On the wetting behavior of surfaces in boiling
On the wetting behavior of surfaces in boiling
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DOI：
10.1063/5.0069686
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发表时间：
2021-11
影响因子：
4.6
通讯作者：
K. Ardron;G. Giustini
中科院分区：
文献类型：
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作者：
K. Ardron;G. Giustini
晶核沸腾换热受表面润湿性的影响很大，如杨氏接触角𝜃𝑌所表征的。接触角通常是通过对水平测试表面上的固着液滴进行测量获得的，但对于高温高压下的水，液滴实验的𝜃𝑌值似乎通常比解释类似表面和温度的气泡偏离大小所需的值高出30-50度。我们用Adamson的表面吸附理论解释了液滴和汽泡的𝜃𝑌值之间的差异。这一理论表明，在高压沸腾中形成气泡的情况下，由于气泡内部的非湿润表面与饱和蒸汽接触，它将被一层纳米级厚度的吸附液层覆盖。另一方面，液滴实验通常使用由蒸汽压力远低于饱和的永久性气体加压的高压灭菌器：在这些相对干燥的气体中，预计不会出现被吸附的液体纳米层。我们认为，在汽泡的情况下，吸附层的存在将增加新的润湿表面的形成功，其量与液体表面张力相当，从而导致𝜃𝑌的显著降低。我们表明，通过应用Adamson模型以及对未知参数的合理选择，可以解释气泡和液滴实验中𝜃𝑌差异的大小，并解释为什么在离开蒸气气泡的情况下，𝜃𝑌对表面材料条件的敏感度比固定液滴的情况低得多。
Nucleate boiling heat transfer is strongly influenced by surface wettability as characterised by the Young’s contact angle, 𝜃 𝑌 . The contact angle is usually obtained from measurements on sessile droplets on horizontal test surfaces, but in the case water at high temperatures and pressures, 𝜃 𝑌 values from droplet experiments appear to be typically 30-50 degrees higher than values needed to explain bubble departure sizes for similar surfaces and temperatures. We explain the differences between 𝜃 𝑌 values for droplets and vapour bubbles by using the surface adsorption theory of Adamson. This theory suggests that in the case of bubble formation in high pressure boiling, as the non-wetted surface inside the bubble is in contact with a saturated vapour it will be covered by an adsorbed liquid layer of nanoscale thickness. Droplet experiments on the other hand generally use autoclaves pressurised by permanent gases in which the vapour pressure is far below saturation: in these relatively dry gases the adsorbed liquid nanolayer is expected to be absent. We suggest that the presence of the adsorbed layer in the case of vapour bubbles will increase the work of formation of new wetted surface by an amount comparable to the liquid surface tension, resulting in the significant reduction in 𝜃 𝑌 . We show that by applying Adamson’s model with plausible choices for unknown parameters it is possible explain the magnitude of the differences in 𝜃 𝑌 in bubble and droplet experiments and to explain why 𝜃 𝑌 appears much less sensitive to surface material conditions in the case of departing vapour bubbles than in the case of sessile droplets.