1. If we have a gas diffusing through say a solid metal, or gas diffusing into a liquid, what phase is the diffusing molecules considered to be in (or is it even possible to define this)? Is it possible to define state properties for molecules diffusing through another substance.

  2. Is it possible for liquid (or even solids) to diffuse through solids, like for water in a metal container? If so how would one define the density gradient? In my mass transfer book every problem deals with gas diffusion through a liquid/solid species so I am not sure if this is possible.

  • $\begingroup$ 1) The phase of a substance is a macroscopic property and hence is not defined for individual atoms/molecules. 2) Yes, both solids and liquids can diffuse through solids, though the corresponding diffusion coefficients are extremely small typically, and for solid-solid diffusion, the diffusion mechanisms may also be anomalous. $\endgroup$ Commented Oct 27, 2014 at 21:40
  • $\begingroup$ @surajshankar so it wouldn't make sense/not possible to define a temperature/pressure (or any thermodynamic state property) for something like dissolved air or molecules diffusing through a solid? What if we have single phase diffusion (gas through gas or liquid through liquid)? $\endgroup$
    – Yandle
    Commented Oct 27, 2014 at 21:51
  • $\begingroup$ It depends on what you are talking about. If, say, you are talking of diffusion at the continuum level, then there are well defined macroscopic properties associated with it as the material, though diffusing, still has a large amount of constituent atoms or molecules and hence their phase is in principle the same as that of the pure material (though the idea of phase is not very useful while thinking of diffusion, but temperature/pressure can be defined). $\endgroup$ Commented Oct 27, 2014 at 23:09
  • $\begingroup$ Only when you are talking of low density diffusion (like Knudsen diffusion) or probing the system at microscopic length scales, then the continuum diffusion description fails and one must resort to a more appropriate detailed theory (say, kinetic models) and then what I said before about macroscopic properties being undefined, applies. $\endgroup$ Commented Oct 27, 2014 at 23:11
  • $\begingroup$ How does one differentiate whether it is low density scale diffusion or a continuum? Intuitively I am thinking that gas dissolving in liquid or through solids is low density, but something like the Stefan problem isn't? $\endgroup$
    – Yandle
    Commented Oct 28, 2014 at 0:31


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