Thixotropic liquids

Many suspensions, and also some polymer solutions, change in time (Figure C4-15). This is usually because stmctures are broken or formed by shearing. The result can be that the viscosity decreases in time (a thixotropic liquid) or increases (a rheopectic liquid). Yoghurt is a good example of a thixotropic liquid rheopectic fluids are rare. The changes in viscosity are often, but not always, reversible. Note that these time effects are not the same that we saw in viscoelastic liquids. 

Pseudoplastic and thixotropic liquid detergents as emulsions are the subjects of U.S. Patent 5,851,979. Equilibrium viscosity values measured using a Brookfield viscometer with Spindle no. 2 at 20°C are cited at 12, 30, and 60 r/min. For one example containing hydrogen peroxide, the viscosity is 1020, 400, and 220 cP at 12, 30, and 60 r/min, respectively. Pseudoplasticity is clearly evident, as the viscosity decreases with increasing rotational speed. 

U.S. 5691292 (1997) [53] Marshall et al. (Procter Gamble) Thixotropic liquid chlorine bleach free Thickener for enzyme-based ADDs enzyme stabilizing... 

U.S. 4950416 (1990) [50] Baxter (Vista Chemical) Thixotropic liquid aqueous alumina dispersion long-chain fatty acid or salt and mixtures Nonpolymeric and nonclay compositions... 

EP 304328 (1989) [124] Kreischer (Unilever) Nonsoap anionic surfactant and soap electrolyte level > 20% Thixotropic liquid structured by surfactants only... 

Thixotropic Liquids - These liquids exhibit lower viscosity as shear rate increases. A practical example is house paint which appears thinner when stirred. 

Thixotropic liquids, e.g. paints, tomato ketchup, salad cream. Here the apparent viscosity reduces with time as the material is sheared at a constant shear rate Figure I.JI). Usually after a large time of shearing, an equilibrium apparent viscosity is reached. If the shearing is now stopped the... 

When the shear stress changes in Newtonian, dilatant, or pseudoplastic liquids, as well as in Bingham bodies or fluids above the flow limit, the corresponding shear gradient or the corresponding viscosity is reached almost instantaneously. In some liquids, however, a noticeable induction time is necessary, i.e., the viscosity also depends on time. If, at a constant shear stress or constant shear gradient, the viscosity falls as the time increases, then the liquid is termed thixotropic. Liquids are termed rheopectic or antithixotropic, on the other hand, when the apparent viscosity increases with time. Thixotropy is interpreted as a time-dependent collapse of ordered structures. A clear molecular picture for rheopexy is not available. 

In practice, it is advantageous to be able to decrease the viscosity (thus decreasing the resistance to flow) by increasing pressure or velocity, as well as to modify the flow geometry in order to increase shear rates. In some cases, there is an effect of the history of previous deformation. This is exemplified by thixotropic liquids that show a low viscosity after a shear history (like stirring), while a high viscosity prevails at rest. This phenomenon is much desired in the case of paints and varnishes. 

Deviations from Newtonian flow can occur when shear stress does not increase in direct proportion to shear rate. Such deviation may be in the direction of thickening (called dilatent flow) and in the direction of thinning (called pseudo plastic). Related to non-Newtonian flow is the behavior of thixotropic liquids when subjected to shear, as explained above. Flow behavior can be represented by the following equation ... 

Two types of flow are recognized thixotropy, defined as a decrease of apparent viscosity under shear stress, followed by a gradual recovery when the stress is removed, and its opposite, anti-thixotropy, or rheopexy. Both are related to molecular or macroscopic changes in interactions. In thixotropic liquids, the aggregate bonding must be weak enough to be broken by flow-induced hydrodynamic forces. If dispersion is fine, even slight interactions may produce thixotropic effects. When... 

Thixotropic liquid a liquid that exhibits a decrease in viscosity as a function of time. 

At high particle concentrations, slurries are often non-Newtonian. For non-Newtonian fluids, the relationship between the shear stress and shear rate, which describes the rheology of the slurry, is not linear and/or a certain minimum stress is required before flow begins. The power-law, Bingham plastic and Herschel-Bulkley models are various models used to describe the flow behaviour of slurries in which these other types of relationships between the shear stress and shear rate exist. Although less common, some slurries also display time-dependent flow behaviour. In these cases, the shear stress can decrease with time when the shear rate is maintained constant (thixotropic fluid) or can increase with time when the shear rate is maintained constant (rheopectic fluid). Milk is an example of a non-settling slurry which behaves as a thixotropic liquid. 

E) Thixotropic liquid. The viscosity of these liquids depends on the time of flow and decreases with increasing time at a constant flow gradient. This effect can be both reversible or irreversible, and is also shown by many polymer solutions. 

Figure 13 The behaviour of a thixotropic liquid, initially at rest, and then subjected to a high shear rate followed by a lower shear rate.

The majority of common unsaturated fatty acids are viscous thixotropic liquids (see Section 7.8.3.2.2). Thixotropy is the property of certain gels or fluids that are thick (viscous) under normal conditions, but flow (less viscous) over time when agitated or... 

Thixotropic liquids are unanimously pseudoplastic, i.e. the slope coefficient in the shear stress-shear rate diagram decreases with increasing shear rate. Pseudoplastic fluids are, however, not necessarily thixotropic. Dilatant liquids show generally rheopexy, but there are also exceptions to this rule. Rheopectic liquids are, on the other hand, always dilatant. 

Sichel (Henkel) Sicomet 9000 Liquid Sichel (Henkel) Sicomet Thixotrope Liquid ... 

Epoxy - One-part thixotropic paste Epoxy - Two-part thixotropic liquid ... 

Most shear-thinning fluids like paints and certain polymer solutions are also thixotropic, i.e. the viscosity decreases with time (at constant shear rate). Yoghurt is also an example of a thixotropic liquid. The opposite, i.e. rheopectic liquids (viscosity increases with time). 

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