Flow and Viscosity

When reviewing the subject of plastic melt flow, the subject of viscosity is involved. Basically viscosity is the property of the resistance of flow exhibited within a body of material. Ordinary viscosity is the internal friction or resistance of a plastic to flow. It is the constant ratio of shearing stress to the rate of shear. Shearing is the motion of a fluid, layer by layer, like a deck of cards. When plastics flow through straight tubes or channels they are sheared and the viscosity expresses their resistance. 

The melt index (MI) or melt flow index (MFT) is an inverse measure of viscosity. High MI implies low viscosity and low MI means high viscosity. Plastics are shear thinning, which means that their resistance to flow decreases as the shear rate increases. This is due to molecular alignments in the direction of flow and disentanglements. 

Viscosity is usually understood to mean Newtonian viscosity in which case the ratio of shearing stress to the shearing strain is constant. In non-Newtonian behavior, which is the usual case for plastics, the ratio varies with the shearing stress (Fig. 8-5). Such ratios are often called the apparent viscosities at the corresponding shearing stresses. Viscosity is measured in terms of flow in Pa s, with water as the base standard (value of 1.0). The higher the number, the less flow. 

Newtonian flow It is a flow characteristic where a material (liquid, etc.) flows immediately on application of force and for which the rate of flow is directly proportional to the force applied. It is a flow characteristic evidenced by viscosity that is independent of shear rate. Water and thin mineral oils are examples of fluids that posses Newtonian flow. 

Non-Newtonian flow Plastic melts are non-Newtonian. They have basically abnormal flow response when force is applied. That 

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The electroviscous effects and the other effects discussed in Sections 4.7a-c lead to what is called non-Newtonian behavior in the flow of dispersions. In the next section, we begin with a brief review of the basic concepts concerning deviations from Newtonian flow behavior and then move on to consider how high particle concentrations and electroviscous effects affect the flow and viscosity. 

This advantage is due to molecular structure and the lack of crystalline wax particles, present in some refined petroleum oils. Fully synthetic engine oil lubricants offer excellent low temperature flow and viscosity properties.(Demmin etal., 1992 Lakes, 1999). 

Normal red blood cells are deformable biconcave disks. Their shape is determined by the external environment of the cell, the metabolic activity of the cell, the nature of hemoglobin, the membrane skeleton (see below), and the age of the cell. A normal human red blood cell has a life span of about 120 days and travels a distance of about 175 miles. Much of this travel occurs in capillary channels of the microcirculation, where flow rates are very slow. Here, particularly at branch points, the shape of the cell undergoes striking deformations and can squeeze through openings as small as one-twentieth the cell diameter. Thus, the primary determinant of blood flow and viscosity is... 

As discussed earlier, LADDs are complex, multicomponent mixtures consisting of both organic and inorganic compounds dispersed in a liquid matrix. Such compositions can exhibit a broad range of rheological characteristics from simple Newtonian to complex pseudoplastic flow. Shown in Figure 9.6 and Figure 9.7 are flow and viscosity profiles of Newtonian and non-Newtonian fluids as a function of applied shear rate. A number of mathematical models have been proposed [76] to describe the flow characteristics of various systems. These equations are called constitutive equations and are used to predict flow behavior in complex systems. 

In summary, miniaturized NMR spectrometers have been demonstrated to be useful for concentration, flow and viscosity measurements on materials in microchannels and should be useful detectors in high fhroughtput systems. 

Flow and viscosity studies in nematic liquid crystals... 

Wall shear stress is the tangential drag force produced by the horizontal movement of the fluid across the surface. It is a function of the velocity gradient of the fluid near the surface. Its magnitode is directly proportional to fluid flow and viscosity and inversely proportional to the cube of the radius. 

Fig. 2.9 (a) Shear flow and viscosity of the solution. Polymers flow toward downstream while they rotate,... 

Rheological measurements were carried out with a computer-controlled Rheometer MCI (Paar Physica) rotational viscometer, with a Z4 concentric cylindrical measurement system at 25 0.1 °C. The complete flow and viscosity curves of the samples were plotted, and the yield value and the size of the thixotropic area were determined. 

Rheological studies give information about the structure, flow properties and stability of coherent emulsions. With rheological measurements, by plotting flow and viscosity curves the circumstances of structure formation and the changes arising in the structure can be characterised in an exact manner. 

KoUer, A., Sun, D., and Kaley, G., Role of shear stress and endothelial prostaglandins in flow- and viscosity-induced dilation of arterioles in vitro, Circ. Res., 72,1276,1993. 

Fig. 1. Flow and viscosity curves for fluids exhibiting Newtonian behaviour Any deviation from Newtonian behaviour is said to be non-Newtonian.

The influence of deformation rate, type of flow, and viscosity ratio on the deformation of droplets in shear and extensional flow. At high deformation rates (right-hand shapes) drop breakup can occur. Adapted from Rum-scheidt and Mason fl961). 

Make sure valve sizing calculations take into account any transition to laminar flow and viscosity correction. 

This relationship is based on experiments of water flow through a range of sand beds. Here, Q, A, and fx, represent the flow rate, cross-sectional area of flow, and viscosity of the fluid. The actual permeability of a given phase is given by k, and AP/L is the pressure gradient across the porons medium having a thickness of L. A common unit for permeability is the darcy (1 darcy 10 m ) in petroleum science, whereas it is represented by centimeters squared or meters squared in SI nnits. When a measured value of absolute permeability is unavailable, the Carman-Kozeny eqnation can be used to make an estimate of the absolute gas-phase permeability of the medium ... 

Technical uses are in - ceramics, where it aids green strength. M. acts as a flow and viscosity control in paints, and as an emulsifying and suspending agent in drilling muds. It is also used in chromatography. 

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