Hydrodynamic shear effect

Publication gravure printing inks apparently show elastic behavior at D = 105 s the modulus of shearing G is 103 Pa. This indicates that at high shear rates D, between 105 and 106 s gravure printing inks develop normal stress effects and thus show a hydrodynamic lubricating effect. These deductions are relevant to an... 

Slip is not always a purely dissipative process, and some energy can be stored at the solid-liquid interface. In the case that storage and dissipation at the interface are independent processes, a two-parameter slip model can be used. This can occur for a surface oscillating in the shear direction. Such a situation involves bulk-mode acoustic wave devices operating in liquid, which is where our interest in hydrodynamic couphng effects stems from. This type of sensor, an example of which is the transverse-shear mode acoustic wave device, the oft-quoted quartz crystal microbalance (QCM), measures changes in acoustic properties, such as resonant frequency and dissipation, in response to perturbations at the surface-liquid interface of the device. 

The design and operation of a bioreactor are mainly determined by biological needs and engineering requirements, which often include a number of factors efficient oxygen transfer and mixing, low shear and hydrodynamic forces, effective control of physico-chemical environment, easy scale-up, and so on. Because some of these factors can be mutually contradictory, it is difficult to directly employ a conventional microbial reactor to shear-sensitive plant tissue cultures. 

The positive effect of velocity on the permeate flux is a result of enhanced hydrodynamic effects at the membrane surface, since high velocities lead to high shear and turbulent flow, which results in the formation of vortices and eddies that minimize the concentration polarization effects and the development of a fouling layer. The bigger the thickness of this layer, the higher its flow resistance and the smaller the permeate flux through the membrane becomes. Under turbulent flow conditions, shear effects induce hydrodynamic diffusion of the particles from the boundary layer back into the bulk, with a positive effect on the permeate flux. 

In practice, however there could be differences between the observed and estimated flux. The mass transfer coefficient is strongly dependent on diffusion coefficient and boundary layer thickness. Under turbulent flow conditions particle shear effects induce hydrodynamic diffusion of particles. Thus, for microfiltration, shear-induced difflisivity values correlate better with the observed filtration rates compared to Brownian difflisivity calculations.Further, concentration polarization effeets are more reliably predicted for MF than UF due to the fact diat macrosolutes diffusivities in gels are much lower than the Brownian difflisivity of micron-sized particles. As a result, the predicted flux for ultrafiltration is much lower than observed, whereas observed flux for microfilters may be eloser to the predicted value. 

As is well known, a lot of effects of surfactants, like damping of surface waves, the rate of thinning of liquid films, foaming and stabilisation of foams and emulsions, cannot just be described by a decrease in interfacial tension or by van der Waals and electrostatic interaction forces between two interfaces. The hydrodynamic shear stress at an interface covered by a surfactant adsorption layer is a typical example for the stimulation of an important surface effect. This effect, shown schematically in Fig. 3.9., is called the Marangoni effect. 

Davison P F (1959). The effect of hydrodynamic shear on the deoxyribonucleic acid from T2 and T4 bacteriophages, Proc. of the National Academy of Science. 45 1560-1568. 

In Agarwal s model, the supramolecular interactions just tie the component chains together and do not bear any load as they are lying perpendicular to the flow direction. The main reasons for the improved shear stability are the redistribution of the hydrodynamic force (because of topology) to the associated chains and the hydrodynamic shielding effect. This may help us to understand the mech-anochemistry of biomacromolecules such as double strand DNAs. 

The theory of hydrodynamics similarly describes an ideal liquid behavior making use of the viscosity (see Sect 5.6). The viscosity is the property of a fluid (liquid or gas) by which it resists a change in shape. The word viscous derives from the Latin viscum, the term for the birdlime, the sticky substance made from mistletoe and used to catch birds. One calls the viscosity Newtonian, if the stress is directly proportional to the rate of strain and independent of the strain itself. The proportionality constant is the viscosity, q, as indicated in the center of Fig. 4.157. The definitions and units are listed, and a sketch for the viscous shear-effect between a stationary, lower and an upper, mobile plate is also reproduced in the figure. Schematically, the Newtonian viscosity is represented by the dashpot drawn in the upper left comer, to contrast the Hookean elastic spring in the upper right. 

As mentioned above, the lyotropic lamellar phases of soap-alcohol-water mixtures [15] or of phospholipid-water dispersions may also be ordered homeotropically. Ordered layers up to 0.5 mm thickness may be produced. The orientation is effected simply by sucking the dispersions into flat glass cells with an inner diameter 0.5 X 10.0 mm. The torque generated by the hydrodynamic shear orients the membranes in such a way that their normals are directed perpendicular to the glass surface. 

Simply note that the general hydrodynamic problem of depleted layer flow including shear effects is a highly non-linear problem which must be solved using an iterative numerical scheme. Using a Carreau A curve based on experimental data for xanthan and an Auvray-type model of the tube concentration profile, calculations have been performed of rj ff as a function of 7n by Sorbie (1989, 1990), where is the Newtonian shear rate in a capillary and is given by... 

The models developed in previous sections are for fiber jrartides rising in a static fluid. They do not consider lateral motion and deformation of the fluid, as well as hydrodynamic diffusion effects. Eqns. (16) and (17) are appropaiate for rmderstanding rising behavior of fibers under static conditions. However, this study is to determine the hole cleaning efficiency of the fiber particles in real world situations such as fluid circulating up the annulus and drillstring rotation. The shearing motion of the fluid in the aimulus wfll affect the apparent viscosity that subsequently influences the behavior of fiber particles in the base fluid. To accurately model the behavior of fiber imder dynamic conditions, the overall shear rate must be computed from the piimaiy and secondary flow shear rates, and the annulus is modeled as a narrow slot to obtain analytical solutions (Fig. 13). 

Shear effects on mammalian cells due to hydrodynamic forces or bubble aeration in bioreactor systems are still one of the most important aspects in the design of these systems. A number of reviews have summarized the main... 

It should be mentioned that the above results are vahd if the hydrodynamic interactions do not affect particle transport through the adsorption layer of thickness 2a. This seems justified for smaller colloid particles and proteins. However, for micrometer-sized particles placed in shearing flows, the hydrodynamic forces play a significant role due to the coupling with the repulsive electrostatic interactions. This leads to enhanced blocking effects called hydrodynamic scattering effects and discussed extensively in recent review works [7,14]. These results have been interpreted theoretically in terms of the Brownian dynamics simulations [14], which are, however, considerably more time-consuming than the RSA simulations. 

Molecular dynamics, in contrast to MC simulations, is a typical model in which hydrodynamic effects are incorporated in the behavior of polymer solutions and may be properly accounted for. In the so-called nonequilibrium molecular dynamics method [54], Newton s equations of a (classical) many-particle problem are iteratively solved whereby quantities of both macroscopic and microscopic interest are expressed in terms of the configurational quantities such as the space coordinates or velocities of all particles. In addition, shear flow may be imposed by the homogeneous shear flow algorithm of Evans [56]. 

The larger macromolecules can be separated using larger particle size columns. However, the flow rate should be watched carefully. As the effective hydrodynamic size of the macromolecules may be reduced due to the deformation by shear (23). Figure 22.8 shows that the effective hydrodynamic size of a 12-15 X 10 MW polyacrylamide sample will not reach its maximum, or the size without shear, unless the flow rate is reduced to 0.01 ml/min. A... 

---