When a drop of salt water is evaporated on a superhydrophilic surface, a ring of crystals forms at the outer edge due to evaporative flux. This process is similar to the coffee-ring effect. Because the crystalline ring is hydrophilic, it creates an even stronger force for contact line pinning, which allows for a significant amount of water to evaporate before the liquid finally depins from the outer ring. The very thin fluid film evaporates and retracts, and is subject to a number of different instabilities caused by both solutal and thermal gradients. Micron- and nano-scale crystals continuously form during contact line motion due to evaporation; and these crystals leave a record of the contact line instabilities at the moment of crystallization. Thus, the resulting crystalline patterns formed from thin film instabilities create a number of extraordinary patterns.
This work is licensed under the Creative Commons Attribution-NonCommercial 4.0 International License. Any reuse must credit the author(s) and provide a link back to this page.