Advanced Studies of Water Evaporation Kinetics over Teflon-Coated Tungsten Nanorod Surfaces with Variable Hydrophobicity and Morphology

Here, we present the process of water droplet evaporation over hydrophobic/superhydrophobic tungsten nanorod (WNRs) surfaces with various nanoscale morphologies and porosities. The WNR surfaces were fabricated by varying both Ar pressure and substrate tilting angle in radio-frequency magnetron sputtering by using the glancing angle deposition technique; their characteristics were analyzed by electron/atomic force microscopy and spectroscopy. The variation in the droplets' contact angle, contact line diameter, and central height as a function of time showed that the evaporation process was highly influenced by the nanomorphology of the. substrate. The surface roughness correlating with the wetting regime (Wenzel and/or Cassie) and the subsequent variation in the contact angle hysteresis (CAR) of the surfaces had a significant effect on the duration of each of the three evaporation modes that were identified. A strong agreement for the CAR determined by using two approaches-dynamic method (adding/withdrawing water to/from surfaces) and natural evaporation process-was observed. In addition, these nanoscale rough surfaces have shown no abrupt transition from dewetting (Cassie) regime to wetting (Wenzel) regime, and the surfaces are less vulnerable to the transition in the case of very small-sized water droplets. Such studies could be the foundation for the development of highly tunable surface platform technologies with applications in water or possibly ice mitigation, biology, aerospace.