The broad class of dynamical systems that display self-organised criticality are hypothesised to be in a universality class named for the 'directed' type of this process. For 10 points each:
[10h] Name this process, whose 'invasion' type can be described by the time dependent forming of bonds on a lattice.
ANSWER: percolation
[10m] The steady state of percolation in certain materials including soils can be described by this equation, which sets the volume flow rate proportional to the pressure difference.
ANSWER: Darcy's law
[10e] Darcy's Law often describes the flow of water through this kind of material, which has many holes. Cork is an example of a material with this property, due to the large number of voids in its structure.
ANSWER: porous [or reasonable equivalents]