Hydrodynamic mechanical stress

A cell subjected to a stress of any kind can potentially exhibit a wide range of responses. Severe stress may lead to cell death and, ultimately, to cell lysis imposition of less severe conditions may result in a metabolically perturbed system, which may either revert to its initial state or adapt in some way to the imposed conditions. Figure 10 shows a hypothetical scheme, presented by Prokop and Bajpai [12], for the signal-response cascade associated with hydrodynamic shear stress. The signal reception/transduction mechanisms are, as yet, poorly understood. While Fig. 10 can be applied to any biological system, Namdev and... 

In the case of Brownian particles detachment by the hydrodynamic mechanism is not possible due to the much higher values of critical shear stress required for release [120]. For colloidally induced release, in the absence of an energy barrier, the rate of particle detachment is based on the assumption that the rate-limiting step is the diffusion of detached colloids across... 

High-intensity ultrasonication can be considered as a mechanical method for producing cellulose nanofibers with hydrodynamic forces [72]. In this process, ultrasonic waves create strong mechanical stress because of cavitations, and therefore, cause the disaggregation of cellulosic fiber to nanofibers [73]. Several attempts have been made to isolate cellulose nanofiber by ultrasonication from various cellulose sources such as microcrystalline cellulose, regenerated and pure cellulose fibers [72], kraft pulp [74], flax, wood, wheat straw and bamboo [75] (Figure 11.5), para rubberwood sawdust [76], and poplar wood powders [15]. 

Laminar flow devices are commonly used to characterize cell adhesion, and include rotating disc, parallel-plate (and variants), and radial-flow devices. These devices all employ tangential fluid flow to exert hydrodynamic drag and torque on adherent cells [98]. Among the different devices, the parallel-plate flow chamber (Figure 34.5a) is described most extensively, and has been used primarily to characterize cell and tissue phenomena under a well-defined hydrodynamic shear stress, r. Here, r can be predicted from fluid mechanics. 

It is also noted that unfolded T4DNA in the rod-like micelle solution with [CTAB] = 0.137 M can be stretched out under a hydrodynamic flow, which is induced by a mechanical stress over the sample (Fig. 6). In this case the... 

Walcheck, B., K. L. Moore, R. P. McEver, and T. K. Kishimoto. 1996. Neutrophil-neutrophil interactions under hydrodynamic shear stress involve L-selectin and PSGL-1 a mechanism that amplifies initial leukocyte accumulation on P-selectin in vitro. J. Clin. Invest. 98 1081-1087. 

In this chapter the regimes of mechanical response nonlinear elastic compression stress tensors the Hugoniot elastic limit elastic-plastic deformation hydrodynamic flow phase transformation release waves other mechanical aspects of shock propagation first-order and second-order behaviors. 

Eredictions. A rotating cyhnder within a cyhnder electrode test system as been developed that operates under a defined hydrodynamics relationship (Figs. 25-15 and 25-16). The assumption is that if the rotating electrode operates at a shear stress comparable to that in plant geometry, the mechanism in the plant geometty may be modeled in the laboratory. Once the mechanism is defined, the appropriate relationship between fluid flow rate and corrosion rate in the plant equipment as defined by the mechanism can be used to predict the expected corrosion... 

When considering boundary conditions, a useful dimensionless hydrodynamic number is the Knudsen number, Kn = X/L, the ratio of the mean free path length to the characteristic dimension of the flow. In the case of a small Knudsen number, continuum mechanics will apply, and the no-slip boundary condition assumption is valid. In this formulation of classical fluid dynamics, the fluid velocity vanishes at the wall, so fluid particles directly adjacent to the wall are stationary, with respect to the wall. This also ensures that there is a continuity of stress across the boundary (i.e., the stress at the lower surface—the wall—is equal to the stress in the surface-adjacent liquid). Although this is an approximation, it is valid in many cases, and greatly simplifies the solution of the equations of motion. Additionally, it eliminates the need to include an extra parameter, which must be determined on a theoretical or experimental basis. 

The techniques that have been used to characterise the mechanical properties of microparticles may be classified as indirect and direct. The former includes measurement of breakage in a "shear" device, for example, a stirred vessel (Poncelet and Neufeld, 1989) or bubble column (Lu et ah, 1992). However, the results from these indirect techniques are rather difficult to use since the mechanical breakage depends not only on the mechanical properties but also the hydrodynamics of the processing equipment, and the latter are still not well understood. To overcome this problem, a cone and plate viscometer that can apply well-defined shear stresses has been used to study breakage of hybridomas (Born et ah, 1992), but this is not a widely applied or applicable technique because the forces are too small to break most cells. 

So far the micro-mechanical origin of the Mullins effect is not totally understood [26, 36, 61]. Beside the action of the entropy elastic polymer network that is quite well understood on a molecular-statistical basis [24, 62], the impact of filler particles on stress-strain properties is of high importance. On the one hand the addition of hard filler particles leads to a stiffening of the rubber matrix that can be described by a hydrodynamic strain amplification factor [22, 63-65]. On the other, the constraints introduced into the system by filler-polymer bonds result in a decreased network entropy. Accordingly, the free energy that equals the negative entropy times the temperature increases linear with the effective number of network junctions [64-67]. A further effect is obtained from the formation of filler clusters or a... 

The above interpretations of the Mullins effect of stress softening ignore the important results of Haarwood et al. [73, 74], who showed that a plot of stress in second extension vs ratio between strain and pre-strain of natural rubber filled with a variety of carbon blacks yields a single master curve [60, 73]. This demonstrates that stress softening is related to hydrodynamic strain amplification due to the presence of the filler. Based on this observation a micro-mechanical model of stress softening has been developed by referring to hydrodynamic reinforcement of the rubber matrix by rigid filler... 

In view of an illustration of the viscoelastic characteristics of the developed model, simulations of uniaxial stress-strain cycles in the small strain regime have been performed for various pre-strains, as depicted in Fig. 47b. Thereby, the material parameters obtained from the adaptation in Fig. 47a (Table 4, sample type C60) have been used. The dashed lines represent the polymer contributions, which include the pre-strain dependent hydrodynamic amplification of the polymer matrix. It becomes clear that in the small and medium strain regime a pronounced filler-induced hysteresis is predicted, due to the cyclic breakdown and re-aggregation of filler clusters. It can considered to be the main mechanism of energy dissipation of filler reinforced rubbers that appears even in the quasi-static limit. In addition, stress softening is present, also at small strains. It leads to the characteristic decline of the polymer contributions with rising pre-strain (dashed lines in... 

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