Reverse order-disorder transition of Janus particles confined in two dimensions.

Janus particles with different patch sizes, confined to two dimensions, generate a series of patterns of interest to the field of nanoscience. Here we observe reverse melting, where for some densities the system melts under cooling. For a broad range of hydrophobic patch sizes $({60}^{\ensuremath{\circ}}l{\ensuremath{\theta}}_{0}l{90}^{\ensuremath{\circ}})$, a reentrant transition from solid to liquid and then to an ordered phase emerges as temperature $(T)$ decreases due to the formation of rhombus chains at low $T$. This reentrant phase has pseudo long-range orientational order but short-range translational order, similar to a hexatic phase. Our work provides guidelines to study the melting and assembly of Janus particles in two dimensions, as well as mechanisms to generate phases with specific symmetry.