Is the driving force for diffusion chemical potential and not concentration gradient? Is it possible that diffusion happens in such a way that transfer is from high potential to low potential but concentration wise, it is from low concentration to high concentration?
Can someone also let me know about Anomalous diffusion?
If you consider the reverse osmosis (for water purification) you have a case where water is moving from a site of lower concentration to a site of higher concentration by using a semipermeable membrane, nevertheless the chemical potential of water is higher in the side of lower water concentration because of the effect of the high pressure applied on this side.
Dear Bhubesh Murugappan
In the general definition, the driving force of diffusion is the chemical potential gradient. So yes, under some conditions, you could have a diffusion flux from a point with lower concentration (higher chem. potential) to a point with higher concentration (lower chem. potential). However, in common systems, the direction of chemical potential and concentration gradient will be the same.
Regarding anomalous diffusion, this term is not exactly defined. In polymer related literature, it is often used as a general term for non-Fickian diffusion.
Regards
Josef
Regarding anomalous diffusion in metals, you can find information in:
P.G. Shewmon, Diffusion in Solids, 2nd edition, The Minerals, Metals & Materials Society, Warrendale, USA, 1989
The term driving force is referred to as an agent/force which compels a reaction
to occur. In case of diffusion, the driving force compels diffusion to happen. For Fick's first law of diffusion, the driving force is concentration gradient. Both terms, concentration gradient and chemical potential are used depending on the context.
See if this is useful:
https://www.frontiersin.org/articles/10.3389/fphy.2019.00018/full
"Fick’s laws describe the diffusive transport of matter as an empirical fact without claiming that it derives from basic concepts" H. Mehrer [1] . Fick’s first law is applicable only in the simplest cases. The equations for diffusion fluxes in general cases (under external fields, the case when diffusion occurs in a chemical composition gradient, temperatures gradient and in some other cases ) have a more complex form and are obtained on the basis of Thermodynamics of Irreversible Processes (TIP) [2,3]. A consideration of the consequences of phenomenological equations for isothermal diffusion based on TIP can be found in [1,4].
In particular the flux equations for solid-state diffusion via vacancies in a substitutional binary alloy have the form of linear function of the chemical potential gradients of alloy components including vacancies. An analysis of the equations shows [1] that Fick’s first law is valid for tracer self-diffusion in a uniform alloy. The flux equations have a much more complex form [1,4] when diffusion occurs in a chemical composition gradient.
The general analysis of the phenomenological equations for isothermal diffusion in a binary system is carried out in [5]. The approach is developed in which:
1. the equations for fluxes are derived in terms of the concentration gradients and the relations between diffusion coefficients and Onsager coefficients are established;
2. nonequilibrium vacancies are taken into account in contrast to the Darken theory of interdiffusion [1, 4] and an equation for the interdiffusion coefficient is obtained that depends in particular on the off-diagonal Onsager coefficients.
1. Mehrer H. Diffusion in solids – Fundamentals, methods, materials, diffusion-controlled processes. Textbook, Springer Series in Solid-State Sciences, Springer-Verlag, Berlin Heldeberg, 2007
2. S.R. de Groot, P. Mazur, Non-equilibrium Thermodynamics, North-Holland,Amsterdam, 1962
3. R. Haase, Thermodynamics of Irreversible Processes, Addison Wesley, Reading,1969
4. Paul, A., Laurila, T., Vuorinen, V., Divinski, S.V. Thermodynamics, Diffusion and the Kirkendall Effect in Solids Springer Cham Heidelberg New York Dordrecht London 2014
5. Nazarov A.V.,Gurov K.P., Taking account of non-equilibrium vacancies in phenomenological theory of interdiffusion. Phys. Metals. and Metallography, 1979, v.45, №4, pp.185-188. https://www.researchgate.net/publication/264384369_Calculation_of_non-equilibrium_vacancies_in_the_phenomenological_theory_of_mutual_diffusion
The word diffusion takes its origins from the Latin word, diffundere (to spread out). Diffusion is being used different in the context of different disciplines, such as, phenomenon that heat flows from hot to cold body dictated by concentration gradient. The diffusion of atoms in solid state phase is mainly driven/defined by/through chemical potential and concentration as well.
Diffusion is a mass form of transport process. Other two transport processes are heat and momentum. Any matter and its particle has three inherent properties, which are mass, momentum (velocity), and heat. Therefore, there are three for types of transport processes attached to the matter or particle, namely, mass transport, momentum transport, and heat transport. The fundamental law governing these transport processes is same, which is flux (property flow per unit area ) =property constant x gradient. Apply this law, and you get Fick's Law of Diffusion for mass transport, Newton's Law of Viscosity for momentum transport, and Fourier's Law of Heat Transport. The inherent property of matter for mass is how many particles are at a location, and it can be described as concentration. The inherent property of matter at a location is momentum and often described as density.velocity of fluid. The inherent property of matter for heat is temperature a location.
In summary, the driving force of diffusion (molecular mass transport) is number of molecules in a defined space.
- Badges
- Science topic