Molecular Diffusion Theory Research Articles

Determination of the effective diffusion coefficient of glucose in callus tissue

Immobilization of plant cells in reticulate foam matrices creates a milieu which is similar to that prevailing in callus tissues. Since many valuable plant secondary metabolites are found mainly in the callus as opposed to suspension cultures, there is considerable interest in immobilized plant cell cultures. In the design of bioreactors for such systems, there is a need to know the effective diffusion coefficients through the integral mass of the immobilized cells. In this work, diffusion of glucose through callus tissue was determined using 4C-labelled glucose. Application of one-dimensional unsteady state molecular diffusion theory to the experimental results indicated that the value of the effective diffusion coefficient was in the range (0.028 – 0. 28) × 10 −5 cm 2 s −1.

Tracer diffusion of crown ethers in cyclohexane

Tracer diffusion coefficients in cyclohexane of s-trioxane, 12-crown-4, 15-crown-5, 18-crown-6, dicyclohexano-18-crown-6, and dicyclohexano-24-crown-8 have been measured with the Taylor dispersion technique from 298 to 358K. It is demonstrated that the deviation from the Stokes-Einstein relationship is explicable with a microfriction factor which accounts for the solute-solvent size discrepancy; the non-spherical nature of crown ethers is averaged out by rapid molecular rotation. As a result, the measured diffusivities are successfully interpreted with a rough-hard-sphere theory of molecular diffusion. The Arrhenius equation and Hildebrand's free-volume model are also capable of correlating quite well the experimental results reported here.

Far infrared power absorption in the low friction limit

A theory of molecular diffusion in liquids that takes into account the shape of intermolecular potential wells with a cosine potential produces fine spectral detail in the far infrared. The effect of a static external electric field is to shift this panoply of peaks to higher frequency, increase the number of peaks, and shift their individual relative frequencies and intensities.

Molecular dynamics simulation of induced anisotropy. I. Equilibrium properties

A simple mechanical torque is applied to 108 C2v triatomics in a molecular dynamics computer simulation. The effect of the torque on equilibrium autocorrelation functions is evaluated for vectors of interest to the analytical theory of molecular diffusion in anisotropic liquids. The results of the simulation do not, in general, support the conventional viewpoint based on Debye’s theory of rotational diffusion. In particular, the angular momentum acf becomes more oscillatory in the anisotropic liquid, which is not the case in theories of liquid anisotropy such as those of Bénoit, Ullman, and others in the literature.

The physicochemical transmission of two components of a multiple chemical signal in the African weaver ant, ( Oecophylla longinoda)

The behavioural thresholds of the two principal components of the mandibular gland secretion in major workers of the weaver ant Oecophylla longinoda, hexanal and 1-hexanol, were found to be 1.99×10 10 and 3.35×10 11 molecules cm −3 respectively. The mandibular gland contains an average of 2.896×10 15 molecules of hexanal, and 1.274×10 16 molecules of 1-hexanol. From these measurements, molecular diffusion theory predicts behavioural active spaces for 1-hexanol (an attractant) which agree with those observed in response to the whole secretion, but there is no such agreemnet for haxanal (a releaser of behaviour with no directionial compnent). The effect of microturbulence on the transmission of releaser and chemotactic stimuli is discussed.

The gas dynamics of a conical nozzle molecular beam sampling system

We report on the performance of a miniature conical nozzle for a molecular beam mass spectrometry (MBMS) system. The gas dynamics for the expansion through a conical nozzle sampling orifice of the MBMS system was characterized by probing the gas density distribution of the flow field inside the nozzle with a movable conical skimmer orifice. The observed experimental density distributions for air (25°C) and hydrogen (25°C and 410°C) were formulated in a simple analytical relation, with a similarity parameter for each gas. The phenomenon of mass separation upon sampling was studied using a calibrated sample of argon and nitrogen in hydrogen and comparing the modulated beam signals with the calibrated compositions. It was found that the observed mass separation was in agreement with predictions from molecular diffusion theory.

Experimental Verification of Diffusion Battery Theory

The diffusion battery, an assembly of circular tubes or rectangular channels, is one of the best devices available for measuring the size and size distribution of submicron aerosols in the diameter range 0.002 to 0.2 µ m. The performance of these batteries is known from molecular diffusion theory, but until now has not been checked experimentally in this size range because of the lack of the necessary monodisperse aerosols. Experimental measurements on singly charged monodisperse aerosols from 0.01 µm to 0.1 µ m are described using a General Electric and a Pollak condensation nucleus counter to measure the aerosol penetration through the stages of a set of portable diffusion batteries in series. Particle sizes in the range tested could be selected at will by adjusting the voltage of an electric mobility classifier. The fraction of aerosol of a given size passing through each battery stage was found to agree closely with the penetration calculated from molecular diffusion theory for that size. This shows t...

Calibration of a porometer in terms of diffusive resistance

The rate of pressure drop in a convenient leaf-porometer is shown to be a simple function of several separable components of flow resistance. Those due to stomata are arrived at by measuring or estimating the others. Stomatal resistance is also calculated from viscous flow theory and the observed dimensions of sample stomata or their replicas. Measured and calculated values for barley agree moderately well, thus justifying the model used in calculation. With standard molecular diffusion theory, the same dimensions give the diffusive resistance of the stomata, which is the quantity of interest in transpiration or assimilation studies. With suitable averaging procedures to relate the properties of single pores to those of whole leaves, a simplified theoretical relation between viscous and diffusive resistance is found permitting conversion of porometer readings directly into diffusive resistances.

A theory of eddy diffusion in the atmosphere

The theory of eddy diffusion in the atmosphere put forward almost simultaneously by G. I. Taylor and L. F. Richardson in England and by W. Schmidt in Austria is a direct generalisation of the classical theory of molecular diffusion. It is assumed that the mass effect of the eddies is entirely similar, except for a scale difference, to that of the molecules ; thus we find an eddy-diffusivity of the order of 10 2 to 10 11 cm. 2 /sec. replacing a molecular diffusivity of the order of 10 -1 cm. 2 /sec. in entirely similar differential equations. Recent researches,§ however, have shown that the difference between the eddy structure of a turbulent fluid and the molecular structure of a fluid at rest is more than one of scale, and it is now clear that there is need of an extended theory to express this difference. The failure of the earlier theory to account for the phenomenon of atmospheric diffusion has been made evident by the enormous variations found in K, the eddy conductivity. Richardson has evaluated K for the diffusion of smoke over short distances, for the distribution of volcanic ash, and for the scatter of small balloons, and has found K ’s varying from 10 4 to 10 8 cm. 2 /sec. Other writers have given estimates varying from 10 2 to 10 11 cm. 2 /sec., and in general it has been found that K increases rapidly with the scale of the phenomenon. The present paper is concerned with an attempt to define a new diffusion coefficient which is constant over a field of a few hundred metres to hundreds of kilometres. The basic idea, that the rate of diffusion is governed by what is termed the “ effective eddy ” is not entirely new ; it is inherent in much of Richardson’s work. In the earlier theory it was assumed that the size of the effective eddy remained constant ; here it is assumed that it varies with the distance travelled by the diffusing cluster.