Two-dimensional ferrofluid foam in an external force field: gravity archesand topological defects

When subjected to an external force field, a two-dimension ferrofluid foam (whose cell boundaries are made of ferrofluid) becomes deformed. New morpho logies are then observed. If the foam is tilted with respect to the horizon tal position, the cells rearrange in arches whose base is the bottom of the system. Furthermore, when submitted to a radial force field, the structure can be described as a topological defect of the hexagonal lattice in the c entre of the system. In all these patterns, which characterize the response of systems whose elements repel via forces of infinite range, the order re mains locally hexagonal. We depict these experimental structures, and we su ggest that they could be described theoretically as conformal transformatio ns of the hexagonal lattice.