受限幾何形狀中及不同濕潤性表面上的複雜流體行為

Abstract

這篇論文分為三個部分。第一部分我們探討了高濃度單分散乳液在微通道中的軟玻璃流動。這些乳液具有堵塞結構，表現出複雜的流動特性，目前尚未完全被理解。我們使用耗散粒子動力學法對其流動行為進行了詳細的研究。我們對不同分辨率和示蹤劑尺寸的局部流動曲線進行分析，發現在低分辨率下，速度曲線與實驗觀察一致，但在高分辨率下則有所偏離，因其在剪切區域呈現階梯狀的變化。液滴分佈表明，在剪切區域形成了分層結構，且隨著壓力降的增加而擴展。肇因於不同層間液滴交換不頻繁發生，導致速度產生突然變化。此外，在高壓力降下，之前在塞狀區域中的均勻液滴分佈也轉變為分層結構。這些發現顯示了高濃度乳液在受限環境中的非均勻結構，對其流動行為有顯著影響。 第二部分我們使用耗散粒子動力學模擬研究了在平滑和粗糙表面上高度變形液滴的滑動行為。在平滑表面上，隨著驅動力增加，出現了四種不同的狀態：幾乎球形液滴、橢圓形、沒有頸部的拉長形狀，以及具有波動滑動速度的振盪拉長形狀。粗糙表面上出現了在平滑表面上不存在的角狀液滴，且在較高的驅動力下，形成尖峰和珍珠狀結構。平滑表面上的混亂破碎，與粗糙表面上的收縮不同，是由於不穩定的流場。在考慮表面濕潤性和粗糙度效應下，我們推導出基於表面速度的滑動速度和修正驅動力之間的線性關係。 第三部分我們研究了奈米通道中受寬度影響(即通道寬度依賴)的黏度和滑移長度。使用耗散粒子動力學法分別在Poiseuille和Couette流系統中研究了黏度和滑移長度。模擬結果顯示，在較小通道中，黏度和滑移長度隨通道寬度增加而增加，但在較大通道下會趨向體相之定值。隨著表面濕潤性降低，滑移長度增加，而黏度保持不變。這種行為源於受限液體的獨特結構：通道變窄破壞了液體的均勻性，形成振盪的密度分佈。這改變了流體層間的摩擦和流體-固體相互作用，影響了通道寬度依賴的黏度和滑移長度。但我們亦發現表面濕潤性僅影響流體-固體相互作用，並不影響流體層間摩擦。 第四部分我們使用多體耗散粒子動力學研究了不同通道寬度下奈米狹縫中的毛細滲透動力學。對於像石墨烯這樣的光滑通道壁，通道寬度對毛細滲透速率的影響在狹窄和寬闊通道之間呈現相反的變化。觀察到毛細滲透速率的局部最小值，這表明在不同潤濕性條件下，狹窄通道中的滲透速率降低，而寬闊通道中的滲透速率增加。相反，對於粗糙通道壁，在沒有壁面滑移的情況下，Lucas-Washburn (L-W) 方程成立，滲透速率隨通道寬度線性增加。這種差異歸因於光滑表面上的壁面滑移，滑移長度隨通道寬度增加而增加，最終趨於一個漸近值。對於具有光滑壁面的狹窄奈米狹縫，根據帶有滑移條件的 L-W 方程得出的動態接觸角（CA）可能小於靜態接觸角，這挑戰了現有的理解。這種“有效”的動態接觸角並不準確代表液體前沿的彎月面，而是表明了表面潤濕性的增強。

This thesis is divided into four parts. In the first part, we investigate the soft glassy flow of highly concentrated monodisperse emulsions in microchannels. These emul-sions, with jammed structures, exhibit complex flow properties that are not fully under-stood. Using dissipative particle dynamics simulations, we study the flow behavior in detail. Our analysis of local flow curves at different resolutions and tracer sizes reveals that the velocity profile agrees with experimental observations at low resolutions but deviates at high resolutions, displaying stairwise changes in the shear zone. Droplet dis-tributions indicate the formation of a layered structure within the shear zone, which ex-pands with increasing pressure drop. The infrequent exchange of droplets between lay-ers leads to sudden velocity changes. Furthermore, at high pressure drops, the previ-ously uniform droplet distribution in the plug zone transforms into a layered structure. These findings underscore the non-homogeneous structures in highly concentrated emulsions under confinement, significantly affecting their flow behavior. In the second part, we investigate the sliding motion of highly deformed droplets on smooth and rough surfaces using dissipative particle dynamics simulations. On smooth surfaces, increasing the driving force reveals four distinct regimes: nearly spherical droplets, oval shapes, elongated shapes without a neck, and oscillating elon-gated shapes with fluctuating sliding velocities. Rough surfaces exhibit corner-shaped droplets, absent on smooth surfaces, with higher driving force resulting in cusp and pearling formation. Chaotic breakage on smooth surfaces, unlike pinching-off on rough ones, arises from an unsteady flow field. A linear relationship between sliding velocity based on surface velocity and modified driving force is derived, considering surface wettability and roughness effects. In the third part, we examine channel width-dependent viscosity and slip length in nanoslits. Using dissipative particle dynamics simulations in Poiseuille and Couette flow systems, viscosity and slip length were investigated. In smaller channels, viscosity and slip length increase with channel width, stabilizing in larger channels. Decreasing surface wettability increases slip length while viscosity remains constant. This behavior arises from the unique structure of confined fluid: narrowing channels disrupt the uni-form density profile, leading to oscillations. This alters friction between fluid layers and fluid-solid interactions, influencing channel width-dependent viscosity and slip length. Surface wettability impacts only fluid-solid interactions, not friction between fluid lay-ers. In the fourth part, we investigate the dynamics of imbibition in nanoslits using many-body dissipative particle dynamics, focusing on the effects of varying channel widths. For smooth channel walls, such as those made of graphene, the influence of channel width on the imbibition rate differs significantly between narrower and wider channels. Specifically, we observe a local minimum in the imbibition rate, indicating that narrower channels experience reduced imbibition rates, while wider channels show increased rates across various wettability conditions. In contrast, for rough channel walls where wall slippage is negligible, the Lucas-Washburn (L-W) equation accurately predicts the imbibition behavior, with the imbibition rate increasing linearly with chan-nel width. This difference is attributed to wall slippage in smooth channels, where the slip length increases with channel width before reaching an asymptotic value. For nar-rower nanoslits with smooth walls, the dynamic contact angle (CA) derived from the L-W equation, considering slip conditions, can be smaller than the static CA, challeng-ing conventional understanding. This "effective" dynamic CA does not accurately re-flect the meniscus at the liquid front but rather indicates enhanced surface wettability.