- 學位論文
以液氣相共存消散粒子動力法探討接觸角與遲滯現象
Analysis of Contact Angle and Hysteresis Phenomenon on Patterned Substrate using Liquid/Vapor Coexistence Dissipative Particle Dynamics Simulation
摘要
濕潤現象廣泛運用於工程應用中，其中接觸角是濕潤現象中最容易觀察到的物理量，而底板微小的結構變化所造成的遲滯現象導致接觸角可能會有巨大的改變。本論文利用具有凡得瓦迴線(van der Waals loop)的多體消散粒子動力學(Multi-body Dissipative Particle Dynamics, MDPD)模擬液滴在理想的結構板上所造成的遲滯現象，並分析遲滯現象與濕潤理論的關係。 DPD具有類似分子動力法(Molecular Dynamics, MD)的離散力學計算，MDPD進一步改良原DPD理論，可模擬出液相/氣相共存介面，使模擬系統更貼切的描述熱力學系統，也更適合做為研究濕潤現象的工具。本研究DPD程式建構於物件導向設計的模擬程式平台Digital Material-MD架構中，藉由其有效率的底層程式設計，加上平行化程式設計進行DPD濕潤現象模擬，並更進一步實作可節省系統顆粒總量的模擬方式，增進模擬效率。本論文也比較了三種接觸角計算方式，並提出此三種接觸角計算在離散粒子的最佳計算方式。 進行濕潤模擬中，以不同起始液滴形狀做為起始條件，觀察各個收斂的接觸角狀態，本研究發現藉由降低系統溫度能降低接觸角量測誤差，但溫度過低系統將呈現非液體行為。在平板模擬中，我們驗證了Young’s Equation的假設；在結構化底板模擬中，我們模擬出前進角、後退角與其他半穩定狀態。將不同粗糙度的底板遲滯範圍與Wenzel/Cassie理論相比較，θw<130°下能觀察到較大的遲滯範圍，θw>130°時遲滯範圍在15°~30°內變化，與實驗所觀察的趨勢吻合。
並列摘要
Wetting is an important phenomenon, and has been used widely in many engineering applications. The contact angle is often used to describe the degree of wetting. The defect of the surface might induce wide contact angle distribution, often called hysteresis. In this thesis, Multi-body Dissipative Particle Dynamics (MDPD), which can represent the van der Waals loop, is used to simulate the wetting phenomenon on ideal patterned substrates and to analyze the relation between simulated hysteresis and theoretical prediction or experimental observation. DPD is similar to Molecular Dynamic (MD) while the MDPD is improved from DPD to model the liquid/vapor coexistence interface. Such improvement allows us to describe the thermodynamic system associated with the wetting phenomenon. An efficient implementation to reduce the particles for the base is introduced. Three contact angle measurement methods from discrete particles are implemented. For a liquid droplet on an ideal flat substrate, hysteresis will not occur. This assessment was verified in our wetting simulations, in which the same contact angle was reached with different initial liquid shapes. For a droplet on a patterned substrate, the advancing contact angle, receding contact angle, and many meta-stable states have been found in our simulations. We found that θw<130° has more hysteresis. When θw>130°, the hysteresis is limited to a range of 15°~30°.