Shear induced agglomeration

Chimmili, S., Shear induced agglomeration in particulate suspensions. M. Sci. thesis, West Virginia University (1996). 

Hollander, E. D., Shear induced agglomeration and mixing , Ph.D. Thesis, Delft University of Technology, Delft, Netherlands (2002). 

In the stirred tank, the final mean size of particles was reduced by the increase of stirring rate, being consistent with increased fluid shear induced particle disruption relative to aggregation. Use of three different gas velocities in the bubble column, however, results in no significant difference in agglomerate size but since the size is relatively small, it may simply reflect an asymptotic value. 

The object of this study was to clarify some aspects of the mechanism of shear-induced flocculation in colloidal dispersions. Vinyl chloride homopolymer and copolymer latices were prepared by emulsion polymerization using sodium dodecyl sulphate as emulsifier. Agglomeration behavior in these latices was studied by measuring the mechanical stability using a high speed stirring test. The latex particle size was measured by an analytical centrifuge. Molecular areas of emulsifier in the saturated adsorption layer at the surface of homopolymer and copolymer latex particles were estimated from adsorption titration data. 

Once particles are present in a volume of gas, they collide and agglomerate by different processes. The coagulation process leads to substantial changes in particle size distribution with time. Coagulation may be induced by any mechanism that involves a relative velocity between particles. Such processes include Brownian motion, shearing flow of fluid, turbulent motion, and differential particle motion associated with external force fields. The theory of particle collisions is quite complicated even if each of these mechanisms is isolated and treated separately. 

The expression c// d has the dimension 1, with [c] = [m] = [(m X M)/ N x m)] = [surface tension/ pressure]. This correctly reflects the fact that dispersion is the result of a stress (N/m ), a dispersion pressure , induced through the polymer by being applied against a surface tension (N/m) of the material to be dispersed, which works as a counter-pressure induced by interfacial forces between the particles and the matrix and is directed against the dispersion. The dispersion stress is transferred to the agglomerates to be dispersed via the shear force, applied on the polymer matrix (IF, = r( x K x i ). 

Particles that move in a laminar flow field with velocity gradient y experience shear and normal stresses which vary along/across the surface and induce particle rotation and deformation. The rotation of spheres is stable with an angular velocity of CO = 1/2 X 7 (Jeffery 1922 Trevelyan and Mason 1951), whereas aspherical particles or agglomerates rotate in a quasi-periodic or even chaotic manner (Blaser... 

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