Fluid thixotropy

Hlavdek M. The influence of the acetabular labrum seal, intact articular superficial zone and synovial fluid thixotropy on squeeze-film lubrication of a spherical synovial joint. J Biomech 2002 35 ... 

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

Thixotropy Describes those fluids whose apparent viscosity decreases... 

In general, for shear-thinning pseudoplastic fluids the apparent viscosity will gradually decrease with time if there is a step increase in its rate of shear. This phenomenon is known as thixotropy. Similarly, with a shear-thickening fluid the apparent viscosity increases under these circumstances and the fluid exhibits rheopexy or negative-thixotropy. 

Rheopexy, a reversible time-dependent effect like thixotropy, is a rare phenomenon in pigmented systems. Rheopectic fluids increase in viscosity t with time when sheared at a constant shear rate D or a constant shear stress t until they approach a viscosity maximum (Fig. 53). 

Thixotropy Describes those fluids whose apparent viscosity decreases with time to an assymptotic value under conditions of constant shear rate. Thixotropic fluids undergo a decrease in apparent viscosity by applying a shearing force such as stirring. If shear is removed, the material s apparent viscosity will increase back to or near its initial value at the onset of applying shear. 

Since fluid shear rates vary enormously across the radius of a capillary tube, this type of instrument is perhaps not well suited to the quantitative study of thixotropy. For this purpose, rotational instruments with a very small clearance between the cup and bob are usually excellent. They enable the determination of hysteresis loops on a shear-stress-shear-rate diagram, the shapes of which may be taken as quantitative measures of the degree of thixotropy (G3). Since the applicability of such loops to equipment design has not yet been shown, and since even their theoretical value is disputed by other rheologists (L4), they are not discussed here. These factors tend to indicate that the experimental study of flow of thixotropic materials in pipes might constitute the most direct approach to this problem, since theoretical work on thixotropy appears to be reasonably far from application. Preliminary estimates of the experimental approach may be taken from the one paper available on flow of thixotropic fluids in pipes (A4). In addition, a recent contribution by Schultz-Grunow (S6) has presented an empirical procedure for correlation of unsteady state flow phenomena in rotational viscometers which can perhaps be extended to this problem in pipe lines. 

Thixotropy polyurethane, and urea-formaldehyde resins. A flow characteristic of certain fluids where a decrease in viscosity of the fluid occurs when it is stirred at a constant or increasing rate of shear. When the stirring or shearing is discontinued, the apparent viscosity of the fluid gradually increases back to the original value. Changes in both directions are dependent on time as well as shear. 

Control of Thixotropy. Often the adhesive application will require that the product be fluid for mixing and application, but it must not flow or sag once applied. For example, ASTM C920 defines a nonsag sealant as one that permits application in joints on vertical surfaces without sagging or slumping. This property is called thixotropy. Thixotropic materials, such as tomato catsup, toothpaste, etc., undergo a decrease in viscosity when subject to shearing. 

Fig. 8.1 Idealized plots of shear rate (y) against shear stress (x) for fluids of various types. (A) Newtonian fluid. (B) Bingham fluid. (C) Shear thinning, (D) Shear thickening. (E) Positive hysteresis 1, 2, 3A thixotropy 1,2, 3B. rhcodestruction, (F) Negative hysteresis with antithixotropy.

In some cases, an extrudable and injectable paste may consist of 65% vol. ceramic powder and 35% vol. polymeric binder. In others, an extrudable paste may consist of a highly loaded aqueous suspension of clay particles such that its rheology is plastic. Hie low shear (i.e., <100 sec ) viscosity of such a paste is between 2000 and 5000 poise at ambient temperature. Highly nonlinear stress strain curves are typical of ceramic pastes, as well as time dependent thixotropy. In many cases, pastes behave like visco-elastic fluids. This complex rheological behavior of ceramic pastes has made theoretical approadies to these problems difficult. For this reason, the discussion in this chapter is limited to Newtonian fluids where analytical solutions are possible, with obvious consequences as to accuracy of these equations for non-Newtonian ceramic pastes. 

Mewis, J. 1979. Thixotropy—a general review. J. Non-Newtonian Fluid Mech. 6 1-20. 

For Newtonian fluids the viscosity is independent of time. However, for most non-Newtonian fluids the viscosity at a shear rate high enough to place the fluid in the non-Newtonian region evolves with time as schematically indicated by the lower curve of Figure 13.39. The viscosity decreases with time until steady-state conditions are reached. This phenomenon is called thixotropy. The cause of this behavior lies in the fact... 

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