Membrane structure shear rate

Mechanical forces can disturb the elaborate structure of the enzyme molecules to such a degree that de-activation can occur. The forces associated with flowing fluids, liquid films and interfaces can all cause de-activation. The rate of denaturation is a function both of intensity and of exposure time to the flow regime. Some enzymes show an ability to recover from such treatment. It should be noted that other enzymes are sensitive to shear stress and not to shear rate. This characteristic mechanical fragility of enzymes may impose limits on the fluid forces which can be tolerated in enzyme reactors. This applies when stirring is used to increase mass transfer rates of substrate, or in membrane filtration systems where increasing flux through a membrane can be accompanied by increased fluid shear at the surface of the membrane and within membrane pores. Another mechanical force, surface... 

Ismail, A. F., Ng, B. C., and Abdul Rahman, W. A. W. (2003), Effects of shear rate and forced convection residence time on asymmetric polysulfone membranes structure and gas separation performance, Sep. Purif. Technol, 33,255-272. 

With the increase of the concentration of PPESK in the casting solution, the viscosity strongly increases and becomes shear-rate dependent. Then the morphology of the hollow fiber membranes changes from a finger-like structure to sponge-like structure. 

The influence of shear rate on membrane structure is illustrated in Figures 31.4 (Ren et al., 2002) and 31.5 (Chung et al., 2002). Figure 31.4 exhibits the cross-section morphology of hollow-fiber membranes spun at shear rates of 812 and 2436 s. Some macrovoids can be observed near the inner skin of fibers spun at a low shear rate (812 s ), while these macrovoids are apparently eliminated or suppressed when the shear rate increases to 2436 s Clearly, high shear rates modify the precipitation path and retard the formation of macrovoids. In addition, with an increase in shear rate, the membrane structure becomes... 

In addition to high filtration rates, asymmetric membranes are most fouling resistant. Conventional symmetric structures act as depth filters and retain particles within their internal structure. These trapped particles plug the membrane and the flux declines during use. Asymmetric membranes are surface filters and retain all rejected materials at the surface where they can be removed by shear forces applied by the feed solution moving parallel to the membrane surface. The difference in the filtration behavior between a symmetric and an asymmetric membrane is shown schematically in Figure 1.10. Two techniques are used to prepare asymmetric membranes one utilizes the phase inversion process and the other leads to a composite structure by depositing an extremely thin polymer film on a microporous substructure. 

Shear stress also causes a transient structured disruption of the cell membrane, and increases membrane permeability to Ca++ and other ions (Serbest et al., 2002). It can also increase lipid peroxidation. Shear stresses in the range of 8-14 dynes/cm begin to damage cells in human vasculature. The percentages of neuronal cells killed in vitro varies linearly with the shear stress (Figure 6.13.2) and also linearly with the rate of shear stress application (Serbest et al., 2002). 

In addition to high rates of filtration, asymmetric membranes are more resistant to fouling. Membranes act as conventional symmetrical filters and traps particles deep within its internal structure. These particles trapped blocked the pores of the membrane, and thus, the trausmembraue flow decreases during the process. Asyuunetric manbranes behave as filters for surface and retain all rejected particles on the surface, where they can be removed by shearing forces applied by the feed solution during its passage parallel to the membrane surface, the tangeutial filtration mode [2],... 

Despite the complexity of internal cell structure, which comprises the containing membrane, gelatinous interior cytoplasm, internal granular bodies, fibrous skeleton, and nuclei, each cell may be treated approximately as a spherical viscoelastic shell containing a viscous fluid. The model of an outer shell with an inner fluid describes very well the deformation of red blood cells and also of artificial vesicles made by sonicating phospholipids in water. The shell dictates the equilibrium while the fluid contents dictate the rate of approach to equilibrium, A red cell needs three numbers to describe its response time of 0.1 s an area compressibility k— 10 mNm , a shear modulus 10 Nm"", and a viscosity 10 Pa s. ... 

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