微奈米結構表面濕潤行為調控對凝結熱傳及表面優化之分析

Abstract

隨著全球氣候變遷與人口成長導致水資源日益匱乏，發展高效且具永續性的水資源回收技術已成為全球關注的焦點。其中，利用大氣水分凝結進行集水與熱管理的技術，因其可不依賴現有水源基礎建設，展現出極大潛力，尤其在乾旱及偏遠地區更具應用價值。液滴式凝結（dropwise condensation, DWC）因能在固體表面形成離散液滴，避免液膜熱阻覆蓋，顯著提升熱傳效能，成為目前熱能轉換與水資源回收領域的關鍵研究方向之一。因此，提升DWC的穩定性與效率已成為表面材料設計的關鍵課題。 本研究結合半導體微影與軟壓印技術，系統性製備170種具高度可控性的工程化表面，涵蓋多種表面粗糙度（單層微米與雙層微奈米）、表面濕潤性及幾何圖案設計。藉此全面探討各種基材對液滴濕潤行為、凝結機制、集水效率與熱傳效能之影響。實驗與理論分析結果顯示，凝結液滴於這些表面上可呈現Wenzel狀態、Cassie狀態、partial Cassie（Wenzel-Cassie混合型）狀態及三相接觸線不規則的Wenzel狀態，並建立了濕潤行為與接觸角量測之間的對應關係。 研究結果顯示，雖然親水性表面因具較低的成核能障，能有效促進凝結初期的成核速率，但凝結液滴與表面之強烈附著性，限制了液滴的排除與更新，導致整體熱傳效能受限。相對而言，具有微/奈米雙層粗糙度的超疏水表面可實現穩定的Cassie態DWC，顯著提升單位面積的熱傳係數，但由於液滴滾落後不易被再利用，造成整體集水效率偏低，限制其實際應用價值。進一步分析發現，僅具單層微米尺度粗糙度之疏水表面，在微結構幾何參數（如方柱高度控制於3.31至6.56 μm且間距小於5.0 μm）優化設計下，即可穩定實現具高液滴移動性與自發排除能力之Cassie態DWC機制。相較於平坦基材，此類表面不僅展現出顯著提升的熱傳效能（提升幅度達346.4%），亦能同步提升凝結液的集水效率（提升幅度達33.0%），達成性能與實用性兼具的設計目標。然而，若微結構高度過高，則在長時間凝結過程中易形成水膜覆蓋表面，而使凝結機制轉變為薄膜式凝結，進而導致熱傳效能明顯衰退，較平坦基材降低63.9%。綜上所述，本研究建立了單層微米粗糙度表面實現高效Cassie態DWC的設計準則，並為未來大氣水分收集與熱管理材料之開發提供關鍵設計依據與技術指引。

With the increasing severity of global water scarcity driven by climate change and population growth, the development of efficient and sustainable technologies for atmospheric water harvesting and thermal management has become a pressing challenge. Among these, dropwise condensation (DWC), which enables the formation of discrete droplets on solid surfaces and avoids the thermal resistance associated with continuous liquid films, has emerged as a key strategy for enhancing both water collection and heat transfer. Accordingly, improving the stability and efficiency of DWC is a critical objective in surface design. In this study, 170 engineered surfaces with varying surface roughness, surface wettability, and geometric patterns were fabricated via photolithography and soft embossing techniques. The effects of these surfaces on droplet wetting behavior, condensation mechanisms, water harvesting efficiency, and heat transfer were comprehensively investigated. Four distinct wetting states were identified during condensation: Wenzel state, Cassie state, partial Cassie (mixed Wenzel-Cassie) state, and Wenzel state with irregular three-phase contact lines. Experimental results reveal that although hydrophilic surfaces facilitate nucleation due to lower energy barriers, their strong droplet adhesion limits droplet removal and thereby reduces overall heat transfer performance. Conversely, superhydrophobic surfaces with dual-scale roughness can significantly enhance heat transfer through stable Cassie-state DWC but suffer from poor water harvesting. Notably, hydrophobic surfaces with only single-micro-scale roughness can achieve both high heat transfer and efficient water harvesting by optimizing geometric parameters (e.g., pillar height between 3.31 and 6.56 μm, with pillar spacing equals to or less than 5.0 μm). Such surfaces achieve stable Cassie-state condensation with enhancements of 33.0% in water harvesting and 346.4% in heat transfer compared to a flat hydrophobic substrate. However, excessively tall micropillars promote water film accumulation over time, leading to a transition to filmwise condensation and a significant performance decline, with heat transfer reduced by 63.9% relative to a flat surface. This study establishes practical design criteria for achieving efficient Cassie-state DWC on single-micro-scale roughness surfaces and offers valuable insights for the development of advanced materials for atmospheric water harvesting and passive thermal management applications.