以分子動力學模擬研究在不同潤濕度的均質固態表面上奈米氣泡的穩定性

Abstract

在固態表面上的奈米氣泡由於其氣泡表面有著巨大的 Laplace 壓力，理論上應該在數微秒之內迅速消散，然而實驗上卻發現表面奈米氣泡能穩定存在數小時甚至數日。為了解釋其穩定存在的機制，Lohse 提出了疏水性固態表面、過飽和溶液環境以及表面釘扎效應是奈米氣泡穩定存在的必要因素，但是在最近的實驗中卻發現奈米氣泡也可以穩定存在於親水性固態表面，例如石英和矽藻土。在這篇論文裡，我們利用分子動力學模擬來分析此系統，模擬奈米氣泡在不同親疏水性的固態表面上並且研究固態表面和周遭氣體分布產生的效應。 另外，也有實驗提出奈米氣泡表面上的短氫鍵可以抑制氣體的擴散，可以使氣泡穩定存在於非過飽和的系統環境中，所以我們研究了氣泡表面上水的結構以及切線方向水跟氣體的擴散係數，同時和平面的氣液界面做比較以研究氣泡表面的曲面效應。

A satisfactory theory for understanding surface nanobubble formation, long time stability, and various morphologies is still a challenge. To understand the properties of the surface nanobubble, we investigate gas aggregation in water influenced by substrate wettability on MgO and graphite using molecular dynamics simulation. The survival probabilities, OH bond orientation and diffusion coefficient of water molecules are calculated near both curved and planar interfaces. We find consistent results, indicating the methods for the planar gas-water interface can extend to curved interfaces. In addition, we perform three-dimensional surface nanobubble simulations on substrates with various wettability and discuss the effects between the substrates and the gas distributions consequentially formed adjacent to the surface. The hydrophobic effect of substrates is widely accepted because hydrophobic substrates with lower water-solid interaction strength allow more gas molecules gathering at the interfaces whereas hydrophilic ones do not. However, there are some experimental exceptions that surface nanobubbles can be produced on mica-water and hydrophilic particle-water interfaces. Our simulation results also demonstrate that surface nanobubbles can exist on homogeneous hydrophilic surfaces.