Shear induced phenomena

Three different models have been proposed for the explanation of these shear-induced phenomena. The first assumes that such phenomena are due to the formation of a nematic phase under the influence of shear. According to this theory, the systems are in the isotropic micellar phase, but close to the boundary of the nematic phase, and the application of shear leads to the transition into this phase. Nematic phases can... 

Up until here, all three models could in principle explain the shear-induced phenomena. Therefore,... 

Here, the shear-induced phenomena became smaller with increasing amounts of Ci2E2.5S04Na. They disappeared for complete replacement of SDS and could not be restored by increasing the concentration or ionic strength. 

Replacement of C14DMAO by the shorter-chain homologous C12 DM AO leads also to a decrease of the shear-induced phenomena, which disappeared again at a certain amount of C12DMAO. This is due to a decrease of the size of the rods and of the interaction between... 

CM model for this blend should be treated with some caution. From Eqs. 87 and 88, it is clear that one of the important ratios is that of the entanglement degrees of polymerisation. For isotopic blends this ratio is most probably 1, since the ratio is the inverse of the ratio of the plateau modulae, which should be identical for each component. In such a case if both components also have very similar degrees of polymerisation, the CM model predicts that shear will have no effect on stability. In order to overcome this problem in their comparison between the models, CSJVC calculated shifts according to the CM model with the ratio of the entanglement degrees of polymerisation 1. This illustrates another difference between the models. The CM model relies on rheological asymmetry between components, whereas the CSJCV model predicts shear-induced phenomena even without such asymmetry. 

Theoretical representation of the behaviour of a hydrocyclone requires adequate analysis of three distinct physical phenomenon taking place in these devices, viz. the understanding of fluid flow, its interactions with the dispersed solid phase and the quantification of shear induced attrition of crystals. Simplified analytical solutions to conservation of mass and momentum equations derived from the Navier-Stokes equation can be used to quantify fluid flow in the hydrocyclone. For dilute slurries, once bulk flow has been quantified in terms of spatial components of velocity, crystal motion can then be traced by balancing forces on the crystals themselves to map out their trajectories. The trajectories for different sizes can then be used to develop a separation efficiency curve, which quantifies performance of the vessel (Bloor and Ingham, 1987). In principle, population balances can be included for crystal attrition in the above description for developing a thorough mathematical model. 

It is unfortunate that research in the area of polymer solutions has been deserted during the past ten years in favor of blend work. With the advent of shear cells, it is expected that research in polymer solutions will become fashionable again. For instance, the phenomenon of shear-induced apparent demixing of high molecular weight polystyrene in semidilute solutions (in DOP for example) is not understood. Kinetics measurements will hopefully permit a close monitoring of the remixing effect after shear cessation as well. 

The shear-thinning phenomenon in Fig. 6-34 can readily be attributed to the shpping of layers past each other, as described above. In fact, the slope of the viscosity-shear rate curve in Fig. 6-34 for the two more concentrated samples is around — 1 at low shear rates, implying that a yield stress must be exceeded to induce the layers to move over one another (see also Chen et al. 1994). Figure 6-35 shows the scattering pattern in the plane normal to the shear... 

Within the fluctuation-dominated region of the disordered phase of a diblock near Todt shearing can apparently induce a transition to the ordered state (Koppi et al. 1993). Cates and Milner (1989) predicted such a phenomenon, based on a shear-induced suppression of... 

Proteins in solution are also sensitive to orthokinetic aggregation, i.e., shear-induced aggregation. We have observed this phenemenon at very low shear rates in the case of BSA, ovalbumin, and BLG [28,29] in Fig. 1, one sees that the viscosity of the protein solution increases with time till it reaches a plateau the phenomenon becomes less and less pronounced as the shear rate increases, so much so that the solution displays time-independent and Newtonian flow behavior in the usual shear rate range. 

When there is a prominent dilatancy effect in the flow phenomenon arising from the interaction of the shear-induced dilatation, with a prevailing mean normal stress (Tin, this is often considered to make the critical threshold resistance in shear T dependent on through a friction coefficient... 

At the same time that we were confronted with the negative normal force phenomenon, we also discovered an odd morphological phenomenon, that of the shear-induced band structure. Many polymer liquid crystal systems have been seen to exhibit striations perpendicular to the shearing direction upon cessation of shear. To our knowledge, the formation of striations or bands was first mentioned in passing by Aharoni [12], who did not specify the shearing direction. This phenomenon has also been confirmed repeatedly and has also now been satisfactorily explained, as discussed in section 11.6. 

Still in the isotropic phase, but closer to the phase transition temperature, a shear induced transition to the nematic phase occurs, see Fig. 6. Based on the equations presented here, such a behavior has been predicted theoretically quite some time ago [20, 21]. This phenomenon has been observed in lyotropic liquid crystals, in particular with wormlike micelles [5] and in side-chain liquid-crystalline polymers [35]. In Fig. 6, results are presented for = 1.3. For comparison, the highest temperature for which a metastable nematic phase exists, -d = 9/8 = 1.125, is included. For imposed shear rates, shear stress and consequently the viscosity jump smaller values at the induced phase transition. For imposed shear stress there is a jump to higher shear rates. 

The Gaussian scission probability distribution function, with a preference for mid-chain scission, is frequently encountered in shear-induced mechanochemical degradation. The parabolic distribution, on the contrary, indicates a preference for chain-end degradation. This phenomenon has been reported in the hydrolysis of dextran, as a result of chain branching. 

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. 

Summarizing the theoretical approaches and experimental studies, we may state that the exact nature of the shear-induced state and the complicated mechanisms involved are not yet completely understood. One important problem, which is still unsolved, concerns the role of electrostatic interactions. Many experiments show that SIS mainly occtus in the regime of very low ionic strengths, where repulsive forces are pre-dominant. Addition of salt shordd lead to enhanced collision frequency between the micellar particles, and this phenomenon should favor SIS. This assmnption is, however, in contrast to experimental observations, where shear-induced structures usually disappear upon addition of salt. Besides the large number of new results, some phenomena of SIS still remain mysterious. It is interesting... 

As is well-known, the rate of nucleation is enhanced by the application of a deformation to the polymer melt (2/i). But only recently, a theory of shear induced crystallization could be developed (25). This type of crystallization causes highly oriented boundary layers in injection molded articles (cf.ref. 8). Shear induction is an elastico-viscous relaxation phenomenon. So far, however, this perception did not contribute to a simplification of the situation. 

Polyethylene can crystallize during extrusion if the conditions are favorable. This phenomenon, known as shear-induced crystallization, occurs when highly oriented melts are extruded at temperatures at or below their melting points (93). 

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