Shear-thickening material

Nienow and Elson (1988) have reviewed work done mainly by them and their co-workers on the mixing of non-Newtonian liquids in tanks. The above approach for inelastic, shearing thinning liquids has been largely substantiated but considerable doubt has been cast over using this method for dilatant, shear thickening materials. 

In the future, body armor may be flexible. Experimentation is underway with shear thickening material using polyethylene glycol (PEG) with nanoparticles that remain flexible until rapidly struck such as with a bullet whereupon it acts as a solid protecting the wearer from the major impact. Kevlar is also being employed to protect space craft and space men from discarded space junk and small meteors. Thus, experimentation in body armor is being applied to additional areas where impact protection is essential. 

If the response of a sample to a change in shear is reversible and essentially instantaneous within the time frame of the measurement, it is said to be time independent. Most shear-thinning or shear-thickening materials are time independent. Alternatively, the sample can take seconds, minutes, hours, or longer to reach steady state such materials are said to be time dependent. This delayed response to a change in applied shear can have a significant impact on processing considerations. 

The linear ramps (continuous or stepped) have the advantage of speed and work best for simple shear-thinning or shear-thickening materials. They should compare with single-point determinations from other equipment if the sample is Newtonian. If the sample is non-Newtonian then there may be discrepancies. This will occur if the single-point device uses a complex shear field (e.g., Zahn or Ford cup). 

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. ° ... 

Figure 22.3 Stress versus shear rate behavior for (a) shear thinning, (b) Newtonian, and (c) shear thickening materials.

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. 

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