Hydrodynamic shear

Moser WR, Marshik-Geurts BJ, Kingsley J, Lemberger M, Willette R, Chan A, Sunstrom JE, Boye AJ (1995) The synthesis and characterization of solid state materials produced by high shear hydrodynamic cavitation. J Mater Res 10 2322-2335... 

Substantial interest has been raised in the problem of the structure and dynamics of suspensions in shear hydrodynamic fields. ° ° The experiments showed that both shear-induced melting and shear-induced ordering can be observed at different particle volume fractions and shear rates. The nonequilibrium microstructure of the suspension under shear can be investigated in these experiments and compared with the predictions from analytical theories and computer simulations. 

Dispersion of a soHd or Hquid in a Hquid affects the viscosity. In many cases Newtonian flow behavior is transformed into non-Newtonian flow behavior. Shear thinning results from the abiHty of the soHd particles or Hquid droplets to come together to form network stmctures when at rest or under low shear. With increasing shear the interlinked stmcture gradually breaks down, and the resistance to flow decreases. The viscosity of a dispersed system depends on hydrodynamic interactions between particles or droplets and the Hquid, particle—particle interactions (bumping), and interparticle attractions that promote the formation of aggregates, floes, and networks. 

In an ideal fluid, the stresses are isotropic. There is no strength, so there are no shear stresses the normal stress and lateral stresses are equal and are identical to the pressure. On the other hand, a solid with strength can support shear stresses. However, when the applied stress greatly exceeds the yield stress of a solid, its behavior can be approximated by that of a fluid because the fractional deviations from stress isotropy are small. Under these conditions, the solid is considered to be hydrodynamic. In the absence of rate-dependent behavior such as viscous relaxation or heat conduction, the equation of state of an isotropic fluid or hydrodynamic solid can be expressed in terms of specific internal energy as a function of pressure and specific volume E(P, V). A familiar equation of state is that for an ideal gas... 

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... 

The fact that the appearance of a wall slip at sufficiently high shear rates is a property inwardly inherent in filled polymers or an external manifestation of these properties may be discussed, but obviously, the role of this effect during the flow of compositions with a disperse filler is great. The wall slip, beginning in the region of high shear rates, was marked many times as the effect that must be taken into account in the analysis of rheological properties of filled polymer melts [24, 25], and the appearance of a slip is initiated in the entry (transitional) zone of the channel [26]. It is quite possible that in reality not a true wall slip takes place, but the formation of a low-viscosity wall layer depleted of a filler. This is most characteristic for the systems with low-viscosity binders. From the point of view of hydrodynamics, an exact mechanism of motion of a material near the wall is immaterial, since in any case it appears as a wall slip. 

The dependence of Vs on rheological parameters-shear stress on the wall and /notion coefficient — as far as the author knows, for filled polymers was not investigated somewhat completely, though its determination is necessary for a specific solution of hydrodynamic problems related to the flow of filled polymers. 

Several attempts to relate the rate for bond scission (kc) with the molecular stress ( jr) have been reported over the years, most of them could be formally traced back to de Boer s model of a stressed bond [92] and Eyring s formulation of the transition state theory [94]. Yew and Davidson [99], in their shearing experiment with DNA, considered the hydrodynamic drag contribution to the tensile force exerted on the bond when the reactant molecule enters the activated state. If this force is exerted along the reaction coordinate over a distance 81, the activation energy for bond dissociation would be reduced by the amount ... 

Chain degradation in turbulent flow has been frequently reported in conjunction with drag reduction and in simple shear flow at high Reynolds numbers [187], Using poly(decyl methacrylate) under conditions of turbulent flow in a capillary tube, Muller and Klein observed that the hydrodynamic volume, [r ] M, is the determining factor for the degradation rate in various solvents and at various polymer concentrations [188], The initial MWD of the polymers used in their experiments are, however, too broad (Mw/Iiln = 5 ) to allow for a precise... 

For hydrodynamically smooth pipes, through which fluid is flowing under turbulent conditions, the shear stress is given approximately by the Blasius equation ... 

The electroviscous effect present with solid particles suspended in ionic liquids, to increase the viscosity over that of the bulk liquid. The primary effect caused by the shear field distorting the electrical double layer surrounding the solid particles in suspension. The secondary effect results from the overlap of the electrical double layers of neighboring particles. The tertiary effect arises from changes in size and shape of the particles caused by the shear field. The primary electroviscous effect has been the subject of much study and has been shown to depend on (a) the size of the Debye length of the electrical double layer compared to the size of the suspended particle (b) the potential at the slipping plane between the particle and the bulk fluid (c) the Peclet number, i.e., diffusive to hydrodynamic forces (d) the Hartmarm number, i.e. electrical to hydrodynamic forces and (e) variations in the Stern layer around the particle (Garcia-Salinas et al. 2000). 

At the thin film limit, the hydrodynamic pressure will approach a distribution that is consistent with the pressure between the two solid surfaces in dry static contact, while the shear stress experienced by the fluid film will reach a limiting value that is equal to the shear strength of a boundary film. 

The present author has performed computer simulations to examine the transition of pressure distributions and shear response from a hydrodynamic to boundary lubrication. Figure 4(a) shows an example of a smooth elastic sphere in contact with a rigid plane, the EHL pressure calculated at a very low rolling speed coincides perfectly with the... 

Fig. 4—Illustration of the transition from hydrodynamic to boundary lubrication (a) a comparison of pressure of thin EHL film with Hertzian distribution (b) a schematic stress-velocity map showing the dependence of shear stress of lubricating films on sliding velocity.

Unlike traditional textbooks of tribology, in this book we regard boundary lubrication as a limit state of hydrodynamic lubrication when film thickness is down to molecular dimension and independent of the velocity of relative motion. The discussions are based on the existing results, some from literatures but mostly from the authors own work. The topics are mainly focused on the mechanical properties of boundary films, including rheology transitions, molecular ordering, and shear responses. Ordered molecule films, such as L-B films and SAM, are discussed, with emphasis on the frictional performance, energy dissipation and the effects from structural features. Boundary films can be modeled either as a confined substance, or an adsorbed/reacted layer on the... 

Sundararajan et al. [131] in 1999 calculated the slurry film thickness and hydrodynamic pressure in CMP by solving the Re5molds equation. The abrasive particles undergo rotational and linear motion in the shear flow. This motion of the abrasive particles enhances the dissolution rate of the surface by facilitating the liquid phase convective mass transfer of the dissolved copper species away from the wafer surface. It is proposed that the enhancement in the polish rate is directly proportional to the product of abrasive concentration and the shear stress on the wafer surface. Hence, the ratio of the polish rate with abrasive to the polish rate without abrasive can be written as... 

Keywords. Plant cell suspensions. Hydrodynamic shear. Energy dissipation. Aeration, Oxidative burst... 

The purpose of this chapter is to review relevant work in the area, including an evaluation of methods employed for shear sensitivity studies, comparison of bases for data analysis and an outline of current knowledge of the interface between the hydrodynamic environment and the plant cells themselves. Coverage is limited to cell suspension cultures and does not extend to other tissue or organ systems. 

The perceived sensitivity of plant cells to the hydrodynamic stress associated with aeration and agitation conditions is typically attributed to the physical characteristics of the suspended cells, namely their size, the presence of a cell wall, the existence of a large vacuole, and their tendency to aggregate. Table 1 illustrates some of the differences between plant cells and other biological systems. Chalmers [19] attributed shear sensitivity in mammalian cultures at least in part to the fact that these cells occur naturally as part of a tissue, surrounded by other cells. The same is true for plant cells. The more robust microbial systems, on the other hand, exist in nature as single organisms or mycelial structures, very close to the forms they assume in submerged culture. 

Hydrodynamic effects on suspended particles in an STR may be broadly categorized as time-averaged, time-dependent and collision-related. Time-averaged shear rates are most commonly considered. Maximum shear rates, and accordingly maximum stresses, are assumed to occur in the impeller region. Time-dependent effects, on the other hand, are attributable to turbulent velocity fluctuations. The relevant turbulent Reynolds stresses are frequently evaluated in terms of the characteristic size and velocity of the turbulent eddies and are generally found to predominate over viscous effects. 

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