A self-generated maze

Can be explained with simple physics. Remarkable patterns can spontaneously emerge in out-of-equilibrium systems with competing forces. The labyrinthine structures in this illustration were created experimentally (gold and black) and in simulations (black and white) by Biørnar Sandnes and colleagues at the University of Oslo, Norway. The lines indicate a residue of glass beads that remains after the liquid has been slowly drained from a bead–fluid suspension confined between two glass plates. Shortly after the draining begins, air deforms the air–liquid interface at the perimeter of the suspension and produces fingers in the liquid. The slow draining means that viscous forces are not important determinants of the fingering. Sandnes suggests that the pattern results instead from a combination of capillary pressure that acts at the air–liquid meniscus to advance the finger and a retarding frictional force. As the illustration shows, the simulations run by the Oslo team to test that simple model produce patterns that look much like the experimental results. The group tested further by exploring how the characteristic finger width of the labyrinth varies with the concentration of glass beads in suspension. Again, simulated and experimental results were in good agreement, particularly when the residue of beads was significantly thicker than the plate spacing. (B. Sandnes et al. , Phys. Rev. Lett. , in press.)