實驗上的研究發現,水在石墨烯奈米狹縫中流動異常地快速(高達每秒1公尺),這揭示了無滑移邊界條件下的Washburn方程式是無法用來描述此現象的。本研究使用分子動力學模擬來探索水在石墨烯奈米狹縫中的浸潤動力學。狹縫的寬度介於一到十層石墨烯寬。經過分析後發現,浸潤在奈米狹縫中的水的密度會隨著通道寬度而變。除此之外,我們也計算了水和石墨烯的界面張力、平衡接觸角,和奈米狹縫中穩定流的滑移長度。此滑移長度被發現會隨著通道寬度的增加而減小。自發性毛細管流動的分析結果顯示,水隨著時間的滲透高度在定性上會遵循Washburn方程式。此外,模擬得到的流速與文獻中的實驗結果是相當的。然而,浸潤速率隨著通道寬度的變化卻與Washburn方程式的預測不一致。不過,將滑移長度隨通道長度變化的關係引入後,Washburn方程式就可以合理地用來描述浸潤動力學。此與通道寬度相關的現象可以以水的狀態會隨著通道寬度而變化來解釋,而水狀態的變化可以透過分析在不同通道寬度下的水密度和氫鍵來展示。
Unexpectedly fast flow (up to 1 m/s) of water in graphene nanoslits was reported in experiments, revealing the breakdown of Washburn’s equation with no-slip condition. In this work, the imbibition dynamics of water into graphene nanoslits, ranging in width from one to ten atomic planes, is explored via Molecular Dynamics. The density of imbibed water in the nanoslit is analyzed and it depends on the channel width. Moreover, the interfacial tensions, equilibrium contact angle, and slip lengths of steady flow in nanoslits are evaluated. The slip length is found to decrease with increasing the channel width. The analyses of spontaneous capillary flow indicate that the time evolution of the penetration length follows Washburn’s equation qualitatively and the flow rates are comparable to the experimental results. However, the dependence of the imbibition rate on the channel width does not agree with Washburn’s equation at all. As the channel width-dependent slip length is introduced, the imbibition dynamics can be reasonably described by Washburn’s equation. The channel width-dependent behavior can be explained by the variation of the water state with the channel width, which can be realized by the changes of water densities and hydrogen bonds with the channel width.
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