Out-of-Plane Formation of Crystal Deposits on Heated Hydrophobic Surfaces

Inorganic fouling, also termed as scaling, is pervasive in several applications ranging from heat exchangers, cooling towers, desalination membranes to household plumbing. Current methods to remove scaling are not environmentally safe and cause a significant downtime of the equipment. Evaporation of a saline droplet on substrates of different surface wettability offers a simple platform to investigate the effect of the underlying substrate on the nature of scaling. Here, we report on the out-of-plane growth of crystal deposits on hydrophobic substrates during the evaporation of saline droplets comprising of salts with high solubility. The crystals are loosely adhered to the substrate, which results in a self-cleaning behavior irrespective of the crystal morphology (cubic, needle-type, and dendritic) and the initial volume of the saline droplet. We present a generalized mechanism for the out-of-plane growth of the crystal deposit based on the relative wettability of the substrate and the salt crystals.