濃乳液的機械特性及基於石墨烯的奈米通道中的異常滲吸動力學

Abstract

本論文分為四個部分。第一部分探討了液滴大小如何影響壓縮乳液的填充微結構和熱性質。在準備熱力學亞穩乳液時，液滴聚合是一個常見現象，使得研究單分散壓縮乳液在實驗或模擬中變得困難。在固定體積分數下，乳液的性質隨著液滴大小而變化，但相關研究有限。本研究使用耗散粒子動力學在無需事先了解微結構或液滴間的相互作用下，探索高濃度單分散液滴乳液的特性。研究結果顯示，形成堵塞結構的臨界體積分率約為0.65。隨著體積分率的增加，平均配位數也增加，可以用一個簡單的尺度關係式描述。另外，我們亦系統性地研究體積分率、液滴直徑和界面張力對內能和熱容的影響。發現隨著體積分率和界面張力的增加，以及液滴直徑的減小，內能和熱容都會增長。尺度分析表明，模擬結果十分符合我們推導的尺度關係式的預測。 第二部分研究了含有奈米尺寸液滴的濃縮乳液之固態彈性行為，包括楊氏模數和體積模數。由於其固態般的行為，高體積分率的濃縮乳液在食品、化妝品、塗料和製藥等各個行業中有著廣泛的應用。然而，由於液滴的聚合和熱力學不穩定性，研究單分散濃縮乳液的機械性質是具有挑戰性的。我們採用一種介觀模擬方法來探索這些性質，並且無需事先得知乳液的微結構。我們研究了體積分率(𝜙)、液滴直徑(D)和界面張力(𝜎)對楊氏模數(E)和體積模數(K)的影響。當𝜙 < 𝜙c，楊氏模數不存在，而體積模數隨著𝜙的增加而增加。當𝜙 > 𝜙c，隨著𝜙和𝜎的增加，兩者都會增加，特別是當液滴直徑減小時會更為明顯。我們的模擬結果顯示楊氏模數(E)和體積模數(K)能以兩式E~𝜙0.13(𝜙-𝜙c)1.55(𝜎/D) 和 K~𝜙1.06(𝜙-𝜙c)0.15(𝜎/D)表示。此外，對於軟材料，彈性模數和體積模數的關係滿足E=3K(1-2𝜈)，其中泊松比(𝜈)非常接近0.5，但隨著𝜙的增加仍然略有下降。 第三部分探討了在石墨烯奈米通道中全潤濕的乙醇之異常滲吸動力學。研究利用分子動力學探索了乙醇（全潤濕）在石墨烯片上的散擴行為，以及在石墨烯奈米通道中的滲吸過程。在散擴動力學中，存在兩個區段：初始由慣性主導的散擴和之後的黏性散擴，其指數高於坦納定律。全潤濕液體在滲吸動力學中表現出與部分潤濕液體不同的行為。前驅膜和主流隨著它們的長度與時間的平方根成比例地前進，前驅膜的比例常數與通道寬度無關，但主流的比例常數隨著通道變寬而減小。另外，前者的比例常數值較主流的值大。前驅膜的厚度和小於通道寬度的弧面曲率直徑都隨著通道變寬而增加。在非常狹窄的奈米通道中，前驅膜會融入主流，呈現出異常快速的滲吸行為。 第四部分研究了石墨烯奈米通道中通過前驅膜增厚來增強毛細流。由於前驅膜的存在，全潤濕液體在奈米毛細管中表現出與部分潤濕液體不同的滲吸動力學。我們使用分子動力學研究了在石墨烯片上全潤濕液體（異丙醇和二甲基甲酰胺）的潤濕行為以及在石墨烯的奈米通道中的滲吸動力學。自發散擴動力學遵循兩個幂律，其長期行為符合坦納定律。奈米通道中的滲吸偏離了沃什伯恩方程式，呈現出一個獨特的雙階段模式，其轉折點與液體類型相關但與通道寬度無關。在第一階段時，前驅膜的前進速率不隨通道寬度改變而變化。在前驅膜達到通道末端後，第二階段即開始，前驅膜重新變厚。此增厚程序，減少了膜的弧面曲率，因此增加毛細驅動力，加速了第二階段的毛細流，所以第二階段的滲吸速率明顯超過第一階段。

This thesis comprises four parts. The first part explores the effect of droplet size on the packing microstructures and thermal properties of compressed emulsions. In preparing thermodynamically metastable emulsions, droplet coalescence is a common issue, making it challenging to study monodisperse compressed emulsions either experimentally or through simulations. Properties of emulsions vary with droplet size at a specified volume fraction, but relevant studies are limited. Here, dissipative particle dynamics simulations are used to explore highly concentrated emulsions of monodisperse droplets without prior knowledge of microstructure or inter-droplet interactions. The critical packing leading to the onset of the jammed structure is identified at a volume fraction around 0.65. The mean coordination number rises with increasing volume fraction and can be described by a scaling relation. The effects of volume fraction, droplet diameter, and interfacial tension on internal energy and heat capacity are systematically studied, showing growth with in-creased volume fraction and interfacial tension, and decreased droplet diameter. Dimen-sional analysis shows that all data points can be well represented by the scaling relations derived in this study. The second part studies the solid-like elastic behavior, including Young's and bulk moduli, of nanosized concentrated emulsions. Concentrated emulsions with high volume fractions find applications in various industries like food, cosmetics, coatings, and phar-maceuticals due to their solid-like behavior. However, studying the mechanical properties of monodisperse concentrated emulsions is challenging due to droplet coalescence and thermodynamic instability. A mesoscopic simulation method is used to explore these properties without prior microstructure knowledge. The effects of volume fraction (𝜙), droplet diameter (D), and interfacial tension (𝜎) on Young’s modulus (E) and bulk modulus (K) are investigated. Young’s modulus is absent for 𝜙 < 𝜙c, while the bulk modulus increases with 𝜙. For 𝜙 > 𝜙c, both moduli grow with 𝜙 and 𝜎, especially as D decreases. Our simulation results are represented by E~𝜙0.13(𝜙-𝜙c)1.55(𝜎/D) and K~𝜙1.06(𝜙-𝜙c)0.15(𝜎/D). Furthermore, the relationship for soft materials E=3K(1-2𝜈) is satisfied. The Poisson’s ratio (𝜈) is very close to 0.5 but still decreases slightly with increasing 𝜙. The third part investigates the abnormal wicking dynamics of total wetting ethanol in graphene nanochannels. The study explores ethanol's (total wetting) spreading behavior on graphene sheets and the imbibition process in graphene nanochannels using Molecular Dynamics. In spreading dynamics, two regimes are identified: initial spreading dominated by inertia and viscous spreading with an exponent higher than Tanner’s law. Total wetting liquid exhibits distinct behavior from partial wetting liquid in imbibition dynamics. The precursor film and main flow advance with their lengths proportional to the square root of time, but the constant for the precursor film, independent of channel widths, is greater than that of the main flow, which decreases with wider channels. Both the precursor film thickness and meniscus curvature diameter, smaller than the channel width, increase with wider channels. Very narrow nanoslits show surprisingly rapid imbibition behavior, with the precursor film blending into the main flow. The fourth part examines the enhancement of capillary flow via precursor film thickening in graphene nanochannels. Total wetting liquids exhibit different wicking dy-namics in nanocapillaries compared to partial wetting liquids due to the precursor film. We investigate total wetting liquids (isopropyl alcohol and dimethylformamide) on graphene sheets and imbibition dynamics in graphene-based nanoslits using molecular dynamics. Spontaneous spreading dynamics follow two power laws, with long-term behavior con-forming to Tanner’s law. Imbibition in nanoslits deviates from Washburn’s equation, showing a unique two-stage pattern with a turning point related to the liquid type, inde-pendent of channel width. The imbibition rate in the second stage exceeds the first. The precursor film's advancing rate remains constant irrespective of the channel width in the first stage. After the precursor film reaches the channel's end, the second stage begins, and the film re-thickens, reducing meniscus curvature and enhancing capillary flow.