Hi Chiara Monti,

Yes you can. One way, for example is to approximate it to one dimensional diffusion and fit the parameters (e.g. diffusivity) to, for example, the Fick's diffusion model. There is a method described in ASTM D5229. I used it in [1] for thin resin plates. In the paper I also showed that you can fit parameters using the history of mass to FEM results.

[1] Article Measurements of the effects of pure and salt water absorptio...

Chiara,

The mode proposed by Gustavo Quino is applied to homogeneous and isotropic diffusion. Your system is neither homogeneous or isotropic, in the sense that an interface between two materials is encountered by water diffusing in it.

The interface can strongly alter the diffusion dynamics by changing even the 1-D concentration gradient during the same process. The porosity of your coating may play a role in the angular distribuition of the diffusing flow across the layer, altering the isotropy assumption.

Of course, a measurement of absorbed water mass vs time is possible, but it might provide you with a diffusivity value mediated by all the above considerations.

I would try the method suggested by Gustavo just to verify wether a deviation from Fick's Law is observed, and its entity and shape. You could be lucky and find that the above cpnstraints are negligible in your specific case, or you may shed some light on the diffusion pattern (water mass vs time curve may show more than a unic diffusion regime/diffusivity value).

Chiara,

I believe both Gustavo and Giuseppe have significant points here. Here are some questions for you.

What is your substrate? How is your substrate coated? Are you able to do this with multiple samples, each with a different thickness coating? Are you confident that your substrate diffusion coefficient is lower than that of your porous thin-film?

My thinking is that if your substrate has a poor diffusion rate, this will rate-limit your diffusion. In this case, you will be measuring the diffusion through both the film, and into the substrate. What you effectively need is a substrate with instantaneous diffusion which will not dampen the results of the measurement. If you are confident in the substrate, this may be a viable solution.