When gel structures are present in suspensions, shear-thinning rheologies can be expected.
Shear-Thinning Behavior
Gel structures typically produce shear-thinning rheologies.
This occurs because the imposed shear causes gel structures to break down and this lowers apparent viscosities.
When a flocculated suspension is quiescent, the gel structure will build to form a complete structure that extends throughout the whole suspension, such as the structure shown in Figure 6.3D.
Flocculated particle/fluid suspensions containing completed gel structures can be expected to exhibit yield stresses. They are examples of yield shear-thinning, or yield-pseudoplastic rheologies.
When a gel structure is sheared, for example by mixing or pumping, the large gel structure will break down. The intensity of shear will determine the size of the flocs that flow free of the gel structure. Low intensity shear will produce relatively large flocs. At the other extreme, high intensity shear can destroy flocs and cause all particles to report as individuals so each particle can flow independently of all others.
Apparent viscosities will be relatively high when measured under low shear conditions in which large flocs are flowing. Apparent viscosities will be relatively low when measured under high shear conditions in which small flocs or individual particles are flowing. The gel breakdown phenomenon explains the behavior of shear-thinning suspensions.
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A B
C D
Figure 7.1 Example of the Shear-Thinning Process
As mentioned earlier, gelled suspensions exhibit dynamic equilibria. As shear destroys gel structures, attractive interparticle forces attempt to rebuild those structures. When the rates of gelation and breakdown balance, the systems will exhibit constant apparent viscosities during shear. This is the reason why suspensions flowing in pipes achieve steady-state viscosities and flow conditions.
Figures 7.1A through 7.1D demonstrate what happens when a gel structure is sheared. Figure 7.1A is a picture of the quiescent gel structure. Figure 7.1B is a picture of the structure under low shear conditions; Figure 7.1C is a picture of the structure after shear at higher rates; and Figure 7.1D corresponds to high intensity dispersion conditions when each particle reports individually within the suspension.
High intensity dispersion (HID) conditions, defined as mixing with impeller tip speeds greater than or equal to 5000 ft/min,
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should completely destroy all gel structures and set all particles free to travel independently. In fact, high intensity dispersion conditions should strip most, if not all, adsorbed ions and chemical additives from the surfaces of the powders and force them out into the interparticle soup. Under HID conditions, the interparticle soup will contain individual particles, ions, and molecules.
When HID conditions are removed, everything in the suspension will be free to move towards and take equilibrium positions. Ions and additive molecules will adsorb onto particle surfaces in equilibrium positions, the particles will reflocculate, and the gel structure will rebuild.
Lower intensity shear rates should not be expected to destroy all of the gel structure. The nature of the attractive forces within a suspension and the strength of its gel structure will determine how intense the shear must be to cause all particles to report as individuals.
As soon as gelation rates exceed the breakdown rates produced by the imposed shear, gel structures will again grow and/or strengthen.
Thixotropy
The breakdown of gel structures also explains time-dependent thixotropic fluids and suspensions. Just as increased shear rates cause gel structures to break down, constant shear rates applied for even short periods of time will also cause gel structures to break down.
Shear-thinning time-independent rheologies are defined by instantaneous changes in apparent viscosities as shear rates increase.
In practice, however, gel breakdown doesn’t occur instantaneously.
Most shear-thinning suspensions are therefore also thixotropic. The thixotropy appears as gel structures continue to break down with time until they achieve buildup/breakdown equilibrium at the imposed shear rates.
When process suspensions are flocculated, one should expect to see both shear-thinning and thixotropic behaviors. Because both of these are caused by gel breakdown, they occur together.
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Summary
Yield shear-thinning (a.k.a. yield-pseudoplastic) and thixotropic rheologies are characteristic of flocculated particle/fluid suspensions. The apparent viscosities of these suspensions will decrease as shear is applied, and increase again as gel structures rebuild after shear is removed.
The rheologies of flocculated and partially flocculated ceramic suspensions are yield thinning (rather than simply shear-thinning) because the gel structure which produces the yield stress is necessary to allow formed wares to hold their shapes. Pure shear-thinning fluids (without yield stresses) are not only rare, but unimportant within ceramic process systems.
The rheologies of flocculated and partially flocculated ceramic suspensions are also thixotropic for the same reason. It is doubtful that ceramic suspensions that do not exhibit yield stresses would ever show indications of thixotropy.
Gel structures, yield stresses, yield-shear thinning behaviors, and thixotropy are all closely related. If measurements show the presence of any one, the others should also be present.
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