在少量幾何與化學缺陷石墨烯奈米管道中的毛細現象與穩流

Abstract

在奈米流體學的應用中，幾何和化學缺陷時常在光滑石墨烯表面被發現或是創造。本研究使用分子動力學模擬來探索水在附有微量缺陷石墨烯奈米管道中的浸潤動力學與穩流。首先，幾何缺陷（孔洞和突起）略微提高了水接觸角，但化學缺陷（羥基和環氧基團）顯著降低了水接觸角。在穩態流動中，平均速度和滑移長度總是會因微量缺陷而減小，並且化學缺陷的影響相較幾何缺陷的影響更顯著。然而，有趣的是，無論滑移長度如何，幾何缺陷的速度分布都呈柱狀流，而化學缺陷則總是拋物線。再來，石墨烯奈米狹縫上的微量缺陷也顯著影響著浸潤動力學，且通常遵循帶有滑移長度的 Washburn 方程。對於化學缺陷，表面摩擦（滑移長度）相較表面潤濕性（接觸角）驅使的驅動力來說更佔優勢。儘管如此，對於突起缺陷，可以觀察到由接觸線遲滯和熱擾動引起的滯滑行為。我們的模擬結果表明，缺陷性質在奈米級流動和浸潤過程中至關重要，且傳統流體力學理論無法描述。

Geometric and chemical defects are frequently found or created on smooth graphene for applications of nanofluidics. In this work, imbibition dynamics and steady flow of water in graphene nanochannels with sparse defects are explored by molecular dynamics. The water contact angle is raised slightly by geometric defects (hole and protrusion) but lowered significantly by chemical defects (hydroxyl and epoxide groups). In steady flows, the mean velocity and slip length are always reduced by sparse defects and the effect of chemical defects is more significant than that of geometric defects. However, it is interesting to find that the velocity profile is plug-like for geometric defects but becomes parabolic for chemical defects, regardless of the slip length. Sparse defects on graphene nanoslits also affect the imbibition dynamics remarkably, which generally follows Washburn’s equation with the slip length. For chemical defects, surface friction (slip length) dominates over the driving force associated with surface wettability (contact angle). Nonetheless, for protrusion defects, the stick-slip behavior caused by contact line pinning and thermal fluctuations can be observed. Our simulation results indicate that the defect nature is crucial in nanoscale flows and imbibition processes, which the conventional hydrodynamic theory fails to depict.