Spirals are one of the most common geometric motifs in both art and in the natural world, with hundreds of natural phenomena yielding spiral patterns including hurricanes, seed arrangements, seashells, and more. Here, we report on the unguided crystallization of gypsum needles into spiral formations; and specifically, into Fermat’s spirals, which have the property of zero curvature at the origin and decreasing distance between subsequent arcs. These patterns are created from the evaporation of a drop on solid substrates with the unusual wetting property of having a hydrophobic advancing angle and hydrophilic receding angle. Forced crystallization via fast evaporation can induce patterns that would not regularly be observed for kinetically-limited crystal growth, and we find that spiral patterns form from a careful balance between evaporation rate and the surface pinning force. We quantify this behavior through modeling, and present a phase diagram describing when spiral patterns dominate over the more typical formation of concentric rings. In addition to being aesthetic, spirals are functional motifs with broad applications in engineered systems from antenna to microfluidics, and this technique introduces exciting possibilities for the assembly of micro-scale spiral patterns and templates.

Spirals from drops

256th National Meeting (2018)

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