Observation of Coexisting Upflow and Downflow Hexagons in Boussinesq Rayleigh-Bénard Convection

We experimentally observed patterns consisting of domains of upflow hexagons coexisting with domains of downflow hexagons in Boussinesq Rayleigh-Bénard convection. The sequence of patterns, as the control parameter $\epsilon =\Delta T/\Delta T_c-1\mathrm{}$ is increased, is from ordered or disordered rolls at onset to extended targets and spirals at $\epsilon \sim 1$, to coexisting hexagons at $\epsilon \sim 3$. Hexagons occur for all values of the Prandtl number $2.8\le P\le 28$ investigated. Surprisingly, they appear in a range where only rolls were known to be stable, and their wave number differs substantially from the roll wave number they coexist with.