| 摘要: | 液滴蒸發的現象無所不在，其應用範圍亦相當廣泛，如薄膜沉積、DNA晶片、噴墨列印、晶體陣列、噴霧冷卻等領域。固著液滴的蒸發過程為暫態系統，其行為受基板濕潤性、液滴或基板溫度、溶質濃度等條件影響，於液滴內部產生多變的流動模式。
本文以數值計算的方式，模擬膠體固著液滴的蒸發過程，透過改變初始液滴溫度的方式，探討溫度與流場分佈隨時間的演化，提供非揮發性成分的液滴的流動現象參考。結果顯示，初始蒸發時間於液氣介面附近因蒸發通量不均，產生數處溫度極值造成多個渦流的產生，隨後逐漸整合成兩個熱馬蘭根尼渦流。當液滴初始溫度高於常溫時，液滴內出現兩個流動階段：週期性流動與過渡期流動。週期演變為，液滴內部出現兩個流向相反的馬蘭根尼渦流競爭，隨後逐漸由下方沿液氣介面流向頂部的馬蘭根尼流主導整個液滴流動，而後再次分裂成兩個馬蘭根尼渦流開始下一輪的循環。單次週期於早、中、晚期所需時間隨液滴初始溫度升高而縮短，週期次數也越多，從週期行為進入過渡期的時間也越晚。蒸發通量隨液滴初始溫度升高，不均勻分佈越強，隨液滴溫度下降，不均勻性逐漸減弱。過渡期間，溫度逐漸接近常溫，週期性流動模式逐漸消失，形成上下雙熱馬蘭根尼渦流持續存在於液滴內競爭，此競爭最終分裂為三個渦流，由位於中間的渦流逐漸主導，將液滴底部流體沿液氣介面逐漸帶往頂部，最終形成常溫液滴流動模式，結束過渡期流動。此時期的結束時間隨液滴初始溫度升高而延後。PS濃度場於升溫液滴中因蒸發通量不均造成多個局部濃度極值，多個渦流將部分PS往液滴內移動，卻受限於液氣介面附近循環；隨後液滴進入週期性流動，PS濃度聚集於頂部，並受限於下方熱馬蘭根尼渦流的週期性流動。進入過渡期後，PS濃度產生兩處較高區分布於兩個渦流中，持續至最終轉變為常溫液滴流動模式的濃度場分布。;The phenomenon of droplet evaporation is ubiquitous and has wide-ranging applications, such as thin film deposition, DNA chips, inkjet printing, crystal arrays, and spray cooling. The evaporation process of sessile droplets is a transient system, and its behavior is influenced by factors such as the substrate wettability, temperature difference due to droplet or substrate, solute concentration, etc. These factors create varied flow patterns inside the droplets.
This study built numerical model to simulate the evaporation process of colloidal sessile droplets by varying the initial droplet temperature, the evolution of temperature and flow field distribution over time are investigated, providing a reference for the flow pattern of non-volatile component droplets. The results show that during the initial evaporation time, multiple vortices are generated near the liquid-gas interface due to non-uniform evaporation flux, which gradually integrate into a pair of thermal Marangoni vortices. When the initial droplet temperature is above ambient temperature, two flow periods are observed: periodic flow field and transition flow field. The periodic behavior involves the competition between the upper and lower thermal Marangoni vortices, eventually leading to a dominant upward Marangoni flow along the liquid-gas interface, which then splits into a new cycle of vortices. The cycle time shortens as the initial temperature of the droplet increases, and the greater the number of cycles, the later the transition from periodic behavior to the transition period. The evaporation flux becomes more uneven as the initial temperature rises and gradually becomes more uniform as the droplet temperature drops. During the transition period, the droplet temperature approaches to the ambient, and the periodic flow fades, the upper and lower Marangoni vortices compete continuously in the droplet. Eventually, these vortices split into three, with the central vortex gradually dominating and carrying the fluid from the bottom to the top along the liquid-gas interface, forming a steady ambient temperature flow pattern and concluding the transition period. The end of this period is delayed as the initial droplet temperature increases. The PS concentration field in the heated droplets causes multiple local concentration extremes due to uneven evaporation flux. Multiple vortices move part of the PS into the droplets, but are limited by circulation near the liquid-gas interface; then the droplets enter a periodic flow, the PS concentration accumulates at the apex and is restricted by the periodic flow of the thermal Marangoni vortices below. After entering the transition period, the PS concentration produces two higher areas distributed in the two vortices, which last until it finally transforms into a concentration field distribution of ambient temperature flow pattern. |
