Shear thinning slurries

As is evident from Eq. (3-20) or (3-21), the Bingham plastic exhibits a shear thinning viscosity i.e., the larger the shear stress or shear rate, the lower the viscosity. This behavior is typical of many concentrated slurries and suspensions such as muds, paints, foams, emulsions (e.g., mayonnaise), ketchup, or blood. 

An alternative method of reducing the resistance to filtration is to recirculate the slurry and thereby maintain a high velocity of flow parallel to the surface of the filter medium. Typical recirculation rates may be 10-20 times the filtration rate. By this means the cake is prevented from forming during the early stages of filtration. This can be particularly beneficial when the slurry is flocculated and exhibits shear-thinning non-Newtonian properties. This method of operation is discussed by Mackley and Sherman(21) and by Holdich, Cumming and Ismail(22). 

Slurries are prepared from a mixture of liquid (typically water), milled powders or granules, and additives (see Table 7.6). The compositions of two common ceramic slips are given in Table 7.8. Casting slurries are typically formulated to be shear thinning (cf. Section 4.1.2.2) with an apparent viscosity below 2000 cpoise (2000 mPa s) at... 

Results. In general, the glass fibre slurries exhibited slight shear thinning behaviour, which became more marked with increase in fibre length and with concentration. This is illustrated in Figure 4 with a series of flow curves for different w/w concentrations of 1.5mm, CSG dispersed in PRA1478, (see (1)) measured with a 3° annular cone. 

While many process fluids are Newtonian, some are non-Newtonian (as seen in Figure 9.7). For such cases, it is not sufQcient to use a single value for viscosity to determine the impeller Reynolds number. Concentrated slurries are typically non-Newtonian and particle-size dependent. They are frequently shear-thinning. Dilatant behavior seldom occurs. If the system is simply shear thinning, it is usually possible to describe its rheological behavior with a simple power-law relationship ... 

Another aspect of cement slurry stability is the stability under dynamic conditions. Dynamic conditions are usually more severe than static ones because cement slurries are shear-thinning. This is a problem in the laboratory as the solid particles may settle while the fluid is being sheared (thickening time, rheology), and also in the field especially if the well is deviated from vertical. But there is currently no standard test in the industry to evaluate the stability of cement slurries under dynamic conditions. 

It was decided to use 60 vol. % slurry tor all future work as it was observed to be the most stable over a period of 24hrs., and showed shear-thinning behavior. The slimy was sufficiently fluid to be poured into the molds. 

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