Application of a Controlled Shear

In industrial applications, emulsions are often obtained by exerting a crude stirring, made of a very complex combination of extensional and shearing flows. To study and understand the fragmentation process, the flow must be simplifled before 

Many authors have worked on drop deformation and breakup, beginning with Taylor. In 1934, he published an experimental work [138] in which a unique drop was submitted to a quasi-static deformation. Taylor provided the first experimental evidence that a drop submitted to a quasi-static flow deforms and bursts under well-defined conditions. The drop bursts if the capillary number Ca, defined as the ratio of the shear stress a over the half Laplace pressure (excess of pressure in a drop of radius R. Pl = where yint is the interfacial tension)  

Rgure 1.10. Condition of mptur-ing in quasi static-conditions for (a) a simple shear flow and (b) a pure elongational flow. (Adapted from [139].) 

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Finely divided fillers used as additives in polymers have a tendency to agglomerate into larger structures due to strong, inter-particle attractive forces. As mentioned earher, it is the purpose of the dispersive mixing process to reduce the size of these agglomerates through the application of a controlled shear stress. 

Fig. 1. Images of a a crude polydisperse emulsion with d= 23 pm and 17=40%, and b of the same emulsion after application of a controlled shear, d= 1 pm and 17=12%...

The process leading to monodisperse emulsions was initially described by Mason and Bibette [ 1,24,26-28]. For that purpose, they first prepare a crude mother emulsion obtained by progressively incorporating oil into the surfactant-water phase. In a second step, they apply a simple and well-controlled shear to this crude emulsion that becomes monodisperse after no more than a few seconds. Figure 1 shows microscope images before and after application of a shear under the same conditions used by Mason and Bibette. The shear has the effect of reducing both the average diameter and the distribution width of the mother emulsion. 

The relation between the kinetic energy of the particle acquired from Brownian motion and Vsm. the depth of the well, is an important one. The enra-gy of the particle will be of the order kT and consequently well depths greater than this increase the probability of secondary minimum pair formation, i.e. association at a distance. This situation is one which can be reversed, however, by the application of an external force, for example, shaking a botde or applying a controlled shear flow in a rheometer. 

Shear controlled orientation in injection moulding (SCORIM) is based on the progressive application of macroscopic shears at the melt-solid interface during solidification in the moulding of a polymer matrix. 

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