Recent work has discovered an unusual phenomenon in which sodium chloride crystals self-eject from hydrophobic surfaces during evaporation of saline water drops. Crystals formed during evaporation eject via growth of crystalline legs formed by evaporative flows. This unusual phenomenon is surprisingly robust and will occur when just a few criteria are satisfied. First, crystals must not adhere to the underlying substrate during evaporation. Thus, nano-structured superhydrophobic surfaces will induce ejection while crystal intrusion into micro-scale textures on micro-structured superhydrophobic prevents ejection. The second criteria for ejection to occur is that evaporation must induce flows towards the substrate.  Materials which reliably eject salt crystals may enable new process design for sustainable systems. For example, one could imagine new evaporative heat exchange processes in which saline waters -perhaps from desalination waste brines- are used instead of fresh water in order to simultaneously preserve fresh water resources and provide a useful function for waste brines. Another possibility could be designing salt-resistant materials that are immune to damage from corrosive sea sprays. In order to realize these possibilities, it is important to understand the role of complex and realistic water compositions on the self-ejection effect. We explored the influence of model contaminants including calcium salts, surfactants, and colloidal particles on the ejection effect in order to understand whether real water sources may be used in the ejection effect for sustainable process design. None of the tested contaminants disrupt ejection; and surfactants actually promote self-ejection. This result is likely due to altered solubility of sodium chloride crystals, which influences crystallization induction time and ultimately leads to altered ejection dynamics.

Effect of water composition on self-ejection of salt crystals from hydrophobic and superhydrophobic surfaces

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