Salt Creeping and Crystal Lifting in Ternary Droplets on Hydrophobic Substrates

Salt creeping during evaporative crystallization is generally associated with hydrophilic substrates, where capillary spreading of the saline solution promotes lateral crystal growth. In contrast, hydrophobic surfaces are typically expected to suppress such spreading. Here, we demonstrate the unexpected and sustained occurrence of salt creeping on both hydrophobic and superhydrophobic substrates during the evaporative crystallization of ternary saline droplets. To establish the generality of this phenomenon, multiple ternary systems are investigated, including water-NaCl-KCl, water-NaCl-KNO₃, water-NaCl-NH₄Cl, water-KCl-KNO₃, water-KCl-NH₄Cl, and water-KNO₃-NH₄Cl, over a broad substrate temperature range of 40-135 °C. Detailed experiments with water-NaCl-KCl mixtures on heated substrates reveal distinct creeping deposition patterns and crystal lifting, accompanied by systematic changes in contact angle, deposit height, and lateral footprint. Morphological analysis shows that ternary crystallization promotes elongated and hopper-type crystals that form interconnected porous networks rather than simple cubic structures. These networks act as efficient capillary pathways, sustaining liquid transport and enabling lateral redistribution of the solution, even on hydrophobic substrates. A local mass-balance between the capillary wicking flux and crystallization indicates that the extent of creeping is governed by the competition between these two processes. At elevated temperatures, this balance shifts, producing a transition from dominant creeping to a mixed regime of creeping and crystal lifting, revealing distinct temperature-dependent deposition behavior. Overall, this work proposes a generalized mechanism based on capillary-driven mass flux to explain salt creeping on hydrophobic substrates and highlights the critical role of crystal morphology in controlling liquid transport and deposit evolution during multicomponent evaporative crystallization.