Homogenization, shear-induced

Illustration Effect of flow type on shear induced collisions in homogenous linear flows. The collision frequency for a general linear flow [Eq. (15)] is obtained following Smoluchowski s (1917) approach as (Bidkar and Khakhar, 1990)... 

Before discussing theoretical approaches let us review some experimental results on the influence of flow on the phase behavior of polymer solutions and blends. Pioneering work on shear-induced phase changes in polymer solutions was carried out by Silberberg and Kuhn [108] on a polymer mixture of polystyrene (PS) and ethyl cellulose dissolved in benzene a system which displays UCST behavior. They observed shear-dependent depressions of the critical point of as much as 13 K under steady-state shear at rates up to 270 s Similar results on shear-induced homogenization were reported on a 50/50 blend solution of PS and poly(butadiene) (PB) with dioctyl phthalate (DOP) as a solvent under steady-state Couette flow [109, 110], A semi-dilute solution of the mixture containing 3 wt% of total polymer was prepared. The quiescent... 

The decrease in expansion seems to occur at moisture levels above 30°/o moisture for both cereal (starch-based) polymer systems, and protein (soy grits). This corresponds to a point on their adsorption curves where water activity rises rapidly with added moisture that is, at a level where the water added to a mix has little effect on primary hydration of polymers, but behaves as a diluent. In mechanical terms, this may be explained by proposing that at above levels of 25°/o-30% water plasticisation of the polymers is complete, and further added water acts as a lubricant, reducing the shear-induced temperature rise and particle damage necessary for the formation of homogeneous melts. 

Many interesting samples are not laterally homogeneous. The calculation of the average shear-induced stress is not an easy task, but it seems feasible in some cases. High-frequency fluid dynamics calculations should be helpful. 

The first observation of shear-induced increase of the LCST was reported for PS/PVME by Mazich and Carr [1983]. The authors concluded that shear stress can enhance miscibility by 2-7°C. Larger effects, AT < 12°C, were reported for the same system in hyperbolic flow [Katsaros et al., 1986]. In a planar extensional flow at 8 = 0.012 - 26 s the phase separated PS/PVME was homogenized at temperatures 3 to 6°C above... 

Another example of shear-induced mixing comes from the mbber field [Izumitani and Hashimoto, 1985]. A solution cast film of SBR/BR blend with two-phase stmcture was folded and pressed by hand. After the folding and pressing process was repeated several times, a homogeneous film was obtained. Note that this process is similar... 

The relation predicts that, initially, as y increases the system undergoes homogenization, then phase separation, followed by another homogenization and demixing. These predictions were confirmed by experiments on blends of ethylene-co-vinyl acetate (EVAc) with chlorinated polyethylene, CPE, and PS/PVME [240-244]. The shear-induced mixing was also observed in PS blends with polyisobutylene, PIB [245]. 

TAK Takebe, T., Sawaoka, R., and Hashimoto, T., Shear-induced homogenization of semi-dilute solution of polymer mixtirre and irnmixing after cessation of the shear, J. Chem. Phys., 91, 4369, 1989. 

Some samples (B, C, and D) were also melt-homogenized in a larger machine specially designed for mixed plastics waste, at the Cadauta plant in Italy. The machine ( Revive compounder from Cerrini, Busto Arsizio, Italy) has a screw diameter, D, of 120 mm, and a cylinder length of 30D. It is a slow running machine (33 rpm). The melt temperature was set at 240°C, but due to the special construction of the machine a high shear-induced heat gradient in the melt was... 

Aslund I, Medronho B, Topgaard D, Soderman O, Schmidt C (2011) Homogeneous length scale of shear-induced multilameUar vesicles studied by diffusion NMR. J Magn Reson 209 291-299... 

Figure 3 Changes in steady-state scattering patterns with increasing shear rate. The change from pattern (a) to (b) indicates shear-induced homogenization or singie-phase formation, and that from pattern (c) to (d) indicates shear-induced phase-separation or concentration fluctuations. The arrow indicates the fiow direction (x-axis) and the bar corresponds to the scattering angle 0= 7° in the solutions. The arrow and the bar are common to all the patterns. Based on Hashimoto, T. In Soft Matter Characterization Borsali, R., Pecora, R., Eds. Springer New York, 2008 Vol. 1, Chapter 8, pp 378-462. ...

When droplets are initially not uniformly dispersed inside the flow cell, i.e. the emulsion is not homogeneous, the presence of a shear flow will induce mixing and the flow behavior of the system will be dependent on the spatial distribution of both phases. Therefore, in order to study the flow and mixing of an initially non-homogeneous emulsion it is necessary to obtain information on how both phases... 

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