| ZHOU Peng,HU Jian-hua,LI Bei.Effect of Bio-inspired Surface Structure Design on Droplet Condensation and Harvesting Behavior[J],52(8):355-362, 379 |
| Effect of Bio-inspired Surface Structure Design on Droplet Condensation and Harvesting Behavior |
| Received:July 22, 2022 Revised:November 22, 2022 |
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| DOI：10.16490/j.cnki.issn.1001-3660.2023.08.030 |
| KeyWord:bio-inspired surface nanoarray hybrid wettability ratio wedge angle condensation directional motion molecular dynamics simulation |
| Author | Institution |
| ZHOU Peng |
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan , China |
| HU Jian-hua |
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan , China |
| LI Bei |
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan , China;State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan , China |
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| Abstract: |
| Bio-inspired water harvesting surfaces present superb self-cleaning and self-propelling properties, high efficiency, and low energy consumption, and are thus of remarkably importance in numerous industrial applications, including dew formation, power generation, seawater desalination and thermal management, etc. Inspired by nature, i.e., desert beetles and cactuses, these surfaces generally involve micro/nanostructures with unique wettability and shapes that make use of alternating hydrophilic and hydrophobic regions and/or conical shape induced wettability gradients. However, a complete and direct picture for underlying principles and mechanisms of droplet condensation and harvesting on the bio-inspired surfaces at small scales is still an imperative requirement. For this, molecular dynamics (MD) simulation is a powerful tool to investigate liquid thermodynamics from an atomic/molecular perspective. Therefore, in this work, we utilized MD simulations to study water vapor condensation and harvesting behavior on the bio-inspired surfaces. The water vapor condensation model coupled with unique micro/nanostructures and surface wettability was constructed. The effects of surface morphology, hybrid wettability ratio (θ) and wedge angle (α)on droplet condensation and harvesting were then systematically investigated. By constructing hydrophobic square and rectangular nanoarrays on solid substrates, the effect of the surface morphologies on droplet condensation behavior was at first explored. It was shown that the water droplet was prone to get pinned in the square nanostructures and hard to be shed from the surfaces; while the rectangular nanorods offered more supports for the droplet, thereby improving its mobility and effectively alleviating the pinning effect. The total condensed water molecules on the surface with rectangular nanorods were also observed to be increased by 30.8% as compared to those on the surface with square nanostructures. Moreover, either dropwise or filmwise condensation typically occurred on a cold surface depending on the surface wettability. It was thus significantly important to design a stable and controllable surface that was capable of dropwise condensation. To achieve it, a reasonable surface chemistry or hybrid wettability ratio θ should be designed apart from the unique nanostructures. It was found that, as the θ increased, the pinning effect became more severe and the stuck water molecules presented the filmwise condensation mode, which impeded the droplet flow and deteriorated the harvesting performance. Conversely, the droplet could follow the dropwise condensation mode at a smaller θ (i.e., 1/6 and 2/6), which promoted droplet mobility and collection with a Cassie state. In addition, a wedge-shaped structure was beneficial for droplet repellency and directional transportation. Therefore, by combining the hybrid wettability and wedge-shaped nanostructure, the effect of the wedge angle α on the droplet condensation and harvesting was examined. The results showed that, the wedged nanostructure could produce sufficient Laplace pressure difference to drive the directional motion towards to the end of the wedges at α=3° or 6°; while the droplets were found to aggregate and merge into larger droplets directly at the end of the wedges at α=9° or 12°, due to the decreasing surface areas at the front of the wedges. As compared with the rectangular nanostructure (i.e., α=0°), the amount of the condensed water molecules was improved by 210.7% and 193.0% at α=9° or 12°, respectively. More importantly, the dual bionic surface inspired from the desert beetles (i.e., hybrid wettability) and cactuses (i.e., wedge-shaped structures) exhibited excellent water condensation and harvesting performance, in terms of the condensation quantity and the largest droplet size, thereby signifying the feasibility and superiority of dual or multiple bionic surfaces over single bionic or hydrophobic ones. The findings in this work are thus believed to provide the atomic/molecular understanding of water droplet condensation and harvesting behaviors on bio-inspired surfaces. |
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