Shear thickening, viscosity measurements

Dilatant fluids (also known as shear thickening fluids) show an increase in viscosity with an increase in shear rate. Such an increase in viscosity may, or may not, be accompanied by a measurable change in the volume of the fluid (Metzener and Whitlock, 1958). Power law-type rheologicaJ equations with n > 1 are usually used to model this type of fluids. 

For smaller particles, smaller stresses are exerted. Thus, in order to predict sedimentation it is necessary to measure the viscosity at very low stresses (or shear rates). These measurements can be carried out using a constant stress rheometer (Carrimed, Bohlin, Rheometrics, Haake or Physica). Usually, a good correlation is obtained between the rate of creaming or sedimentation, v, and the residual viscosity rj 0), as will be described in Chapter 21. Above a certain value of ri(0), v becomes equal to 0. Clearly, in order to minimize sedimentation it is necessary to increase rj 0) an acceptable level for the high shear viscosity must be achieved, depending on the application. In some cases, a high rj[0) may be accompanied by a high rj (which may not be acceptable for apphcation, for example if spontaneous dispersion on dilution is required). If this is the case, the formulation chemist should seek an alternative thickener. 

Arheopectic pigmented bleach (alkali metal hypochlorite) hard surface cleaner formulated with bentonite clay is disclosed in U.S. Patent 5,688,435. Examples of time-dependent shear effects determined from constant shear rate measurements at 1, 10, 50, and 100 sec-1 are provided in the patent and shown in Figure 4.2 and Figure 4.3. The viscosity data show evidence of shear thickening as a function of time at constant shear rates of 1 and 10 sec-1 and thixotropy occurs at 50 and 100 sec-1. The formulation is rheopectic at 10 sec-1. Dynamic mechanical data are also contained in the patent and the storage and loss modulus as a function of strain amplitude is shown in Figure 4.4, for one patent example. 

The constant of proportionality in equation 2.10 is the viscosity of the liquid tf). Some fluids, such as water, olive oil and sucrose solutions obey this equation and are said to be Newtonian. Their viscosity does not depend on the velocity gradient, i.e. how fast the liquid is sheared - known as the shear rate, More complex fluids (e.g. solutions of polymers) have a viscosity that does depend on the shear rate. Such fluids are called non-Newtonian . Many complex fluids, for example tomato ketchup and ice cream mix, become less viscous when they are sheared and are described as shear-thinning . Tapping the bottom of the bottle applies shear to the ketchup, which becomes less viscous and flows more easily onto your plate. Other fluids, such as a concentrated solution of cornstarch or quicksand, become more viscous (i.e. they are shear-thickening ). Experiment 7 in Chapter 8 gives some examples of non-Newtonian fluids. A single viscosity is not sufficient to describe the flow properties of non-Newtonian liquids and if a viscosity is stated, the shear rate at which it was measured must also be given. 

Concentrated dispersions may be shear thickening, as opposed to the shear thinning of dilute polymer solutions. Some materials, such as latex paints, tend to form a structure. As the structure breaks down with shearing action, the viscosity decreases. Such materials are thixotropic. Some fluids have a yield stress. A thorough characterization of the rheology may require a number of different measurements. 

Measurements of a mineral, base oil (Figure 3) show significant shear thickening properties at stress levels above 10 kPa (17). Viscosity did Increase with a factor of about 4, when shear stress was raised to 1 MPa. The spread of the measured points from a continuous flow curve... 

If the measured viscosity rj is constant with respect to shear rate, then the liquid is said to be Newtonian as described above. However, as is usually the case for structured liquids such as polymer solutions and suspensions, the viscosity decreases with increasing shear rate such liquids are described as shear thinning. Occasionally, situations arise where the opposite is true, and the viscosity increases with increasing shear rate these are called shear-thickening liquids, see chapter 15. 

In shear measurements one expects the described solutions behave like normal Newtonian aqueous solutions. This is in fact the case for small shear rates (Fig. 11.32). In Fig. 11.32 the shear viscosity, which was measured in a capillary viscometer, is plotted vs.the shear rate. One observes a sudden rise of the viscosity at a characteristic shear rate and for y> % the solutions show some shear thickening behaviour. Obviously something dramatic has happened to the micelles in the solutions. Some conclusions about what has happened can be drawn from flow birefringence measurements. Some typical results of flow measurements from a Couette system are shown in Fig. 11.33. We note a sudden increase of the flow birefringence at a critical shear rate. For y < yc no flow birefringence could be detected. 

Usually, sols behave as Newtonian liquids, when the viscosity is low due to low concentration of particles, small particle size and/or separated particles. On the other hand, when the viscosity becomes high due to growth or connection of the particles, the sols behave as non-Newtonian liquids, exhibiting viscoelastic properties, such as shear thinning or shear thickening. These behaviors can be found by measuring the viscosity as a function... 

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