Shear thickening materials viscosity

In processes with very small liquid dimensions and high speeds, such as spraying or coating processes, shear rates can reach up to 105 1/s. If shear thinning or shear thickening materials are used, the viscosity can therefore vary greatly depending on shear rate. 

Shear thickening materials show an increase in viscosity with increasing shear strain rate. An idealized flow curve is presented in Fig. 6, and the viscosity as a function of shear strain rate is depicted in Fig. 7. The shear thinning region usually extends only over about one decade of shear rate (power law index n > 1) in contrast to shear thinning, which usually covers at least two or three decades. Also, in many cases, shear thickening is preceded by a short phase of shear thinning at low shear strain rates. ° ... 

Shear thinning (thixotropy) is one of the most common manifestations of non-Newtonian behavior in polymeric liquids [61], Increased shear can lead to partial alignment of polymers or colloid particles with the flow, thus decreasing viscosity. Examples include latex paint, blood, and syrups. Shear thickening is the opposite phenomenon (antithixotropy) whereby the material becomes more viscous or stiffer with increasing shear, often due to shear-induced organization, such as partial crystallization. Quicksand and aqueous solutions of cornstarch are examples of shear-thickening materials. 

Oscillatory shear experiments are the preferred method to study the rheological behavior due to particle interactions because they directly probe these interactions without the influence of the external flow field as encountered in steady shear experiments. However, phenomena that arise due to the external flow, such as shear thickening, can only be investigated in steady shear experiments. Additionally, the analysis is complicated by the different response of the material to shear and extensional flow. For example, very strong deviations from Trouton s ratio (extensional viscosity is three times the shear viscosity) were found for suspensions [113]. 

Some materials will increase in viscosity as the flow rate increases, and, although this is relatively rare, it is crucial to be aware of this if processi ng is to be effective. Shear thickening occurs as a result of one or more of the following situations ... 

As the shear rate increases, the viscosity of some dispersions actually increases. This is called dilatancy, or shear-thickening. Dilatancy can be due to the dense packing of particles in very concentrated dispersions for which at low shear, the particles can just move past each other but at high shear they become wedged together such that the fluid cannot fill (lubricate) the increased void volume, and the viscosity increases. An example of this effect is the apparent drying of wet beach sand when walked on, the sand in the footprint initially appears very dry and then moistens a few seconds later. Other examples include concentrated suspensions (plastisols) of polyvinyl chloride (PVC) particles in plasticizer liquid and the commercial novelty product Silly Putty (which is a silicone material). 

In the case of fluids without yield stress, viscous and viscoelastic fluids can be distinguished. The properties of viscoelastic fluids lie between those of elastic solids and those of Newtonian fluids. There are some viscous fluids whose viscosity does not change in relation to the stress (Newtonian fluids) and some whose shear viscosity T] depends on the shear rate y (non-Newtonian fluids). If the viscosity increases when a deformation is imposed, we define the material as a shear-thickening (dilatant) fluid. If viscosity decreases, we define it as a shear-thinning fluid. 

If)/ is independent of shear history, the material is said to be time independent. Such liquids can exhibit different behavior patterns, however, if, as is frequently the case with polymers, )/ varies with shear rate. A material whose viscosity is independent of shear rate, e.g., water, is a Newtonian fluid. Figure 11-26 illustrates shear-thickening, Newtonian and shear-thinning rj-y relations. Most polymer melts and solutions are shear-thinning. (Low-molecular-weight polymers and dilute solutions often exhibit Newtonian characteristics.) Wet sand is a familiar example of a shear-thickening substance. It feels hard if you run on it, but you can sink down while standing still. 

In some cases the very act of deforming a material can cause rearrangement of its microstructure such that the resistance to flow increases with an increase of shear rate. In other words, the viscosity increases with appHed shear rate and the flow curve can be fitted with the power law. Equation (20.3), but in this case n> 1. The shear thickening regime extends over only about a decade of shear rate. In almost all cases of shear thickening, there is a region of shear thinning at low shear rates. 

Simple classifications of fluids can be made on the basis of their rheological profiles. Figure 3.78 shows the (a) shear stress and (b) viscosity profiles for various systems. From Figure 3.78 one may define the following systems. Newtonian systems have a constant viscosity with respect to shear rate. Dilatant (or shear-thickening) systems have a viscosity that increases with respect to shear rate. Pseudo-plastic (or shear-thinning) systems have a viscosity that decreases with respect to shear rate. Yield-stress materials are materials that have an initial structure that requires a finite stress before deformation can occur. The stress that initiates deformation is defined as the yield stress. 

When the shear stress of a liquid is directly proportional to the strain rate, as in Fig. 2.4a, the liquid is said to exhibit ideal viscous flow or Newtonian behavior. Most unfilled and capillary underfill adhesives are Newtonian fluids. Materials whose viscosity decreases with increasing shear rate are said to display non-Newtonian behavior or shear thinning (Fig. 2.4b). Non-Newtonian fluids are also referred to as pseudoplastic or thixotropic. For these materials, the shear rate increases faster than the shear stress. Most fllled adhesives that can be screen printed or automatically dispensed for surface-mounting components are thixotropic and non-Newtonian. A second deviation from Newtonian behavior is shear thickening in which viscosity increases with increasing shear rate. This type of non-Newtonian behavior, however, rarely occurs with polymers. ... 

The viscosity of a liquid is its internal property that offers resistance to flow. Actually a liquid can be also defined as a material that deforms as long as it is subjected to a tensile or shear stress. Under shear, the rate of deformation (or shear rate) is proportional to the shearing stress. According to Newton s idea, the ratio of the stress to the shear rate is a constant, called viscosity, which is independent of the shear rate. For ideal or Newtonian liquids, e.g., water, this viscosity is independent of the shear rate. However, the viscosities of many liquids are not independent of the shear rate, which is called non-Newtonian liquids. Such non-Newtonian liquids can exhibit either shearthinning or shear-thickening and can be classified according to their viscosity behavior as a function of the shear rate. Generally, viscosity can be defined in two ways ... 

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