Velocity gradient agglomeration

If colloids are sufficiently large or the fluid shear rate high, the relative motion from velocity gradients exceeds that caused by Brownian (thermal) effects (orthokinetic agglomeration). 

TJhe aggregation of particles in a colloidal dispersion proceeds in two distinct reaction steps. Particle transport leads to collisions between suspended colloids, and particle destabilization causes permanent contact between particles upon collision. Consequently, the rate of agglomeration is the product of the collision frequency as determined by conditions of the transport and the collision efficiency factor, the fraction of collisions leading to permanent contact, which is determined by conditions of the destabilization step (2). Particle transport occurs either by Brownian motion (perikinetic) or because of velocity gradients in the suspending medium (orthokinetic). Transport is characterized by physical parame-... 

From a comparison of the two collision frequency terms, described in detail in the Equations 3 and 6, one obtains the relative contributions of the perikinetic and orthokinetic transport to the total particle agglomeration. The ratio is a function of the radius of the colloid, r, and the absolute value of the velocity gradient du/dz ... 

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... 

The agglomeration kernel j3 depends on the mechanism of a lomeration and on the particle size (Figure 4.5). For small particles, the Brownian motion due to fluctuations in the solution suffices for an effective a omeration this is called orthokinetic a lomeration. For larger particles, a velocity gradient in the mother phase is necessary for particles to collide and thus to agglomerate, this is the perildnetic agglomeration. 

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