Personally, since you're dealing with cross-flow I wouldn't hesitate to use "degree of openness," which is abundant in many contexts in Google. But then that's me. Probably 50 percent of the scientific/technical words I use today didn't exit when I start translating 56 years ago, so using new words/combinations doesn't bother me.
INTERFACE - point where two substances are. in contact with one another. POROSITY is the degree of openness or a sponge-like quality, usually in percentage. PERMEABILITY is the capacity of a porous rock/sediment/soil for transmitting a fluid, a measure of the relative ease of fluid flow, usually measured in millidary