缺陷密度對六角氮化硼奈米流道中表面潤濕性與流體流動之影響

Abstract

二維材料中的原子尺度缺陷會顯著影響奈米尺度下的流體行為。本研究使用 NAMD（Nanoscale Molecular Dynamics）進行分子動力學模擬，探討單空孔（monovacancy）與凸起（protrusion）缺陷對於水在六角氮化硼（h-BN）表面潤濕性及奈米流道中流動動力學的影響。透過針對兩種幾何構型：表面駐留的奈米液滴（sessile nanodrop）與管道內奈米水栓（liquid nanoplug）的模擬，結果顯示，單空孔缺陷可藉由降低水的接觸角（water contact angle）與系統內能來提升親水性；相對地，凸起缺陷則因亂度降低而提高接觸角。於奈米狹縫流動情境下，空孔缺陷導致速度分布由類塞狀轉變為拋物線型，並伴隨明顯的滑移速度下降。局部黏度分布呈現出與密度振盪一致的層狀結構。由 Hagen-Poiseuille 擬合得之有效黏度與滑移長度，皆在缺陷濃度趨近零時顯著上升。當引入空孔缺陷後，靠近壁面的密度分布產生肩狀特徵，進而擾亂受限水分子的空間排列並提升層間分子移動性，導致黏度下降，最終於高缺陷濃度下趨於飽和。

Atomic-scale defects in two-dimensional materials can substantially affect fluid behavior at the nanoscale. In this study, nanoscale molecular dynamics simulations are employed to investigate the effects of monovacancy and protrusion defects on the surface wettability and flow dynamics of water in h-BN nanoslits. By considering two configurations—sessile nanodroplet and liquid nanoplug—monovacancy defects are found to enhance hydrophilicity by lowering the water contact angle (WCA) and reducing internal energy, whereas protrusion defects increase the WCA by decreasing entropy. For nanoslit flows, the presence of vacancy defects induces a transition from plug-like to parabolic velocity profiles, accompanied by a pronounced reduction in slip velocity. The local viscosity distribution exhibits layering consistent with density oscillations. Both the effective viscosity and slip length—extracted via Hagen-Poiseuille fitting—increase rapidly as the defect concentration approaches zero. The introduction of vacancy defects produces shoulder-like features in the density profile near the walls, disrupting the spatial ordering of confined water. This effect leads to a decrease in viscosity, which eventually plateaus at sufficiently high defect concentrations.