Viscosity Pivotal Role

We note that our previous descriptions of flow processes have tacitly assumed laminar flow. For example, flow in capillaries was described by balancing pressure-derived forces against viscous forces, ignoring acceleration (inertial) effects. Darcy s law, Eq. 4.18, is also based on laminar flow. With turbulence, flow resistance increases the pressure gradient is no longer linearly related to flow (see Eqs. 4.18 through 4.20) but increases more rapidly as expressed by 

Early in this chapter we defined viscosity and discussed its role in opposing flow. By resisting flow, viscosity often limits the flowrate to less than optimal values. Higher pressure drops can be used to offset viscous drag but the pressure drop necessary to force fluid at the desired rate through narrow pores is often unattainable. Thus the rate of flow is often viscosity limited. For example, the rate of fluid transfer through both membranes and chromatographic columns is often limited by viscous resistance this in turn limits the speed of separation. 

Along with limiting flow, our analysis has shown that viscous effects control the detailed form of flow. This form, which exhibits itself in the various flow velocity profiles already noted, is important to many methods of separation. 

In the previous sections of this chapter we have discussed flow in relationship to viscous effects. In the next few sections we will discuss the 

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The pivotal role that superplasticizers play in the formulation of self-leveling mortars is due to the dramatic effects they produce on flow behavior. Such effects are believed to be derived by the adsorption of the admixture on the surfaces of cement grains, thereby providing surfaces of a similar or zero charge which are mutually repulsive. They thus fully disperse cement particles, freeing more water for lubrication and reducing interparticle attraction. Both yield stress and plastic viscosity are decreased and the decrease is greater for yield stress it may be completely eliminated if sufficient admixture is added so that Newtonian behavior is observed (Fig. 7.25) [75, 76]. 

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