Adiabatic but permeable piston

[arpon]

How can a permeable piston be adiabatic? If substances can go in and out of the cylinder and the substances have heat energy, heat can be exchanged through a permeable piston.
I came across this term in the book, but cannot understand.

 

[Vanadium 50]

How can a permeable piston even be a piston?

 

[DrDu]

You could use an ice cube as a piston which is permeable for water at 0 deg. , but impermeable for heat. Better even, use some porous material which strongly absorbs the substance it shall be permeable for. The substance will loose its thermal energy when first binding to the material, and then diffuse against the temperature gradiet. Microporous substances are permeable for the superfluid component of liquid Helium which has S=0, but not the non-superfluid phase, which has S>0, so they can act as nearly ideal permeable adiabatic pistons for helium.

 

[Chestermiller]

It's an idealization. Substances can flow through the pores, but the material comprising the piston does not conduct heat. This would be the same deal as an array of insulated capillaries in parallel.

 

[lychette]

How can a permeable piston even be a piston?

A Stirling engine involves the use of a permeable 'piston' although the term 'displacer' is used to distinguish it from the type of 'piston' you have in mind.