Thixotropic restoration

The thixotropic rheological behavior of structured disperse system to a significant extent depends on the direction towards which the equilibrium between rupture and re-establishment of interparticle contacts is shifted. Because of the finite rate of contact formation due to Brownian motion, a certain amount of time is needed for the equilibrium to establish. Consequently, some time is required for a spontaneous thixotropic restoration of structure destroyed by mechanical action. Due to a complete structure disintegration taking place in region IV (Fig. IX-24), the strength, i.e. the... 

At rest, the system with time regains its strength, i.e. restores solid-like behavior (Fig. IX-27). The strength (critical shear stress, x ) of a completely restored structure does not depend on a number of structure disintegration cycles. The time required for a complete thixotropic restoration of a primarily disintegrated structure is referred to as the period of thixotropy, tT. 

Increase in the flow of materials under acoustic treatment is conditioned by different factors. Investigation into rheological and molecular-mass characteristics of polymers having been subjected to acoustic treatment has revealed that, in the case of a low-intensity treatment, the effect is of a reversible (thixotropic) character. However, at high intensities of acoustic treatment, rheological characteristics of the material are not restored completely after the vibration effect is terminated and the... 

Thixotropic Time-dependent pseudoplastic flow. At constant applied shear rate, viscosity decreases. In a flow curve, hysteresis occurs. Paint, quicksand. In bentonite clay gels which liquefy on shaking and solidify on standing, there is a time-dependent aligning to match the induced flow. After the shear rate is reduced it takes some time for the original alignments to be restored. 

If in non-Newtonian liquids the structure of the liquid is destroyed upon increasing y, hysteresis curves are observed as shown in Fig. 1.29. The behaviour of these liquids depends not only on the time of shear but also on the past shear and thermal history. Pseudoplastic liquids of this kind are named thixotropic, and dilatant liquids are referred to as rheopectic. The longer the duration of shear, the stronger is the destruction of the liquid structure, and the longer it takes to restore it. 

Thixotropy This refers to the reversible time-dependent decease of viscosity. When the system is sheared for some time the viscosity decreases, but when the shear is stopped (the system is left to rest) the viscosity of the system is restored. Practical examples of systems that show thixotropy include paint formulations (sometimes referred to as thixotropic paints), tomato ketchup, and some hand creams and lotions. 

The restoring force for a dispersion to return to a random, isotropic situation at rest is either Brownian (thermal fluctuations) or osmotic. The former is most important for submicrometer particles and the latter for larger particles. Changing the flow conditions changes the structure, and this leads to thixotropic effects, which are especially strong in flocculated systems. 

Fig. IX-27. The critical shear stress, t , as a function of time, t, required for a thixotropic structure restoration...

Thixotropic Pseudoplastic flow that is time-dependent. At constant applied shear rate, viscosity gradually decreases, and in a flow curve hysteresis occurs. That is, after a given shear rate is applied and then reduced, it takes some time for the original dispersed species alignments to be restored. Thixotropy in gels is sometimes termed reversible sol-gel transformation. 

The rheological action of the above additives is based on the fact that they form three-dimensional networks in the paint. These lattice structures are destroyed by shear forces but are restored when the forces are removed. This recovery is not, however, immediate. The rising viscosity initially allows leveling of the surface but subsequently prevents sagging. This thixotropic behavior allows to adjust the balance between sagging and levelling. 

Thixotropy is the most relevant time-dependent rheological behavior. In thixotropic fluids, interparticle interactions break down under the influence of a stress and the viscosity decreases. Hence, it is shear thinning. Upon reducing the shear stress, interactions are restored with a concomitant increase in viscosity. Figure 17.8... 

Coagulation of clay suspensions forms a coagulation structure specified by the presence of far-and close-distance coagulation contacts between structural elements. They control the physical-mechanical properties of soils, i.e., their liquid or semi-liquid consistency, high porosity (up to 97%), very low strength, as well as thixotropic destruction and restoration. Factors controlling structure formation in clay sediments. 

Hydrogel materials that show thixotropic behavior are very interesting for biomedical applications and are potentially useful candidates for the direct injection of delivery matrices. Under stress (shaking or agitation, for example), a thixotropic system thins, becomes less viscous and flows, but after the period of stress the viscosity increases and thickening is restored over time. For the encapsulation and delivery of delicate or thermally non-robust goods (cells or proteins), this appears to be a particularly useful concept they can be injected through a thin needle under pressure into a tissue because of their thixotropic character. 

FIGURE 3.29 The illustration of a schematic yield stress, x as a function of the time, f, necessary to restore the thixotropic structure. 

In practice, when working with thixotropic systems, one often uses some apparent values of the rheological characteristics, rather than the equilibrium ( true ) ones. The apparent characteristics can be evaluated at a particular time, for instance, after a complete degradation of the thixotropic-reversible structure. In some instances, a prolonged constant-rate deformation of the system is required for the equilibrium between the contact rupture and restoration to be established. Such conditions may not be always achievable in the laboratory. 

The thixotropic properties of pigment structures in oil-based paints provide the paint with the necessary rheological properties. Mixing results in the destruction of the coagulation structure and allows one to apply the paint as a thin layer on the surface. Quick restoration of the coagulation structure prevents the paint from gravity-caused draining downward. 

When that stress is exceeded, the shear rate grows. Further stress leads finally to linear (Newtonian) behaviour. Examples of plastic systems are chocolate, butter, cheese, various spreads and ice cream. In pseudoplastic systems the observed viscosity decreases with an increase in shear stress. An example of a pseudoplastic system is pudding. Dilatant systems resist deformation more than in proportion to the apphed force. The shear rate is growing much faster than that of Newtonian fluids and viscosity increases with an increase in shear stress. At low apphed forces, the system behaves as a Newtonian fluid. Examples of dilatants systems are honey with added dextran and a slurry of wet beach sand. Thixotropic systems become more fluid (they have lower viscosity) with increasing time of an apphed force. If the apphed force ceases to operate, the original viscosity of the system is restored due to a reversible transformation of the sol gel type. Examples of thixotropic systems are mayonnaise, ketchup, whipped and hardened fats, butter and processed cheeses. Rheopectic systems exhibit behaviour opposite to that of thixotropic systems. Their viscosity increases with increasing time of apphed force. An example is whipped egg white. 

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