Gravity invasion percolation in two dimensions: Experiment and simulation

We present experiments and computer simulations of slow drainage in a two-dimensional porous medium. The effect of gravity is systematically varied by tilting the system from the horizontal position. The width of the front between the fluids is found to scale with the dimensionless Bond number Bo (ratio between gravitational and capillary forces) as Bo-0.57, as predicted by theory. The external perimeter of the invaded structure is shown to be fractal with the fractal dimension De1.34 for length scales smaller than the front width.