Nanostructured superhydrophobic surface has long been recognized to have potential for condensation enhancement by achieving extreme wetting properties, which is of great importance in a wide range of applications including building environment control, water harvesting, desalination, and industrial power generation. Small coalescing droplets (<100 μm) on superhydrophobic surface can spontaneously jump away from the surface due to the released surface energy during coalescence. This jumping-droplet condensation mode results in an enhanced removal of condensate droplets compared to the conventional filmwise and dropwise condensation. However, the superhydrophobic surface suffers from a condensate flooding problem in the moist environment, leading to the degradation of condensation efficiency. Moreover, the low surface free energy of liquid-repellent coating typically induces a huge water nucleation barrier on the superhydrophobic surface, which is unfavourable for capturing the moisture in a dry atmosphere. To address these issues, in this thesis, we developed the different superhydrophobic TiO₂ nanostructured surfaces via tuning the surface roughness and the surface chemistry for enhancing the water condensation rate.

To promote a high condensation efficiency under humid condition, superhydrophobic TiO₂ nanorod surface with different geometric parameters was fabricated. In comparison to the unmodified flat TiO₂ surface, 166% increase in water harvesting rate was achieved on the TiO₂ NR-6μm surface. In addition, the topographical effec...[ Read more ]

Nanostructured superhydrophobic surface has long been recognized to have potential for condensation enhancement by achieving extreme wetting properties, which is of great importance in a wide range of applications including building environment control, water harvesting, desalination, and industrial power generation. Small coalescing droplets (<100 μm) on superhydrophobic surface can spontaneously jump away from the surface due to the released surface energy during coalescence. This jumping-droplet condensation mode results in an enhanced removal of condensate droplets compared to the conventional filmwise and dropwise condensation. However, the superhydrophobic surface suffers from a condensate flooding problem in the moist environment, leading to the degradation of condensation efficiency. Moreover, the low surface free energy of liquid-repellent coating typically induces a huge water nucleation barrier on the superhydrophobic surface, which is unfavourable for capturing the moisture in a dry atmosphere. To address these issues, in this thesis, we developed the different superhydrophobic TiO₂ nanostructured surfaces via tuning the surface roughness and the surface chemistry for enhancing the water condensation rate.

To promote a high condensation efficiency under humid condition, superhydrophobic TiO₂ nanorod surface with different geometric parameters was fabricated. In comparison to the unmodified flat TiO₂ surface, 166% increase in water harvesting rate was achieved on the TiO₂ NR-6μm surface. In addition, the topographical effect of nanostructures on water condensation was also studied via preparation of hollow TiO₂ nanotubes on surface. Water harvesting results along with the optical microscopy observation revealed that the nanorods was a more appropriate surface topography than the nanotubes for the water harvesting applications. The role of each geometric parameters of nanorods on water harvesting efficiency was investigated. Appropriate nanorod length (~ 6μm) and tip size (~ 200nm) can accelerate droplet departure, while increasing droplet nucleation density.

Using the insights gained from the surface optimization, a superhydrophobic nano-hierarchical TiO₂ nanorod surface was developed, which can surprisingly sustain the superior jumping-droplet condensation mode over a wide range of supersaturation. This led to 120% higher heat flux compared to the superhydrophobic one-tier TiO₂ nanorod surface at supersaturation, S = 1.044. Due to the remarkable water repellency and better homogeneity, nano-hierarchical TiO₂ nanorods could be a promising surface for application in capturing the water from air at a high humid condition.

To facilitate the water collection rate in a moisture-lacking air, a TiO₂ nanorod surface with novel PDMS coating (named as DCDMS 2-NR in this work) was designed by manipulating the grafting techniques. This PDMS coated nanorod surface exhibited an ultra-efficient water harvesting performance. In contrast to the superhydrophobic surface coated with fluoroalkylsilane (FAS), DCDMS 2-NR sample displayed a more than 10-fold water collection rate enhancement in the arid climate (RH = 30%). Our experimental investigation suggests that the PDMS is a very promising coating material for enhancing the water condensation and solving the freshwater scarcity in drought affected areas.

Additionally, we experimentally demonstrated that the wettability of superhydrophobic surface can be changed by the ion implantation of hydrogen and fluoride ions before FAS coating. Hydrogen ion doping makes the surface more hydrophilic, while doping fluoride ions result in a more hydrophobic surface. After the hydrogen ions implantation on the nanorod surface, 57% improvement in water collection rate was attained compared to the unmodified sample. As a comparison, the fluoride-doped TiO₂ nanotube surface achieved a more efficient droplet-jumping condensation, resulting in a 66% enhancement of water harvesting efficiency compared to the unmodified surface.

This thesis presents improved fundamental understandings of wetting and condensation on nanostructured surface as well as practical optimization of surface structures for enhanced condensation heat transfer. The insights gained demonstrate the potential of new surface engineering approaches to improve the performance of various dehumidification and water harvesting applications.