  The invasion of air into porous systems in drying processes is often
localized in soft materials, such as colloidal suspensions and granular pastes,
and it typically develops in the form of cracks before ordinary drying begins.
To investigate such processes, we construct an invasion percolation model on a
deformable lattice for cohesionless elastic systems, and with this model we
derive the condition under which cracking occurs. A Griffith-like condition
characterized by a dimensionless parameter is proposed, and its validity is
checked numerically. This condition indicates that the ease with which cracking
occurs increases as the particles composing the material become smaller, as the
rigidity of the system increases, and as the degree of heterogeneity
characterizing the drying processes decreases.
