Concentration in channel

Joining (Fig. 4)—The cap and body portions are aligned concentrically in channels, and the two portions are slowly pushed together. 

Channel Flow for a Unit Cell 9.5.1. Gas Concentrations in Channels... 

Diffusion and Mass Transfer During Leaching. Rates of extraction from individual particles are difficult to assess because it is impossible to define the shapes of the pores or channels through which mass transfer (qv) has to take place. However, the nature of the diffusional process in a porous soHd could be illustrated by considering the diffusion of solute through a pore. This is described mathematically by the diffusion equation, the solutions of which indicate that the concentration in the pore would be expected to decrease according to an exponential decay function. 

When a dilute solution of a polymer (c << c ) is equilibrated with a porous medium, some polymer chains are partitioned to the pore channels. The partition coefficient K, defined as the ratio of the polymer concentration in the pore to the one in the exterior solution, decreases with increasing MW of the polymer (7). This size exclusion principle has been used successfully in SEC to characterize the MW distribution of polymer samples (8). 

Enterochromaffin cells are interspersed with mucosal cells mainly in the stomach and small intestine. In the blood, serotonin is present at high concentrations in platelets, which take up serotonin from the plasma by an active transport process. Serotonin is released on platelet activation. In the central nervous system, serotonin serves as a transmitter. The main serotonin-containing neurons are those clustered in form of the Raphe nuclei. Serotonin exerts its biological effects through the activation of specific receptors. Most of them are G-protein coupled receptors (GPCRs) and belong to the 5-HTr, 5-HT2-, 5-HT4-, 5-HTs-, 5-HT6-, 5-HT7-receptor subfamilies. The 5-HT3-receptor is a ligand-operated ion channel. 

Tetrodotoxin (TTX) is a toxin derived from bacteria which is concentrated in the gonads and liver of certain pufferfishes (fugu). Similar to saxitoxin, tetrodotoxin is a very potent blocker of most voltage-sensitive Na+ channels. 

Willis GH, McDowell LL, SouthwickLM, et al. 1987. Methoxychlor and endosulfan concentrations in unit-source runoff and in channel flow of a complex watershed. Transactions of the American Society of Agricultural Engineers 30 394-399. 

One notes in Table 1.2 a uniform increase in the adsorption energies of the alkanes when the microspore size decreases (compare 12-ring-channel zeohte MOR with 10-ring-channel TON). However, at the temperature of hydroisomerization the equilibrium constant for adsorption is less in the narrow-pore zeohte than in the wide-pore system. This difference is due to the more limited mobility of the hydrocarbon in the narrow-pore material. This can be used to compute Eq. (1.22b) with the result that the overall hydroisomerization rate in the narrow-pore material is lower than that in the wide-pore material. This entropy-difference-dominated effect is reflected in a substantially decreased hydrocarbon concentration in the narrow-pore material. 

In a different context, a micropipette has been applied to monitor the current through a single-ion channel in a biological membrane. The patch-clamp technique invented by Sackmann and Neher [119] led to their Nobel Prize in medicine. The variations in channel current with voltage, concentration, type of ions, and type of channels have been explored. While the functions of specific channels, in particular their ionic selectivity, have been well known, only a handful of channels have the internal geometry and charge distribution determined. The development of a theory to interpret the mass of channel data and to predict channel action is still lacking. 

The membranes of nerve cells contain well-studied ion channels that are responsible for the action potentials generated across the membrane. The activity of some of these channels is controlled by neurotransmitters hence, channel activity can be regulated. One ion can regulate the activity of the channel of another ion. For example, a decrease of Ca + concentration in the extracellular fluid increases membrane permeability and increases the diffusion of Na+. This depolarizes the membrane and triggers nerve discharge, which may explain the numbness, tinghng, and muscle cramps symptomatic of a low level of plasma Ca. ... 

Similar convection-diffusion equations to the Navier-Stokes equation can be formulated for enthalpy or species concentration. In all of these formulations there is always a superposition of diffusive and convective transport of a field quantity, supplemented by source terms describing creation or destruction of the transported quantity. There are two fundamental assumptions on which the Navier-Stokes and other convection-diffusion equations are based. The first and most fundamental is the continuum hypothesis it is assumed that the fluid can be described by a scalar or vector field, such as density or velocity. In fact, the field quantities have to be regarded as local averages over a large number of particles contained in a volume element embracing the point of interest. The second hypothesis relates to the local statistical distribution of the particles in phase space the standard convection-diffusion equations rely on the assumption of local thermal equilibrium. For gas flow, this means that a Maxwell-Boltzmann distribution is assumed for the velocity of the particles in the frame-of-reference co-moving with the fluid. Especially the second assumption may break dovm when gas flow at high temperature or low pressure in micro channels is considered, as will be discussed below. 

Tybsre m is the total mass of analyte collected, D the molecular diffusion coefficient, A the area of the diffusion channel, L the diffusion path length, C the analyte concentration in the air, and Tt the sampling time. In deriving equation (8.7) it was assumed t. that the sorbent is effective sink for the analyte and,... 

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