased. Thixotropic systems show an up–down curve, called hysteresis loop, such that for a given shear stress, the flow response is a function of the history of the sample—increasing the shear stress (up curve) leads to different flow behavior than if the shear stress were decreas-ing (down curve). Typical rheograms for pseudoplastic and dilatant systems exhibiting this behavior are shown in Figure 12.2.

Figure 12.2 Thixotropy in pseudoplastic and dilatant flow systems: (a) thixotropy in pseudoplastic material and (b) thixotropy in dilatant material.

Rheograms of pseudoplastic thixotropic materials are highly dependent on the rate at which shear is increased or decreased and the length of time for which a sample is subjected to any one rate of shear or shear stress. As shown in Figure 12.3 for a pseudoplastic thixotropic system shown in Figure 12.2a, the shear rate (or flow) increases from point “a” to point “b” with an increase in shear stress and decreases from “b” to “e” with a decrease in the shear stress. This forms the hysteresis loop “abe.” However, if the sample was taken to point “b” and the shear rate was held constant for a certain period of time (say, t₁ seconds), the rate of shear (and hence the consistency and the flow) increases for the same shear stress (vertical upward movement of line from point “b,” not shown in the figure). 

Figure 12.3 Relationship between shearing stress and rate of shear for a plastic system possessing thixotropy.

Consequently, to maintain the same desired rate of shear (as in point “b”), the amount of shear stress required is progressively lower as time passes (t₂ > t₁)—represented by the horizontal line from “b” to “c” for t₁ and “b” to “d” for t₂ in Figure 12.3. This decrease in the required shear stress to maintain the same rate of shear is attributed to reduction in the degree or amount of structure in the sample.

Decreasing the rate of shear to zero after having reached the state “b,” “c,” or “d” (depending on the time for which shear stress is applied at con-stant shear rate) would create hysteresis loops aba, aca, or ada, respectively. Therefore, in contrast to pseudoplastic or dilatant materials, the rheogram of a thixotropic material is not unique but depends on the rheologic history of the sample and the approach used to obtain the rheogram. For example, keeping the constant shear stress at point “b” for times t₁ and t ₂ would result in very different rheograms than the rheogram shown in Figure 12.3, where the shear rate was kept constant at point “b.” Student exercise: draw the rheograms for the case where shear stress is kept constant for times t₁ and t₂ at point “b” and compare them with that in Figure 12.3. This property of thixotropic systems is important to bear in mind when attempting to obtain a quantitative measure of viscosity and flow of thixotropic systems.