Racetrack for competing viscous fingers

We have observed viscous fingering flow in a Hele-Shaw cell, starting from a carefully prepared initial state of fourteen almost identical fingers. As the flow progresses, killing off thirteen of the fingers on its way to the single-finger steady state, several systematic effects stand out, including (1) a characteristic trajectory for a finger which depends not on its position in the ``starting gate'' but rather on its final position in the ``race'' and, most importantly, (2) a cascade of reverse bifurcations, stages in which the number of competing fingers is reduced, typically by a factor of 2.