Avalanches of imbibition fronts: Towards critical pinning

High-resolution fast imaging reveals that imbibition fronts driven at constant flow rate (mean velocity v) and roughened by capillary forces move by local avalanches — spatial clusters of large local velocity. Our measurements show that the lateral extents, amplitudes and durations of the avalanches are power-law distributed, with an exponentially decaying cutoff related to the lateral correlations of the interface. As the value of v is systematically reduced the exponential cutoffs diverge, suggesting the presence of a critical pinning transition at v = 0. We verify that the measured statistical distributions (specifically the power-law exponents and the divergence of the exponential cutoffs as v→0) fulfill a number of scaling relations expected to hold for slowly driven systems close to a depinning transition.