探討三維親疏水混和矽表面之冷凝熱傳

Abstract

三維親疏水混和表面可以透過親疏水之間的高度差與潤濕性的不同，將液滴與液膜限制於親水溝槽內，限制了液膜的厚度並降低液膜在表面上的熱阻，更能避免橋接水滴(Bridging Droplets)現象發生，並且藉由更改親疏水區間之寬度可以限制液滴離開直徑，因此，如何透過三維親疏水混和表面提升單位面積下的冷凝熱傳係數，已成為現今增強冷凝熱傳能力的主要研究方向。 有文獻指出親疏水高度差為50 μm的三維親疏水混和表面的熱傳係數優於親水以及超疏水表面，其中在親水溝槽寬度為1300 μm之表面中，能夠有效抑制橋接水滴現象，因此相較於超疏水表面，在中、高過冷度時，冷凝熱傳係數分別提升了130%以及 85%。此外，親水溝槽寬度為600以及300 μm之表面，於各個過冷度表面上都出現橋接水滴，因此在降低親疏水區間寬度的同時，仍無法避免橋接水滴現象發生，且過長的矽奈米線陣列使橋接水滴不易滑離疏水區間，出現橋接水滴滯留於冷凝表面的現象，導致淹沒(flooding)面積比增加，形成額外的熱阻使熱傳效率降低。 因此，本研究所設計之三維親疏水混和矽表面，親水區間採用氮化矽親水溝槽，為了避免液滴Wenzel以及淹沒現象，疏水區間採用矽短奈米線陣列搭配Teflon之疏水表面，矽短奈米線長度約為1.5 μm，透過親疏水之間的高度差以及潤濕性的不同，除了能夠促進液滴由疏水區間滑離至親水溝槽內，限制液滴離開最大直徑以外，更能將液滴與液膜限制於親水溝槽內，並且藉由縮短奈米線長度，避免橋接水滴的出現以及滯留於實驗表面上，進一步提高冷凝熱傳性能。 實驗結果發現，矽短奈米線陣列之疏水表面不易淹沒的特性可以在各個過冷度都維持著滴狀冷凝，因此三維親疏水混和矽表面於中、高過冷度時，冷凝熱傳係數幾乎維持定值，並無明顯的下降。於中、高過冷度時，N1300、N600以及N300表面上並沒有出現滯留或是成長後向下滑動的橋接水滴，並且可以得知親水溝槽內，液膜的流動速度隨著會隨著親水溝槽寬度降低而下降，這是因為當溝槽寬度越小，流動阻力會增加，導致液膜的流動速度有所下降，當液膜的流動速度越快，水就可以越快的離開實驗表面，所得到的熱傳效果也越好，因此在沒有橋接水滴的影響之下，N1300表面於中、高過冷度的冷凝熱傳係數優於N600以及N300，於高過冷度時，N1300表面的冷凝熱傳係數相對於膜狀冷凝有178%的增益。 另外，本研究發現在三維親疏水混和矽表面中，橋接水滴是影響冷凝熱傳的重要現象，於各個過冷度下，N100表面上依然出現大量的橋接水滴，但實驗結果並未如先前文獻中所述，橋接水滴的出現會產生巨大的熱阻進而降低冷凝熱傳效果，反而因橋接水滴擁有較短的循環時間及以及較高的離開頻率，橋接水滴能迅速且不間斷地刷新冷凝表面，降低表面淹沒的風險，確保持續冷凝並提高熱傳性能，於高過冷度時，N100表面的冷凝熱傳係數相對於膜狀冷凝有216%的增益。

Three-dimensional hybrid surfaces can confine liquid droplets and films within hydrophilic grooves by leveraging the height difference between hydrophilic and hydrophobic regions, as well as differences in wettability. This restricts the thickness of the liquid film and reduces the thermal resistance on the surface. Moreover, it helps to prevent the phenomenon of bridging droplets. By adjusting the width of the hydrophilic and hydrophobic regions, the departure diameter of liquid droplets can be limited. Thus, enhancing the condensation heat transfer coefficient per unit area through three-dimensional hybrid surfaces has become a primary research direction for improving condensation heat transfer capability. Literature indicates that the heat transfer coefficient of three-dimensional hybrid surfaces with a height difference of 50 μm outperforms that of hydrophilic and superhydrophobic surfaces. Particularly, on surfaces with a hydrophilic groove width of 1300 μm, the phenomenon of bridging droplets can be effectively suppressed. Consequently, compared to superhydrophobic surfaces, the condensation heat transfer coefficients are increased by 130% and 85% respectively at medium and high subcooling. Furthermore, the surfaces with hydrophilic groove width of 600 μm and 300 μm, bridging droplets appear on each surface at various subcooling. Therefore, while reducing the width of the hydrophilic and hydrophobic region, the phenomenon of bridging droplets cannot be avoided. The excessive length of the silicon nanowire array hinders the bridging droplets from easily sliding off the hydrophobic regions, resulting in the retention of bridging droplets on the condensation surface, leading to an increased flooded area and additional thermal resistance that reduces heat transfer efficiency. Therefore, the three-dimensional hybrid silicon surface designed in this study utilizes silicon nitride hydrophilic grooves in the hydrophilic regions. To avoid Wenzel state and flooding phenomenon, Hydrophobic regions are constructed with silicon short nanowire arrays combined with a Teflon. the length of the silicon short nanowires is approximately 1.5 μm. Through the height difference between hydrophilic and hydrophobic regions as well as their different wettability, this design not only facilitates droplets transitioning from hydrophobic regions to hydrophilic grooves, limiting their maximum departure diameter, but also confines liquid droplets and films within the hydrophilic grooves. Furthermore, by shortening the length of the nanowires, bridging droplets are prevented from forming and lingering on the experimental surface, thereby further enhancing condensation heat transfer performance. The experimental results revealed that the characteristic of being less prone to flooding on the hydrophobic surface of silicon short nanowire array allows dropwise condensation to be maintained at various subcooling. Therefore, the condensation heat transfer coefficient of the three-dimensional hybrid silicon surface remains almost constant at medium and high subcooling, without significant decrease. At medium and high subcooling, surfaces N1300, N600, and N300 did not exhibit any retained or growing bridging droplets sliding downward. It is also observed that the flow velocity of the liquid film within the hydrophilic grooves decreases with the decrease in groove width. This is because as the groove width decreases, flow resistance increases, leading to a decrease in the flow velocity of the liquid film. With faster flow of the liquid film, water can leave the experimental surface more quickly, resulting in better heat transfer effects. Therefore, without the influence of bridging droplets, the condensation heat transfer coefficient of surface N1300 is superior to N600 and N300 at medium and high subcooling. At high subcooling, the condensation heat transfer coefficient of surface N1300 exhibits a gain of 178% relative to film condensation. Furthermore, this study found that bridging droplets are a crucial phenomenon affecting condensation heat transfer on the three-dimensional hybrid silicon surface. At various subcooling, a significant number of bridging droplets still appeared on the N100 surface. However, contrary to previous literature, the experimental results did not show that the presence of bridging droplets generates substantial thermal resistance, thereby reducing condensation heat transfer efficiency. Instead, due to the shorter cycle time and higher departure frequency of bridging droplets, bridging droplets can quickly and continuously refresh the condensation surface, lowering the risk of surface flooding, ensuring continuous condensation, and enhancing heat transfer performance. At high subcooling, the condensation heat transfer coefficient of the N100 surface exhibited a gain of 216% relative to film condensation.