Time dependent diffusion coefficient in a disordered medium

1996 ◽  
Vol 104 (1) ◽  
pp. 206-209 ◽  
Author(s):  
Thomas M. de Swiet ◽  
Pabitra N. Sen
Keyword(s):  
Diffusion Coefficient ◽  
Time Dependent ◽  
Disordered Medium

scholarly journals Time-Dependent Diffusion Coefficients for Chaotic Advection due to Fluctuations of Convective Rolls

Fluids ◽  
2018 ◽  
Vol 3 (4) ◽  
pp. 99 ◽  
Author(s):  
Kazuma Yamanaka ◽  
Takayuki Narumi ◽  
Megumi Hashiguchi ◽  
Hirotaka Okabe ◽  
Kazuhiro Hara ◽  
...  
Keyword(s):  
Diffusion Coefficient ◽  
Liquid Crystals ◽  
Diffusion Coefficients ◽  
Coarse Graining ◽  
Chaotic Advection ◽  
Time Dependent ◽  
Local Order ◽  
Weak Turbulence ◽  
Normal Diffusion ◽  
Convective Rolls

The properties of chaotic advection arising from defect turbulence, that is, weak turbulence in the electroconvection of nematic liquid crystals, were experimentally investigated. Defect turbulence is a phenomenon in which fluctuations of convective rolls arise and are globally disturbed while maintaining convective rolls locally. The time-dependent diffusion coefficient, as measured from the motion of a tagged particle driven by the turbulence, was used to clarify the dependence of the type of diffusion on coarse-graining time. The results showed that, as coarse-graining time increases, the type of diffusion changes from superdiffusion → subdiffusion → normal diffusion. The change in diffusive properties over the observed timescale reflects the coexistence of local order and global disorder in the defect turbulence.

The concentration and pressure dependent diffusion of carbon dioxide in nitrile rubbers

1992 ◽  
Vol 339 (1655) ◽  
pp. 497-519 ◽  
Keyword(s):  
Carbon Dioxide ◽  
Diffusion Coefficient ◽  
High Pressures ◽  
Ambient Pressure ◽  
Weight Fraction ◽  
Gas Pressure ◽  
Time Dependent ◽  
Initial Increase ◽  
Parametric Function ◽  
Ambient Gas

The paper reports the results of an experimental and associated analytical study of the time dependent adsorption of carbon dioxide gas into two nitrile elastomers. The mass gas sorption has been measured using a device based on a vibrating reed to a weight fraction accuracy of ca . 0.05 % at 47 °C in the ambient gas pressure range 0.1-34 MPa. The experimental method is described and data are provided. These data are used to compute the most effective description of the diffusion process by invoking a number of different diffusion coefficient, D(θ), characteristics, where θ denotes lapsed time, ambient pressure and local ambient gas concentration within the elastomers. The numerical procedures adopted to perform the fitting of the experimental data with various D(θ) characteristics are described and the quality of the fit is assessed. The D(θ) characteristics chosen have no particular physical basis but follow established empirical precedents. The characteristics of the parameters associated with the various D(θ) functions generally indicate that as the gas is embibed with progressively increasing ambient pressures the diffusion coefficient increases. At high pressures the diffusion is arrested and the coefficient decreases. We have associated the initial increase with gas induced plasticization and the eventual decreases with the effect of the hydrostatic component of the ambient gas pressure. The parameter fitting also indicates that the diffusion is arrested with lapsed time which is tentatively associated with time dependent volumetric relaxations. These interpretations apart, the data and analyses clearly indicate that the transport is not simply fickian and a relatively complex parametric function to describe the sensitivity of the diffusion coefficient to time, concentration and pressure is necessary for these systems.

Rationalization and optimization of the hemodialysis procedure. II

1970 ◽  
Vol 48 (6) ◽  
pp. 438-441 ◽  
Author(s):  
Robert H. Stinson ◽  
Jan de Boer
Keyword(s):  
Diffusion Coefficient ◽  
Time Dependent ◽  
Independent Function ◽  
Membrane Area ◽  
Dependent Function ◽  
Flow Situation

The concepts of dialysance and clearance, as applied in hemodialysis, were closely examined. Strict application of Wolf's definition shows dialysance to be a flow-independent function of the membrane area and thickness as well as of the diffusion coefficient of the substance to be removed. Clearance is a flow-dependent function of dialysance except in a non-flow situation, where clearance equals dialysance. Flow-dependent clearance equations and time-dependent equations for removal of substances from blood, both with and without concurrent metabolic production, are developed for Kolff and Kiil hemodialyzers. Since clearance increases with increase of any variable, only practical considerations need be considered in hemodialyzer design.

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