Aggregation shear rate

The dynamics of reactor flow is also important for its effect on the crystal agglomeration, since the intensity of turbulent shear dominates the orthoki-netic mechanism for both processes of aggregation and disruption. The mean shear rate is estimated as (see Harnby etai, 1992)... 

Creation of the correct shear conditions. High shear rate may be harmful to the organism and disrupt the cell wall low shear may also be undesirable because of unwanted flocculation and aggregation of the cells, or even growth of bacteria on the reactor wall and stirrer. 

The native sample E (Fig. 3) contains a relatively large amount of aggregated structures. Most of them appear to rearrange into perfectly matched double-stranded chains after incubation at 80 °C for 2 months (27). This rearrangement is reported to result in a fivefold increase in the apparent viscosity at a shear rate of 1 s-1 (27). A similar rearrangement of xanthan assemblies is also observed in a unpasteurized fermentation broth after exposure of the sample... 

An increase in shear rate and particle size will lead to decreased aggregation. Naturally both can be changed only in a limited range since excessive shear leads to degradation, while large particles easily separate from the matrix under the effect of external load as was pointed out in Sect. 2.1. Equation 7 also explains... 

Figure 1. Effects of the concentration of dextrans with various molecular weights on three indices of BBC aggregation (16). MAI indicates the average number of RBCs in each aggregation unit counted under the microscope. ESR is the maximum rate of sedimentation of erythrocytes in a calibrated tube, with corrections made for changes in viscosity and density of the suspending medium following the addition of dextrans. The relative viscosity (t)r) is the ratio of the viscosity of RBC suspension to that of the suspending medium at a shear rate fo 0.1 sec 1. The RBC concentration of the suspension was 1% for MAI and 45% for ESR and r)r measurements. The vertical bars represent SEM. (A), Dx 40 (O), Dx 80 (M), Dx 150 (A), Dx 500 ( ), Dx 2000.

If particle aggregation occurs in a colloidal system, then an increase in the shear rate will tend to break down the aggregates, which will result, among other things, in a reduction of the amount of solvent immobilised by the particles, thus lowering the apparent viscosity of the system. 

The apparent viscosity of a system which thins on shearing is most susceptible to changes in the shear rate in the intermediate range where there is a balance between randomness and alignment, and between aggregation and dispersion. 

For a given, low applied shear rate, perikinetic collision rates tend to be higher than orthokinetic rates when the dispersed species are quite small, of the order of 100 nm or less. The orthokinetic collision rates tend to dominate for larger-sized dispersed species, of the order of several pm or more. For more information on aggregation kinetics see Refs. [27,292,318,319]. 

Blood is obtained from the test subjects by venipuncture and mixed with K-EDTA (1 mg/ml) or heparin (5 IU/ml heparin sodium) to prevent clotting. Erythrocyte aggregation is determined in whole blood of 40 % haematocrit. A sample of 40 xl blood is transferred to the measuring device. The red cells are dispersed at a shear rate of 600/s. After 20 s, flow is switched to stasis and the extent of erythrocyte aggregation is determined photometrically. 

Determine the particle size above which shear aggregation is dominant for a room temperature aqueous suspension of particles when the shear rate is varied from 1 to 100 sec . ... 

With 0.05 jLim latex particles in a glycerol-salt solution, large-scale shear aggregates have been shown to have the same fractal dimension as that of Brownian aggregates (i.e.. Dp = 1.8 0.1) when the shear rate is less than 1500/sec [88]. Due to shear allegation, the aggregation rate for these experiments was much faster than that of comparative Brownian aggregation rates. 

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