Flow anisotropy

Although the Izod and Charpy tests are widely used for plastics, other types of test are also popular. These include tensile impact tests and flexural plate (falling weight) tests. The latter is particularly useful in situations where the effects of flow anisotropy are being assessed. In addition, arbitrary end-product tests are widely used to provide reassurance that unforseen factors have not emerged to reduce the impact performance of the product. 

Figure 5. Optical micrograph of a coke surface showing an optical texture of coarse-flow anisotropy 30-60 ym length, 5-10 ym width, OTI = 20.

Figure 1. Three steps in the development of optical anisotropy via the mesophase. (a) Nucleation of mesophase spherules. (b) Growth and coalescence of mesophase. (c) Flow anisotropy.

Gasification and Heat Treatment. Examination under the optical microscope showed the Spencer works and Clyde Ironworks cokes to have optical textures mainly consisting of fine- and medium-grained mosaics with some coarse flow anisotropy and isotropic inert material. Of particular interest are the fissures which develop in different types of optical texture and those occurring at the anisotropic-inert interface. SEM examination of these polished surfaces before experimentation shows all of them to be flat and featureless. 

Differences in coke behaviour in the blast furnace (not detectable by cold testing prior to the charging of the blast furnace) may be attributable to differences in the mode of gasification of the coke as a result of combined effects of thermal and gasification fissuring. These results indicate that mosaic optical textures are preferable to flow anisotropy in terms of fissure containment and also show that inert particles can act as centres of fissure generation. 

The A, B and C terms are related to flow anisotropy, molecular longitudinal diffusion and mass transfer processes, respectively. The theoretical support for the Knox equation was derived by Horvath [12]. The A term cannot be expressed simply. The theoretical treatment links A to structural parameters of the column packing, porosity, pore volume, pore diameter and tortuosity [12]. A is related to the flow pattern and the general band spreading due to "eddy" diffusion [13]. The B term (longitudinal molecular diffusion) was written as [13] ... 

As shown by eqs. 6.7, 6.8 and 6.9, the A, B and C terms of the Knox equation are not directly related to the kinetic constants of the three equilibria occurring with micellar mobile phases. The slow exchange of the solute between the micellar and aqueous phases increases the three terms of the Knox equation. It increases the flow anisotropy (A term). It reduces the molecular diffusion increasing the B term (eq. 6.8) and the C term (eq. 6.9). The reduction of the stationary phase-micellar phase solute-exchanges due to the adsorbed layer acts mainly on the C term. 

The use of the Knox plots to study the causes of micellar reduced efficiencies leads to the following conclusions. The micellar phase flow anisotropy seems to be much higher than the flow anisotropy obtained with a hydro-organic phase of comparable viscosity (increased A term). This is only partly due to the micellar viscosity. The main reason of such differences in flow patterns is the partial clogging of the stationary phase pores by adsorbed surfactant molecules [19, 22]. A temperature raise decreases the mobile phase viscosity and the amount of adsorbed surfactant [22]. Both effects decrease the flow anisotropy and the A term. It will be exposed thereafter that alcohol additions to a micellar phase dramatically reduce the amount of adsorbed surfactant. 

Fig. 3 a, Simulation of the average plate height for a TLC layer hy use (Eq. 12) and properties listed in Table 1. The contribution from flow anisotropy is represented by iTa. that from longitudinal diffusion by /fb, and that from resistance to mass transfer by H, , and b, Simulation of the average plate height for an HPTLC layer by use (Eq. 12) and properties listed in Table 1. The contribution from flow anisotropy is represented by that from longitudinal diffusion by /fb, and that from resistance to mass transfer by... 

The most striking change is in the A-term that accounts for multipath dispersion (Equation [3.33] in the van Deemter theory) and decreases rapidly with decreasing i.d. This effect was interpreted (Karlsson 1988) as reflecting a decrease in the extent of the flow rate variation over the column cross-section as the i.d. decreases, together with shorter time for solute molecules to diffuse radially through the flow velocity profile (parabolic in the case of a nonpacked column) together these phenomena will decrease the effect of flow anisotropy that is reflected in... 

Electrical analogy. Again, 9(k(x,y) 9P/9x)/9x + 9(k(x,y)5P/ )/ = 0 was solved for isotropic but variable k(x,y), and results were obtained that behaved anisotropically, as if 9(k h 9P/9x)/9x + 9(k v dPldy)ldy = 0 had been solved. This was the case for high permeability contrasts and dip angles. Now, electric currents satisfy 9(a(x,y) 9V/9x)/9x + 9(a(x,y) 9V/ )/ = 0 in the low-frequency limit, where V and ct represent potential and electrical conductivity in isotropic media. Because the analogy with fluid flow is exact, our conclusions on streamline pattern apply to electric pathlines. Thus, dipping laminates that are isotropic microscopically can behave anisotropically macroscopically electrically. Because fluid and electrical models appear hand-in-hand, fluid flow anisotropy implies electrical anisotropy, and vice-versa. 

The basic factor which causes a decrease in the viscosity in the transition into the LC state is thus the cooperative orientation of the major axes of the macromolecules along the direction of flow (anisotropy of viscosity), but the scale of this decrease will be a function of the presence of phase heterogeneity, the amount of inhomogeneities (disclinations), the range of the effective stress, and in particular, the concentration of the solution. 

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