The default Porosity εp of the solid matrix is taken 
From material. The Porous Matrix node supports the use of a 
Porous Material node, where the porosity is defined in the manner of
 
    where θs,i and 
θimf,i are the volume fractions given in 
Solid and 
Immobile Fluids subnodes under the 
Porous Material node.
 
    For User defined, enter a value or expression for the porosity.
 
    In a Porous Catalyst, the dry bulk density 
ρ of the porous medium is needed. This is used to compute mass phase transfer due to adsorption or desorption.
 
    Select Dry bulk density when this quantity is known. Or select 
Solid phase density to instead compute the dry bulk density from the solid phase one. In both cases the density definition can be taken 
From material or 
User-defined.