Transition from viscous fingers to compact displacement during unstable drainage in porous media

The immiscible displacement of a viscous fluid by less one in porous medium is known to be unstable and often results dendritic fingering patterns, where the pressure gradient screened internally only advanced invading parts grow. We present experiments media, systematically varying imposed injection ($P$) viscosity ratio ($M$) defending fluid. A crossover more stable compact invasion structures found for higher $P$ $M$. This regime onset observed even $M<{10}^{\ensuremath{-}3}$. show that not this patterns exhibit rich internal flow dynamics.