Influence of contact angle on quasistatic fluid invasion of porous media

We present results of detailed simulations of capillary displacement in model two-dimensional porous media as a function of the contact angle \ensuremath{\theta} of the invading fluid. In the nonwetting limit (\ensuremath{\theta}=180\ifmmode^\circ\else\textdegree\fi{}), growth patterns are fractal as in the invasion percolation model. As \ensuremath{\theta} decreases, cooperative smoothing mechanisms involving neighboring throats become important. The typical width of invading fingers appears to diverge at a critical angle ${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$, which depends on porosity. Above ${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$ the invaded pattern remains fractal at large scales. Below ${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$ the fluid floods the system uniformly. Probabilities of local interface instabilities are analyzed to elucidate these findings.