探討液滴黏附力儀器（DAFI）在潤濕性表面的作用現象：結合實驗與分子動力學模擬

Abstract

在微流體控制、塗布和生醫檢測等領域，理解和控制液滴受振動引起的運動行為至關重要。儘管200多年前人們已知固體間的最大靜摩擦力大於動摩擦力，但對液滴在固體表面上橫向運動的阻力尚缺乏全面理解。此外，現有的保持力（retentive force）公式在描述液滴運動行為時存在局限，需進一步修正和優化。 在本研究中，我們利用分子動力學的多體耗散性粒子動力法（mDPD）來模擬液滴在不同粗糙基底結構上的運動行為及相關的Cassie/Wenzel狀態變化。多體耗散性粒子動力法考慮了液-氣體之間的相互作用，能夠有效地描述氣-液共存系統的熱力學和流體力學特性。此外，mDPD方法也能夠模擬液滴與固體表面之間的接觸角和受力行為，以及液滴內部的微觀結構和相變化過程。 在液滴受振動行為的研究中，我們模擬了液滴在粗糙基底結構上受水平與垂直振動所產生的運動行為。研究發現，當振動模態為Rocking mode時，液滴會開始移動，且有兩段不同速度變化。當垂直與水平振動耦合時，液滴質心高度較高，接觸線減小，導致側向黏附力降低，從而在相同水平振動下液滴移動速度更快。此外，我們對液滴黏附力儀器（DAFI）實驗進行了模擬和驗證，成功再現了液滴在潤濕性表面上的靜摩擦和動摩擦現象，並分析了液滴的側向黏附力行為。模擬中發現接觸線、接觸寬度和接觸長度對液滴移動影響顯著，且過往對於保持力公式的假設過於簡化，故我們基於模擬結果對現有保持力公式進行了修正，提出了一個新的計算公式，顯著提高了預測液滴黏附力的準確性。 本研究不僅提供了可準確用於模擬液滴動態行為的方法，且提出了改進的理論公式，這些發現和突破將推動微流體控制、塗布技術和生醫檢測等領域的進一步發展。我們的研究為液滴基礎理論與行為提供了堅實的科學基礎，並為未來的研究和應用開闢了新的方向。

Understanding and controlling the movement of droplets induced by vibration is crucial in fields such as microfluidic control, coating, and biomedical detection. Although it has been known for over 200 years that the maximum static friction between solid surfaces is greater than dynamic friction, the resistance faced by droplets during lateral movement on solid surfaces remains insufficiently understood. Additionally, the current retentive force formulas have limitations in accurately describing droplet movement behaviors, necessitating further refinement and optimization. In this study, we employed the many-body dissipative particle dynamics (mDPD) method to simulate the motion of droplets on various rough substrates and the associated Cassie/Wenzel state transitions. The mDPD method effectively considers liquid-gas interactions, capturing gas-liquid coexisting systems' thermodynamic and fluid dynamic properties. Furthermore, it simulates the contact angles, force behaviors between droplets and solid surfaces, and the internal microstructure and phase changes within the droplets. Our research on droplet behavior under vibration involved simulating droplet motion on rough substrates subjected to both horizontal and vertical vibrations. The study revealed that when the vibration mode is rocking, the droplets initiate movement, exhibiting two distinct velocity phases. Coupling vertical and horizontal vibrations increases the droplet's center of mass height and reduces the contact line, leading to decreased lateral adhesion force, resulting in faster droplet movement under identical horizontal vibration conditions. Additionally, we conducted simulations and validations based on the droplet adhesion force instrument (DAFI) experiments. We successfully replicated the observed static and dynamic friction phenomena of droplets on wettable surfaces and analyzed their lateral adhesion force behaviors. The simulations indicated that the contact line, contact width, and contact length significantly influence droplet movement. We identified that previous assumptions for the retentive force formula were overly simplified. Consequently, based on our simulation results, we refined the existing retentive force formula and proposed a new calculation formula, significantly enhancing the accuracy in predicting droplet adhesion forces. This study not only provides a precise method for simulating droplet dynamic behavior but also introduces an improved theoretical formula. These findings and breakthroughs are poised to advance the fields of microfluidic control, coating technology, and biomedical detection. Our research establishes a robust scientific foundation for the fundamental theory and behavior of droplets, paving the way for future research and applications.