Crystal patterns created by rupture of a thin film

We describe a method of generating remarkable crystal patterns on smooth surfaces such as mica, glass, or calcite. Very dilute droplets of aqueous salt solution (0.1 wt % NaCl) are deposited on the chosen substrate, and if the perimeter of these droplets becomes pinned by crystal deposition, they thin and concentrate by evaporation rather than shrink laterally. When thin enough, the liquid film ruptures by one of two mechanisms. In the first, crystals nucleate on the substrate and puncture the film, and as the holes grow radially from the rupturing crystal, the receding liquid rim deposits two-dimensional crystal dendrites. A second mechanism of rupture can occur with more dilute solutions (0.025 wt % NaCl), where attractive forces across the liquid film lead to rupture before crystals nucleate. This results in a pattern of bare patches surrounded by crystals, because supersaturation is only reached when the neighboring holes in the film are close to coalescence. The method is applicable to any solute in a volatile solvent on any smooth substrate, and the interplay of dewetting, crystal deposition, and symmetry of crystal and substrate leads to complex, system-specific patterns. The technique may be developed into methods of surface patterning with technologically important crystals.