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纳米尺度下表面润湿性如何影响薄液膜沸腾传热仍是一个具有挑战性的研究. 本研究采用分子动力学方法探讨壁面润湿性影响纳米尺度薄液膜沸腾换热的机制. 结果发现: 亲水表面能够显著地提升沸腾换热性能, 有较早的沸腾起始时间, 较高的升温速率、热流密度和界面导热率, 以及较小的界面热阻. 通过建立二维表面势能模型, 揭示表面润湿性影响纳米尺度沸腾换热的机理. 亲水壁面的表面势能为–0.34 eV, 而疏水壁面的表面势能仅为–0.09 eV, 提升表面润湿性强化沸腾传热的本质原因是表面势能绝对值的提高. 此外, 通过计算分子间的相互作用能, 揭示了纳米尺度下亲疏水壁面的成核机理. 水分子-亲水壁面、水分子-疏水壁面和水分子内部的相互作用能分别为1.57, 0.26和0.48 eV/nm2. 亲水表面的界面能大于水分子内部的相互作用能, 因此亲水表面上气泡成核发生在水膜内部; 疏水表面上的界面能比水分子内部的相互作用能弱, 疏水表面的气泡成核发生在固/液界面处. 本研究揭示了表面润湿性如何影响纳米尺度薄液膜沸腾传热和气泡成核的主要机制.How surface wettability affects boiling heat transfer of thin liquid film on a nanoscale remains a challenging research topic. In this work, the effects of wettability on the nanoscale boiling heat transfer for a thin liquid film on hydrophilic surface and hydrophobic surface are investigated by molecular dynamics simulation. Results demonstrate that the hydrophilic surface has better heat transfer performance than the hydrophobic surface. It has a shorter boiling onset time, higher temperature, heat flux, interfacial thermal conductance, and weakened interfacial thermal resistance. The hydrophilic surface throughout has higher critical heat flux than the hydrophobic surface in both macro-system and nanoscale system. Besides, a two-dimensional surface potential energy is proposed to reveal the mechanism of wettability affecting the boiling heat transfer. The absolute value of potential energy in one regular unit of hydrophilicity (–0.34 eV) is much higher than that of hydrophobicity (–0.09 eV). That is the crucial reason why the heat transfer enhancement via improving surface wettability should be primarily the powerful surface potential energy. In addition, the interaction energy is calculated to further address the nucleation mechanism and heat transfer performance for liquid film on different wettability surfaces. The interaction energy values are ordered as Iphi (1.57 eV/nm2) > Iwater (0.48 eV/nm2) > Ipho (0.26 eV/nm2), indicating that the better heat transfer performance of hydrophilic surface is because of the large interaction energy at the solid/liquid interface. Besides, the bubble nucleation on a hydrophilic surface needs absorbing more energy and occurs inside the thin liquid film, while it needs absorbing less energy and triggering off at the solid/liquid interface with hydrophobicity. Those uncover the principal mechanisms of how wettability influences the bubble nucleation and boiling heat transfer performance on a nanoscale.
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Keywords:
- wettability /
- surface potential energy /
- interaction energy /
- thin liquid film boiling /
- molecular dynamics
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图 2 水分子在亲疏水表面上相变行为的快照 (a) 亲水表面; (b) 疏水表面
Fig. 2. Snapshots of phase-change behavior for liquid water on (a) hydrophilic and (b) hydrophobic surfaces.
图 3 亲疏水表面(a)气泡体积和(b)液膜体积变化图
Fig. 3. Variation of (a) bubble volume and (b) water film volume on hydrophilic and hydrophobic surfaces.
图 4 (a)亲水和(b)疏水表面氧原子的径向分布函数
Fig. 4. Radial distribution function of O-O atoms on (a) hydrophilic and (b) hydrophobic surfaces.
图 6 亲疏水表面上水分子的热流密度变化图
Fig. 6. Heat flux for water molecules over hydrophilic and hydrophobic surfaces.
图 7 界面热导变化图
Fig. 7. Interfacial thermal conductance for water molecules on hydrophilic and hydrophobic surfaces.
图 8 界面热阻变化图
Fig. 8. Interfacial thermal resistances for water molecules over hydrophilic and hydrophobic surfaces.
图 11 亲疏水表面1 nm区域内水分子数量变化图
Fig. 11. Variation of the number of water molecules within the 1 nm region of the hydrophilic and hydrophobic surfaces.
图 9 水分子与铜基底的相互作用
Fig. 9. Interactions between water molecules and real copper surface.
图 10 亲疏水表面的相互作用势能分布图 (a) 亲水表面; (b) 疏水表面
Fig. 10. Distribution of interaction potential energy for the (a) hydrophilic and (b) hydrophobic plate surfaces in a periodic cell.
图 12 亲疏水表面铜-氧原子的VDOS
Fig. 12. VDOS of Cu and O for the hydrophilic and hydrophobic surfaces.
表 1 原子间作用势能参数
Table 1. Potential energy and related parameters between particles.
粒子 i, j qe/e σij/Å εij/eV H-H +0.52 0.00 0.0000 O-O –1.04 3.165 0.006998 Cu-Cu — 2.33 0.4096 Cu-H — 0.00 0.0000 Cu-O(亲水) — 2.75 0.034 Cu-O(疏水) — 2.75 0.009 
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