Solutal instabilities for patterning during evaporation*

A number of interfacial instabilities arise during evaporation of drops or thin films on hydrophilic materials due to the coexistence of solutal and thermal gradients. Here, we show that crystallization during evaporation can leave a record of multiple fluid instabilities to create a range of ordered patterns. For example, Marangoni instabilities are capable of generating ordered, hexagonal arrays of droplets from a moving fluid front. In our system, crystal clusters precipitate at the locations of the droplets organized by Marangoni forces to generate a crystalline pattern. We also find a novel instability in which a kink-like wave propagates along the fluid front and delineates regions of uniform but disparate film height. This unusual fluid behavior leads to generation of sawtooth structures composed of crystals of disparate heights with a large and distinct step change between the regions. The patterning regime that emerges can be controlled by alteration of the evaporation rate and surface forces between the drop and underlying substrate, and we present a phase diagram for controlling regimes. By harnessing instability for controlled patterning, we also demonstrate that these water-soluble crystalline patterns can be used as masks for sustainable micro-fabrication.