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Physical and Chemical Properties of Pesticides and Other Contaminants: Volatilization, Adsorption, Environmental Distribution, and Reactivity
Published in James N. Seiber, Thomas M. Cahill, Pesticides, Organic Contaminants, and Pathogens in Air, 2022
James N. Seiber, Thomas M. Cahill
where P is the chemical’s partial pressure. This isotherm adequately describes adsorption at low P, typical of chemical contaminants in an air basin or airborne pesticide residues near treated fields. Chemicals in air can be transported to the soil surface by diffusion, by a process following Fick’s first law, and advection (mass transport). Diffusion is defined as the movement of chemical along a concentration gradient from higher to lower concentration. Movement to the soil is measured as flux, where flux is governed by the chemical’s diffusion coefficient (D [cm2/sec]) and the concentration gradient (dc/dx): Flux =−D dc/dx
Hydrogeology
Published in Mohammad Albaji, Introduction to Water Engineering, Hydrology, and Irrigation, 2022
Diffusion is the net movement of anything (e.g. atoms, ions, molecules, energy) that is driven by concentration gradient, from a higher concentration region to a lower concentration region. This fundamental physical phenomenon depends on the random movement of small particles. Diffusion is important for small distances because it is essential for meeting thermodynamic equilibria while it is ineffective for spreading a solute over macroscopic distances because the necessary time to cover a distance by diffusion is proportional to the square of the distance itself. The diffusion phenomenon is quantified by the diffusion coefficient (D) which can often be considered negligible due to its very small amounts, except in cases where the groundwater flow velocities are extremely low (such as clay aquitards).
Introduction to Nanosensors
Published in Vinod Kumar Khanna, Nanosensors, 2021
At a junction between P-type and N-type semiconductors, holes diffuse from the P-side to the N-side and electrons diffuse from the N-side to the P-side (Figure 1.6). Diffusion is the movement of particles along a concentration gradient, that is, from a high-concentration region to a low-concentration region. The above diffusion of carriers constitutes the diffusion current and leaves negatively charged acceptor ions on the P-side and positively charged donor ions on the N-side. Thus, an electric field is created around the junction. This electric field is directed from the N- to the P-side. It sets up an electric current in the reverse direction to the diffusion current flowing along the concentration gradient. This current set up by the electric field is called the drift current. Ultimately, the number of carriers crossing the junction from one side to the other by diffusion equals the number crossing in the reverse direction, due to the electric field. Then, a dynamic equilibrium is established. The potential difference associated with the electric field at the junction is called the built-in potential.P–N junction diode.
Plane wave propagation in a fiber-reinforced diffusive magneto-thermoelastic half space with two-temperature
Published in Waves in Random and Complex Media, 2022
Sunita Deswal, Sunil Kumar, Kavita Jain
Diffusion is the spontaneous movement of particles from a high concentration region to the low concentration region and it occurs in response to a concentration gradient expressed as the change in the concentration due to change in position. The study of this phenomena has received considerable attention during the last several decades due to its relevance in a wide range of geophysical and industrial applications such as forming base and emitter in bipolar transistors, making integrated resistors and improving the conditions of oil extractions. Thermodiffusion in the solids is one of the transport processes that has great practical importance. The thermoelastic diffusion theory studies the interactions among thermal, mass concentration and mechanical fields in elastic bodies. Thus the process of thermodiffusion has a very considerable influence upon the deformation of the solid phase and vice versa.
Effects of electro-osmotic and double diffusion on nano-blood flow through stenosis and aneurysm of the subclavian artery: numerical simulation
Published in Waves in Random and Complex Media, 2022
A. M. A. Moawad, A. M. Abdel-Wahab, Kh. S. Mekheimer, Khalid K. Ali, N. S. Sweed
Double-diffusive convection is a fluid dynamics phenomena explains convection which powered by two separate density gradients with differing diffusion rates (temperature and concentration). Thermal diffusion creates a concentration gradient when the temperature differential is kept constant. The concept of double diffusion is crucial for comprehending the development of complex systems with several causes for variations of density. It also contributes significantly for nutrient upwelling.In [29–33], there is some remarkable double diffusion that is pertinent to research.
Numerical investigation for non-axisymmetric Homann stagnation point flow of a SWCNT/MWCNT-water nanofluid over a disk
Published in Waves in Random and Complex Media, 2022
Jawad Ahmed, Awais Ahmed, Fouzia Sultana, Masood Khan
Figures 13 and 14 are portrayed to analyze the nature of for the parameters like , , . The variation in concentration distribution in nanofluid due to higher values and are reflected in Figure 13(a and b). Concentration profile enhances for larger values of as (0.1,0.3,0.5,0.7), while concentration profile decays due to higher values of Brownian motion . Since Brownian motion is due to the random movements of nanoparticles. This movement is due to concentration gradient as motion from the area of higher concentration towards the area of low concentration. Thus, the solutal energy transport disturb due to this random motion of particles as results the concentration profile lower down. Due to larger thermophoretic force the in the system the mass specie in fluid transport rapidly. Figure 14, determines the nature of for . The behavior of is comparable with , for concentration profile. Since Schmidt number is the ratio of the momentum diffusivity over the mass diffusivity, it is observed from the figure that a rise in leads to declines in mass transport due to decrease in the mass diffusivity. Therefore, diminishes because a decrease in the molecular diffusion decays the mass diffusion.