I Crum's shell effect - visualisation needed

[rwooduk]

When a bubble in liquid expands or contracts it can be seen as having a liquid shell that gets larger on expansion and smaller on compression.

The idea of diffusion is what I am struggling with, if the bubble was getting bigger and the liquid shell contracts then surely it would become more concentrated and diffusion into the bubble from the liquid would be reduced? likewise if the bubble was getting smaller the liquid shell would get bigger and it would be less concentrated, therefore diffusion from the liquid into the bubble would increase?

BUT Crum did not say this, he said the opposite, i'll quote a paper:

The gas diffusion is controlled by the thickness of a diffusion layer or a shell that is formed in the liquid surrounding the bubble (the bubble is assumed to be surrounded by a constant mass of liquid). When the bubble is expanded, the shell becomes thinner and the concentration gradient increases. In this case, the flow rate of gas to the bubble also increases. When the bubble is compressed, the shell is thicker as a result the concentration gradient decreases.

The problem is the language is vague, it says "to the bubble", do they mean into the bubble, do they mean molecules move to the bubble wall and stay there?

Any help with this idea would really be appreciated. Why would diffusion into the bubble occur more readily if the concentration of the wall was increased? which would intuitively make it harder for gas molecules to penetrate.

Paper source: http://www.sciencedirect.com/science/article/pii/S0307904X0300204X

 

[rwooduk]

I'll update for anyone else who comes across this, now solved. The key was that it occurs from the shell and is the same substance as the bulk solution, therefore it's simply a concentration gradient issue. Here's what I have written:

On expansion, a liquid shell surrounding the bubble may be envisaged that becomes thinner and more concentrated and thus the rate of diffusion from the shell into the bubble increases. On contraction the shell would become larger it’s concentration would decrease therefore the aforementioned diffusion would not readily occur [24].