Fluctuations flow-induced enhancement

More recently, the Onuki-Doi theory has been experimentally verified for the problem of flow-induced enhancement of concentration fluctuations in polymer solutions [60]. This problem is explored in greater detail in section 7.1.7. A similar verification has been achieved in the problem of electric field induced enhancement of fluctuations [61]. 

An alternative explanation of the observed turbidity in PS/DOP solutions has recently been suggested simultaneously by Helfand and Fredrickson [92] and Onuki [93] and argues that the application of flow actually induces enhanced concentration fluctuations, as derived in section 7.1.7. This approach leads to an explicit prediction of the structure factor, once the constitutive equation for the liquid is selected. Complex, butterfly-shaped scattering patterns are predicted, with the wings of the butterfly oriented parallel to the principal strain axes in the flow. Since the structure factor is the Fourier transform of the autocorrelation function of concentration fluctuations, this suggests that the fluctuations grow along directions perpendicular to these axes. 

Fig.4 Borders of instability in the flow-gradient direction for G = 1, according to the model of Clarke and McLeish (dotted /me), and Criado-Sancho et al. (solid line). The CM model predicts shear-induced suppression of fluctuations, i.e., Axc >0, to the left of the dotted line and shear-induced enhancement to the right, i.e., Axc < 0. The CSJVC model predicts shear-induced suppression of fluctuations to the left of the left-most solid curve and to the right of the right-most solid curve and shear-induced enhancement between the two curves. Reproduced with permission from reference [38]...

Different from the secondary flows and flow channel spacer techniques, the pulsed flow method is to generate a pressure fluctuation wave in either the feed or permeate flow channel using certain oscillators. The fluctuating pressure wave can enhance the membrane filtration through reducing the boundary layer or induced instant local backflushing flow as discussed in Section 10.4.1. 

It is believed that, similar to inorganic filler-enhanced nucleation, the shear/ extensional flow in different regions of the die, as well as the expansion of nucleated bubbles near the nanoparticles, would generate a pressure fluctuation around the suspended nanoparticles. The schematic in Figure 3.4 illustrates the induced-extensional flow around the side surface of the nanoclay particle. In extreme situations, such a local pressure held may even be negative and significantly promote cell nucleation. More details about the cell nucleation mechanism of polymer/fillers foaming systems have been presented in our newly submitted journal paper (Zhai et al., 2012), and the readers may get more information from this review paper. 

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