Constrictive effects

Michaels [241], using a model pore shown in Figure 19, considered the constriction effect by solving the steady-state species continuity equations in a model pore consisting of a single constriction. The result for the pore shown in Figure 19 is given by... 

Boyack and Giddings [45] considered the tortuosity effects to be separable from the constriction effects. In their derivation they assumed that the electric field in the constricted channel decreased proportionally to the decrease in cross-sectional area and changes in path length. The field was assumed not to penetrate the barrier. The effect of constriction can be written in terms of porosity and L as... 

The a receptors are generally excitatory, as shown in table 4.2, and mediate a constricting effect on vascular, uterine, and intestinal muscle when stimulated by an agonist. They respond to different adrenergic agonists in the following order epinephrine >... 

The fact that in the case of the bicrystal in Figure 54 the effective thickness calculated from the low-frequency semicircle is very much greater than expected from the width of the boundary, while the activation energy is almost equal to that of the bulk, points towards a frequently overlooked complication, namely to current-constriction effects. Such constriction effects occur287 when the crystal grains are not ideally sintered together, if pores or second phases are included, and interrupt the lateral conductivity of boundaries, as is the case for inhomogeneous electrode contacts. 

These compounds are the first examples for the stabilization of the LS state of iron(II) with saturated nitrogen as the most abundant donors this has been ascribed by the authors to the enhanced ligand field strength due to the constrictive effect of the mechanically confining in-plane macrocyclic ligand . Six-coordinate iron(II) complexes are formed with relatively weak axial ligands such as X = CH3C002 >... 

Minimum downcomer width and minimum downcomer area. As the downcomer becomes smaller, its width decreases faster than its length, turning the downcomer into a long and narrow slot. This geometry increases the resistance to liquid downflow and to upflow of disengaging vapor. Small downcomers are also extremely sensitive to foaming, fouling, construction tolerances, and introduction of debris. Smaller weirs associated with small downcomers distort the liquid flow pattern as it approaches the weir ("weir constriction effect ), which in-... 

These compounds seem to produce some of the paradoxical reactions often observed with amphetamine and thus should be handled with caution. A short pamphlet from Stash, a psychedelic information service, warns against use of MDA by "Persons with heart disease, severe high blood pressure, hyperthyroidism or diabetes mellitus. MDA was described by Alles as being a third as potent as amphetamine in vaso-constrictive effects. The... 

As in the case of the conductivity experiments, current-constriction effects can occur in the diffusion experiments, if lateral inhomogeneities are present. In this way resistivities occur that can be easily misinterpreted in terms of sluggish surface steps. A concise treatment of proper surface kinetics will be given now. 

The area of the mesophyll cell walls across which CO2 can diffuse is considerably larger than the surface area of the leaf (Figs. 1-2 and 8-4). For the constricting effect caused by the stomata, we used Ast/A, the fraction of the leaf surface area that is occupied by stomatal pores. Here we will use the ratio Am s/A to indicate the increase in area available for CO2 diffusion into cells within a leaf compared to the leaf surface area, where Ames is the total area of the cell walls of mesophyll cells that is exposed to the intercellular air spaces, and A is the area of one side of the same leaf. More conveniently, A mesM can refer to the internal and the external areas of a part of the leaf that is examined microscopically. 

Snap Off at Strong Constrictions Effect of Pore Geometry... 

A somewhat different interpretation of such phenomena has been given recently by Falls et al. (14). These investigators use energy arguments to account for the work associated with moving lamellae into the constrictions along the bubble train path. Such an approach is attractive from a mechanistic standpoint however, it predicts that the capillary resistance increases with the length of the porous medium, which has not been observed experimentally. Obviously, more extensive analysis and experimentation are required before such pore constriction effects are fully understood and accurately represented. 

In Fig. 15, it is seen that the sheet made of cotton linters is more conductive than the other sheets. The sheet made with "Seagull W" hardwood pulp is the least conductive while the three other pulps have intermediate conductivity values. This behaviour seems to be related to the size of the different types of fibers used to make the sheets. As mentioned previously, hardwood fibers are shorter than softwood fibers, and consequently, there are more interfiber contacts in the sheets made of hardwood fiber. The surface area of a contact is very important to the conductivity. There may be a constrictive effect due to smaller surface at these contacts which results in a restricted flow of ions through the paper, and therefore a lower conductivity. Cotton linters are chemically different and longer than wood fibers and the sheet made of these fibers is more conductive. The difference between the three softwood pulps is probably related to a different chemical composition of its fibers. 

Since diffusion of the electrons and ions tends to spread the discharge radially, one expects that constriction effects will set in at higher pressures for gases which have larger ambipolar diffusion coefficients as is observed (39, 40) for helium relative to the other rare gases. 

Fig. 2.3. Schematic representation of (a) tortuosity, (b) retardation and (c) constrictive effects [10] during migration of an ion in the stabilized medium.

Amongst the methylated xanthines (Table 3.2), caffeine is included in some analgesic preparations because of its CNS-stimulating and cerebrovascular-constricting effects. Many people consume coffee or soft drinks, which contain caffeine, to maintain alertness and to stay awake. 

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