INTERMITTENT BEHAVIOR IN SLOW DRAINAGE

The water pressure measured during slow, constant rate drainage in a t wo-dimensional porous model exhibits sudden jumps as bursts of air qui ckly displace water from a region. The measured size distribution of t he pressure jumps is exponential. Invasion percolation (IF) simulation s give a power-law size distribution of the connected regions invaded in bursts. In the experiments the meniscii of the fluid-fluid front ad just during a burst, causing the capillary pressure to decrease. Inclu ding this effect in a modified invasion percolation algorithm causes p otentially large bursts to split up into smaller bursts that are expon entially distributed. From the experimental pressure curve it is possi ble to identify groups of bursts that would become a single, ''composi te'' burst in a larger system. These composite bursts are power-law di stributed, consistent with simulations and percolation theory. Differe nt versions of the IP model result in different structures and power-l aw exponents. The best choice of model for the present experiment is d iscussed.