Shear induced collisions

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)... 

In the absence of flow, and in the initial stages of the reaction (p < 0.3) the molecular weight distribution closely follows the Flory distribution [6], though at later times there is a significant deviations with <7<( + p). Thus a narrower distribution is obtained relative to the Flory distribution [6], which is in agreement with experimental results. Application of a shear flow results in acceleration of the reaction (Figure 8), and this results because of orientation of the molecules by the flow rather than an increased number of shear induced collisions. There is a nearly linear increase in degree of polymerization with shear rate, at a fixed time, of reaction which is in concurrence with experiments [32]. 

He have presented a simple procedure whereby one can estimate the stability of a colloidal system undergoing simultaneous creaming and gravity-induced flocculation. This procedure is by no means restricted to only this case. One can easily take into account other particle loss mechanisms, such as shear-induced flocculation or Brownian flocculation. What is required in these cases are the appropriate particle/particle collision kernels, which can be computed by solving the governing convective-diffusion equation. 

For an MSMPR precipitator operating at high sunflower protein feed concentration, the mean particle size decreased as the mean residence time increased, see Figure 3, due to shear-induced breakage and particle-particle collisions. The population density showed a bi-modal PSD with a large CV at short mean residence... 

Thixotropy comes about first because of the finite time taken for any shear-induced change in microstructure to take place. Microstructure is brought to a new equilibrium by competition between, on the one hand the processes of tearing apart by stress diuing shearing, and on the other hand build-up due to flow- and Brownian-motion-induced collision, over a time that can be minutes. Then, when the flow ceases, the Brownian motion (the only force left) is able to slowly move the elements of the microstructure around to more favourable positions and thus rebuild the structure this can take many hours to complete. The whole process is completely reversible. 

Summarizing the theoretical approaches and experimental studies, we may state that the exact nature of the shear-induced state and the complicated mechanisms involved are not yet completely understood. One important problem, which is still unsolved, concerns the role of electrostatic interactions. Many experiments show that SIS mainly occtus in the regime of very low ionic strengths, where repulsive forces are pre-dominant. Addition of salt shordd lead to enhanced collision frequency between the micellar particles, and this phenomenon should favor SIS. This assmnption is, however, in contrast to experimental observations, where shear-induced structures usually disappear upon addition of salt. Besides the large number of new results, some phenomena of SIS still remain mysterious. It is interesting... 

Since we are considering shear-induced adsorption, the adsorption rate constant can be approximated from Smoluchowski s theory [14] for the collision between two... 

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. 

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