Study of organic-coated water droplet coalescence by molecular dynamics simulation

Ph.D.Droplet coalescence is a main mechanism for cloud droplet growth and raindrop formation. It also controls the distribution of atmospheric water droplet size. Organic matter is so prevalent in the atmosphere that the submicron atmospheric aerosols can comprise organic matter up to 18-70% by mass. Such organic matter often forms coatings on the aerosols and complicates the picture of droplet coalescence. In this thesis, we aim to understand better the effects of organic matter on droplet coalescence by molecular dynamics (MD) simulation.In the first part, we first tried to understand more about water droplet coalescence without organic matter. From (quasi-)equilibrium simulations, by studying different droplet sizes, we obtained a linear relationship between the free energy change of coalescence ΔF and the reciprocal of the number of water molecules. We further decomposed each ΔF into energetic ΔU and entropic TΔS contributions in order to elucidate coalescence in terms of the molecular details. We found that the change in Coulomb interactions mainly governed the change in ΔU due to the change in the number of hydrogen bonds during coalescence. A linear relationship between ΔF and the change of the number of hydrogen bonds was obtained, giving a consistent estimate of ΔF per hydrogen bond with the experimental result. A complete explanation for the trend in TΔS is often difficult, but we discovered a positive correlation between TΔS and the mobility of water molecules. To understand the dynamics of coalescence, nonequilibrium simulations in which two water droplets of the same size were allowed to collide at different initial speeds, impact parameters, and collision angles. We classified three outcomes from each collision: coalescence, stretching separation, and shattering. As the impact parameter increased, the critical speed for the transition to stretching separation decreased, which is consistent to the trend found in the previous studies. We also discovered that the time required for thorough mixing between interfacial and bulk water molecules was shorter in head-on collisions, whereas thorough mixing took longer in off-center collision.With a better understanding of pure water droplet coalescence, we then moved on to investigate the coalescence between organic-coated droplets. We studied three different organic coatings formed by benzoic acid, heptanoic acid, and pimelic acid and compared these results with those for pure water droplets. Similar to the uncoated case, the free energy changes of coalescence were all negative, indicating the spontaneity of coalescence in the presence of the studied coatings. A decomposition of ΔF into ΔU and TΔS shows that ΔU was primarily determined by the flexibility and solubility in water of the organic compound, as well as the hydrogen bond network in the system. We reconfirmed the positive correlation between TΔS and the mobility of molecules. For the dynamics, we reported lower critical speeds for any transitions to separation/shattering in organic-coated cases than the uncoated droplets. We also reported the presence of the studied organic coatings tended to produce smaller water fragments when the collision resulted in shattering; such effects will likely have implications for the size distribution of particulate matter in the context of air pollution. Finally, we observed rotational motion only in off-center collisions in the uncoated case, but the presence of organic matter could induce rotational motion also in head-on collisions, changing the rotational dynamics of the process.The content of the following chapters is as follows: in Chapter 1, we give a brief introduction on the background of this research topic, as well as some discussion on the previous studies. Also, we briefly introduce the methodology used in the computer simulations. In Chapter 2, we give the simulation details on both equilibrium and nonequilibrium simulations. In Chapter 3 and Chapter 4, the results and discussion on pure water droplet coalescence and organic-coated water droplet coalescence are presented. Finally, the conclusions are in Chapter 5.In this thesis, most of the content originates from our two published papers: J. Phys. Chem. C. 122 (2018): 22975-22984 and J. Phys. Chem. C. 124 (2020): 8749-8757.液滴聚結不只是雲滴生長和雨滴形成的主要機制，它更控制著大氣水滴大小的分佈。有機物質在大氣中亦非常普遍，當中亞微型大氣氣溶膠中的有機物質佔整體高達18-70% 質量。這些有機物通常都在氣溶膠上形成塗層，並使液滴聚結過程變得更複雜。在本文，我們旨在通過分子動力學 (MD) 模擬理解有機物對液滴聚結的影響。在第一部分，我們首先嘗試了解在沒有有機物存在下的水滴聚結。通過進行準平衡模擬研究不同的液滴尺寸，我們發現聚結過程中的自由能變化ΔF與水分子數目的倒數之間的線性關係。我們進一步將ΔF分解為勢能ΔU和熵TΔS，以便闡明聚結過程中分子程度的細節。我們發現，由於聚結期間氫鍵數的變化，庫侖相互作用的變化主要控制了ΔU的變化。在獲得了ΔF與氫鍵數變化之間的線性關係後，我們從中得到每個氫鍵的ΔF，而計算結果亦與實驗結果一致。要對TΔS 趨勢作出完整解釋通常都是一件很困難的事，但是我們發現TΔS與水分子流動性之間存在正關係。為了理解聚結的動態，我們進行了非平衡模擬，當中兩個大小相同的水滴以不同的初始速度，撞擊參數和碰撞角度進行碰撞。我們將每次碰撞的三個結果分類為：聚結，拉伸分離和破碎。隨著撞擊參數增加，從聚結過渡至拉伸分離的臨界速度降低，這項觀察亦與先前研究中發現的趨勢一致。我們還發現，在對心碰撞中，界面水分子與本體水分子之間需要較短的時間徹底混合，而在偏心碰撞中，則需要較長的時間徹底混合。明白更多關於純水滴聚結的發現後，我們接著會討論有機塗層液滴之間的聚結。我們研究了分別由苯甲酸，庚酸和庚二酸形成的三種不同的有機塗層，並將這些結果與純水滴聚結的結果進行比較。這三種有機塗層的水滴聚結的自由能變化均為負，與純水滴聚結情況類似，說明了在這三種塗層存在下聚結的自發性。下一步，我們將 ΔF分解為ΔU和TΔS，當中ΔU主要由有機化合物在水中的彈性，溶解度，以及系統中的氫鍵網絡所決定。我們亦再次確認了TΔS與分子流動性之間的正關係。對於聚結的動態，我們發現在有機塗層的存在下，任何過渡至分離/破碎的臨界速度都比沒有塗層存在的情況下要低。我們還觀察到，當碰撞導致破碎時，有機塗層的存在往往會導致較小的水碎片產生。在空氣污染的存在下，這種影響可能會改變大氣懸浮微粒的尺寸分佈。最後，在無塗層情況下，我們只在偏心碰撞觀察到旋轉運動，但在有機物的存在下，對心碰撞也引起旋轉運動，改變了聚結過程中的旋轉動態。各章節的內容如下：在第一章節，我們簡要介紹本研究主題的背景，並對先前的研究進行一些討論。此外，我們亦會簡要介紹在電腦模擬中所使用的方法。在第二章節，我們會說明電腦模擬中，平衡和非平衡模擬的細節。在第三和第四章節，我們會討論純水滴聚結和有機塗層水滴聚結的結果。最後，我們會在第五章節進行總結。在本論文，大部分內容都源自我們兩份已發表的文章：J. Phys. Chem. C. 122 (2018): 22975-22984 以及 J. Phys. Chem. C. 124 (2020): 8749-8757.Pak, Chi Yuen.Thesis Ph.D. Chinese University of Hong Kong 2020.Includes bibliographical references (leaves 63-71).Abstracts also in Chinese.Title from PDF title page (viewed on February 17, 2022)