Particle thermal gradient fields

Motion in Thermal Gradient Fields. It has long been observed that particles in a medium move from hotter to colder regions. In a thermal gradient field and at atmospheric pressure, the thermal force acting on a particle is given by (7, 8)... 

A colloidal particle or large molecule can be driven to move in a gas or a liquid by the application of a nonuniform temperature field. The mass flow rate J , of a species or of colloidal particles as the result of a thermal gradient VT in a fluid is given in, e.g., [1],... 

Field flow fractionation (FFF) can also be used for microbial cell separation. In the FFF technique, a field (may be gravitational, centrifugal, thermal-gradient, electrical, magnetic, etc.) is applied perpendicular to the fluid flow, causing particles to migrate with different velocities. Fields of sedimentation, diffusion, and electrical diffusion are manipulated to optimize the separations of microbes. Separation of Pseudomonas putida and E. coli has been achieved by hyperlayer FFF. Fractions of the whole cells were collected after the separation at different time intervals, dif-... 

The external electric field is in the direction of the pore axis. The particle is driven to move by the imposed electric field, the electroosmotic flow, and the Brownian force due to thermal fluctuation of the solvent molecules. Unlike the usual electroosmotic flow in an open slit, the fluid velocity profile is no longer uniform because a pressure gradient is built up due to the presence of the closed end. The probability of the particle position is obtained by solving the Fokker-Planck equation. The penetration depth is found to be dependent upon the Peclet number, which is a measure of significance of the convective electroosmotic flow relative to the Brownian diffusion, and the Damkohler number, which is a ratio of the characteristic diffusion-to-deposition times. 

In thermal field flow fractionation (TFFF), a temperature gradient is applied. The primary potential advantage of this technique is that it can be used to size particles in the range 0.01 pm to 0.001 pm, an order of magnitude smaller than SFFF. Fffractionation market a TFFF polymer fractionator channel module with 286/16 MHz IBM compatible PC, super VGA color monitor workstation to include data acquisition software, hardware and data analysis software. A linear UV detector and single channel high performance pump are optional. 

The force fields of most interest i n particle transport are gravitational, electrical, and thermal. with the last field produced by temperature gradients in the gas. If a balance exists locally in the gas between the force field and the drag on the particle, the two can be equated to give... 

The smaller aerosol particles can be captured from the air for subsequent counting and size measurement by means of so-called thermal precipitators. In these instruments, metal wires are heated to produce a temperature gradient. Aerosol particles move away from the wire in the direction of a cold surface, since the impact of more energetic gas molecules from the heated side gives them a net motion in that direction. The particles captured are studied with an electron microscope. Another possible way to measure Aitken particles is by charging them electrically under well-defined conditions. The charged particles are passed through an electric field and are captured as a result of their electrical mobility (see equation [4.6]). Since size and electrical mobility are related, the size distribution of particles can be deduced. These devices are called electrical mobility analyzers. 

When suspended particles are subject to a temperature field there is a tendency for small particles to travel down the temperature gradient. The phenomenon is usually called thermophoresis, but it may also be referred to as thermal difiusion. The process has been used as a separation process for high value or heat sensitive substances [Bott and Khoo 1967] although in general the separation process is not of particles but of molecules. 

The temperatiue difference between electrons and heavy neutral particles due to Joule heating in the collisional weakly ionized plasma is conventionally proportional to the sqirare of the ratio of the electric field ( ) to the pressure p). Only in the case of small values of E/p do the temperatiues of electrons and heavy particles approach each other. Thus, this is a basic requirement for local thermodynamic equilibrium (LTE) in plasma. Additionally, LTE conditions require chemical equilibrium as well as restrictions on the gradients. The LTE plasma follows the maj or laws of equilibrium thermodynamics and canbe characterized by a single temperature at each point of space. Ionization and chemical processes in such plasmas are determined by temperature (and only indirectly by the electric fields through Joule heating). The quasi-equilibrium plasma of this kind is usually called thermal plasma. Thermal plasmas in nature canbe represented by solar plasma (Fig. 1-4). 

The only external force on the dispersion considered so far has been the earth s gravitational field. On particles of colloidal size and especially tho.se below c. 0.5 pm diameter, and density close to the medium, the effect of gravity is completely outweighed by the thermal motion of the particles Brownian motior, leads to a structure which is close to an equilibrium state. As the particle size increases, e.g. for a suspension, then the effect of external fields has to be considered since under the influence of gravity particles tend to senle. Another importani external field occurs when the system is stirred or sheared. The easiest case to consider is the application of a simple shear gradient The flux J (velocity x... 

Nuclear spin relaxation (NSR) does not require small particles because in certain cases nuclear spin depolarization occms by coupling of the nuclear electric quadmpole moment of the adsorbate to fluctuations in the substrate electric field gradient as the atom moves [95Chrl]. The depolarization of an initially prepared set of nuclear spins is monitored by thermal desorption into a special detector. Mathematical modeling is complicated, and only a small set of substrates and adsorbate nuclei can avoid competing depolarization processes. Spatial resolution depends on the length of diffusion before desorption, but lies near 10 nm. 

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