Applications of Shear Flow

We have thus far discussed the basic foundations of nonequilibrium molecular dynamics, its methodology, and the details of numerically integrating the equations of motion for SLLOD dynamics. The next section presents applications of these methods. 

As discussed in the Introduction, considerable effort has been devoted to the study of transport properties. Specifically, viscosity estimates are of potential importance in the rational design of lubricants. In these simulations, two main approaches have been used. The earlier of the two focused on a linear response theory based Green-Kubo formula  

Given this prescription for parameterization, prototypical lubricants were then explored. One of the first concerns was the investigation of the effects of molecular architecture on viscosity. Such tests were performed by comparing linear alkanes, such as tetracosane (C24) and triacontane ( 30)5 versus a branched alkane squalane (C30, with a 24-carbon-atom backbone).Intuitively, one would expect linear alkanes to have a lower viscosity. Not surprisingly, the simulations showed that the linear alkanes may form layers that 

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In References [6,114,115] shear-induced crystallization of iPP-based nanocomposites with o-MMT was examined. In Reference [114] early stages of flow-induced isothermal spherulitic crystallization of intercalated iPP/PP-g-MA/o-MMT nanocomposites with 2 wt% of the clay were studied after application of shear flow for a short time of up to 30 seconds. The flow induced enhancement of crystallization was stronger in the nanocomposite than in neat iPP. However, acceleration of crystallization in iPP/PP-g-MA blend was also observed. 

Bingham plastics are fluids which remain rigid under the application of shear stresses less than a yield stress, Ty, but flow like a. simple Newtonian fluid once the applied shear exceeds this value. Different constitutive models representing this type of fluids were developed by Herschel and Bulkley (1926), Oldroyd (1947) and Casson (1959). 

Resistance of a material to flow or to undergoing permanent deformation under applications of shearing stresses. 

The range of application of shear cell testing methodology is seen in Tables 2-6. Table 3 relates the flow properties of mixtures of spray-dried lactose and bolted lactose. These mixtures, in combination with the excipients tested, cover a broad range of flow. Tables 4 and 5, for example, show lot to lot variations in the flow properties of several materials, and Table 6 shows the variation in flow properties of bolted starch, sucrose, and phenacetin at different relative humidities (RH). Figure 8 presents the yield loci of sucrose at four different consolidation loads. Also shown in the figure are the shear indices determined at each consolidation load. 

For the case that no convection stream is applied, the time, which elapses between the start of shear flow and the establishment of the temperature maximum in the center of the gap has been reported to be useful for the measurement. This measurement could be carried out within 10 sec (202). The possibility of such a procedure, however, follows from the theoretical treatment only for relatively wide gaps of d > 2 mms. For metal cylinders (199) as well as for glass cylinders (203) times of establishment can be calculated which are definitely too short for the application of this method, when smaller gap widths are used. 

The special elements most commonly used today are for the deliberate application of elongational flows or defined shearing fields, for low-shear plastification, or for retaining un-melted particles. 

Hydrogenated SBCs are often used to modify polyolefins such as polypropylene, polybutylene and polyethylene. One of the unique characteristics of strongly phase-separated block copolymers such as high molecular weight (>50 000) SEBS and SEPS is their response to shear in the melt. These polymers retain their phase-separated structure well above the Tg of the polystyrene because their order-disorder transition temperatures are above processing temperatures. This phase separation strongly inhibits flow in the absence of shear resulting in infinite viscosity at zero shear rates. The application of shear... 

Capillary glass tubes. With this method, radioactively labeled tumor cells are allowed to adhere to the inner surface of glass capillary tubes either uncoated, or coated with different adhesion molecules. After incubation, adherent cells are subjected to variable shear forces by application of variable flows through an automatic... 

To be more precise, the general tensor equation of Newton s law of viscosity should be obeyed by a Newtonian fluid (2) however, for onedimensional flow, the applicability of eq 1 is sufficient. For a Newtonian fluid, a linear plot of t versus 7 gives a straight line whose slope gives the fluid viscosity. Also, a log-log plot of t versus 7 is linear with a slope of unity. Both types of plots are useful in characterizing a Newtonian fluid. For a Newtonian fluid, the viscosity is independent of both t and 7, and it may be a function of temperature, pressure, and composition. Moreover, the viscosity of a Newtonian fluid is not a function of the duration of shear nor of the time lapse between consecutive applications of shear stress (3). 

The applicability of the flow property test used is highly dependent on what the user is trying to capture. As an example, if there is concern that a certain batch of material may arch when transferred into a bin, a shear test may be the most comprehensive QC test. However, a quantitative test such as a shear test may require more time/resources to conduct than is practical, so faster test methods are often desired. One option is abbreviated shear cell testing (1). 

The importance of FIPI is twofold. It can be used to promote phase inversion without changing the thermodynamics of the system to obtain a higher entropy state, or it is possible to delay phase inversion while reducing the system entropy. The characteristics of the microstructure formed (such as emulsion droplet size) are dependent on the type of microstructure and deformation (shear, extension, or combined), as well as the deformation rate. To maximize the fluid micro-structure/flow field interactions, the flow fleld must be uniform, which requires the application of the flow field over a small processing volume, which can be achieved by using MFCS mixers or CDDMs. 

We felt it important to obtain data on the particle network in order to understand the flow of gels quantitatively. It is clear, however, that this is still impossible because an adequate theory going much beyond the ideas expressed by Goodeve (7) has not been developed. The flow properties of gels vary considerably furthermore, some gels become temporarily liquid upon the application of shear, and others become thicker. Quantitatively explaining these various types of behavior on the basis of the particle network model does not appear easy. Nevertheless, the information obtained here should be of some aid, in that the model is now much more closely defined. 

The incorporation of such materials as aluminum stearate, fumed silicas, or certain bentonites gives a paste that shows pronounced Bingham Body behavior (i.e., it only flows on application of shearing stress above a certain value). Such putty-like materials (called pastigels), which are usually thixotropic maybe hand-shaped and subsequently gelled (see Plastisol Casting in Chapter 2). 

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