Crystal critters: Self-ejection of crystals from heated, superhydrophobic surfaces

Mineral or crystal fouling (the accumulation of precipitants on a material and damage associated with the same) is a pervasive problem in water treatment, thermoelectric power production, and numerous industrial processes. Growing efforts have focused on materials engineering strategies (e.g., superhydrophobicity) to prevent fouling. Here, we present a curious phenomenon in which crystals self-eject from heated, nanotextured superhydrophobic materials during evaporation of saline water drops. These crystal structures (crystal critters) have exceedingly minimal contact with the substrate and thus pre-empt crystal fouling. This unusual phenomenon is caused by cooperative effects of crystallization, evaporative flows, and nanoscale effects. The temperature dependence of the critter effect can be predicted using principles of mass conservation, and we demonstrate that self-propulsion can be generated via temperature gradients, which promote asymmetric growth. The insights on confinement-driven evaporative crystallization can be applied for antifouling by self-ejection of mineral foulants, for drop-based fluidic machines, or even for self-propulsion.