| Abstract: | 液滴蒸發的應用範圍相當廣泛，包括晶體陣列、噴墨印刷、噴霧冷卻、薄膜沉積、蛋白質結晶等各個領域。固著液滴的蒸發行為是隨時間變化的動態系統，其模式會受到基板溫度、基板表面粗糙度、環境溫溼度、液滴溶質成分和液滴接觸角等的影響，進而在液滴內部形成不同流場，最終導致不同的沉積圖案。本研究的主要目的為探究在常溫下的奈米流體親水液滴，加入不同濃度的表面活性劑對液滴的初期乾燥過程行為的影響。
本研究以數值模擬的方式，發展可以預測液滴內部流動的簡易模型，透過調整奈米粒子密度、奈米粒子粒徑和增加表面活性劑濃度等方法，進一步提供抑制咖啡環效應的解決方案。由模擬結果可知，親水液滴的水蒸氣在三相接觸線區域擴散較快，因此此處的蒸發通量最大並朝液滴頂點方向蒸發通量逐漸遞減。當液滴內部無添加表面活性劑，其流場為浮力驅動的瑞利流，奈米粒子容易堆積在接觸線區域，透過增加奈米粒子密度和調整奈米粒子粒徑均可以抑制咖啡環效應。當液滴內部加入表面活性劑，其流場為表面張力驅動的馬蘭哥尼流，奈米粒子被帶離接觸線區域，而加入越高濃度的表面活性劑，液滴抑制咖啡環效應的能力越強。
關鍵字：固著液滴、蒸發、瑞利流、馬蘭哥尼流、咖啡環;The application scope of droplet evaporation is extensive, encompassing various fields such as crystal arrays, inkjet printing, spray cooling, thin film deposition, protein crystallization, and more. The evaporation behavior of sessile droplets is a dynamic system. Factors such as substrate temperature, surface roughness, environmental temperature and humidity, solute composition within the droplet, and the contact angle influence the droplet’s deposition pattern. These factors give rise to different flows within the droplet, ultimately leading to distinct deposition patterns. The primary objective of this study is to investigate the short-time behavior of hydrophilic droplets of nanofluid evaporated at room temperature with varying concentrations of surfactants.
This study develops a simplified model for predicting the internal flow of the droplets for numerical simulation. By adjusting parameters such as nanoparticle density, nanoparticle size, and surfactant concentration, this study aims to provide a solution reference for mitigating the coffee ring effect. According to the simulation results, the water vapor from the hydrophilic droplets diffuses faster in the three-phase contact region. Consequently, the highest evaporation flux occurs in this region, gradually decreasing towards the droplet apex. In cases with no surfactant added to the droplet, the buoyancy-driven Rayleigh convection dominated the flow field, leading to nanoparticle accumulation in the contact line region. Increasing nanoparticle density and adjusting nanoparticle size can suppress the coffee ring effect. Conversely, when surfactants are added to the droplet, the flow field is governed by surface tension-driven Marangoni flow, causing the nanoparticle migration away from the contact line region. Higher surfactant concentrations enhance the mitigation of the coffee ring effect.
Keywords: sessile droplets, evaporation, Rayleigh flow, Marangoni flow, coffee ring |
