Fluids, nature dilatant

As a simple depiction of a hpid bilayer, one can consider it as a film or a slab, which may be curved, compressed or dilated, and sheared. At physiological temperatures most natural hpid membranes are fluid. Therefore, within this slab, the hpid molecules are free to move. Below the hpid phase transition temperature, single-component membranes crystalhze. In this so called gel phase, the relative mohon of hpids and membrane inclusions is principally hindered. The fluidity of the membrane and resistance to shear in the plane of the film are characterized by the shear viscosity, % (or the diffusion coefficient of the hpids). Typical values... 

The resistance to flow is a strong function of vessel radius Q (x R. As blood vessels become smaller, the resistance to flow increases dramatically (Figure 6.4b). Therefore, an additional consequence of the branching pattern of blood vessels is that the majority of the overall resistance to blood flow resides in the smallest vessels (Figure 6.2b) the majority of the pressure drop ( 80%) occurs in arterioles and capillaries. This natural consequence of the physics of fluid flows is exploited in regulation of blood flow to organs of the body. Local blood flow to a tissue is controlled by constriction and dilation of... 

CT can be used to distinguish between paralytic (Fig. 5.19) and obstructive ileus or to localize the obstruction but should not be used routinely and care needs to be taken to minimize total radiation dose. Other advantages include the fact that no oral contrast material is needed as the retained intraluminal fluid serves as a natural (negative) contrast agent. It is a rapid, non-invasive, readily available technique with which also extraluminal pathology can be visualized. CT images of an obstructive ileus will identify dilated bowel loops proximally with normal caliber or collapsed loops distally. Sometimes the transition zone, which may resemble a beak, can be identified. This CT sign. 

In addition to the possible existence of interfacial-tension gradients at surfactant-adsorbed fluid interfaces, other interfacial rheological stresses of a viscous nature may arise, such as those relating to interfacial shear and dilatational viscosities (see [10,11]). 

Many technologies and natural phenomena involve processes of fast expansion or compression of fluid interfaces covered with surfactant adsorption layers. The dynamic system properties depend on the mechanisms and rate of equilibrium restoration after a deformation. At small magnitudes of deformation the mechanical relaxation of an interface can be described by the complex dilational viscoelastic modulus [1,2]. For sinusoidal deformations it is deflned as the ratio of complex amplitudes of interfacial tension variation and the relative surface area variation f (I ty) = dy /din A being a function of frequency. This modulus may include... 

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