  This letter describes a periodically oscillating microfoam flow. For constant
input parameters, both the produced bubble volume and the flow rate vary over a
factor two. We explicit the link between foam topology alternance and flow rate
changes, and construct a retroaction model where bubbles still present
downstream determine the volume of new bubbles, in agreement with experiment.
This gives insight into the various parameters important to maintain
monodispersity and at the same time shows a method to achieve controlled
polydispersity.
