Evaporation of Initially Heated Sessile Droplets and the Resultant Dried Colloidal Deposits on Substrates Held at Ambient Temperature
Evaporation of Initially Heated Sessile Droplets and the Resultant Dried Colloidal Deposits on Substrates Held at Ambient Temperature
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DOI：
10.1021/acs.langmuir.0c00756
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发表时间：
2020-07-28
期刊：
影响因子：
3.9
通讯作者：
Bhardwaj, Rajneesh
中科院分区：
文献类型：
--
作者：
Chatterjee, Sanghamitro;Kumar, Manish;Bhardwaj, Rajneesh
本研究实验和数值研究的蒸发和由此产生的图案的干燥沉积物的含水胶体固着液滴时，液滴最初被提升到一个高温，然后被放置在基板上保持在环境温度。然后释放系统进行自然蒸发，而不施加任何外部扰动。红外热成像法和光学轮廓作为重要的工具，界面温度测量和咖啡环尺寸的量化，分别。最初，一旦液滴沉积在衬底上，就沿液-气界面沿着存在显著的温度梯度，这触发了从液滴顶部沿沿着液-气界面朝向接触线引导的马兰戈尼应力诱导的再循环流。因此，流动方向与常规衬底加热情况下所见的方向相反。有趣的是，这种温度梯度在总蒸发时间的前10%内迅速衰减，此后液滴-基底系统与环境达到热平衡。尽管温度梯度快速衰减，但咖啡环尺寸显著减小，导致内部存款。对于悬浮颗粒浓度在0.05和1.0%v/v之间变化，通过将初始液滴温度从25 ℃升高到75 ℃，记录到咖啡环尺寸减小50-70%。咖啡环效应的这种抑制归因于以下事实：初始Marangoni应力引起的再循环流持续到蒸发的最后阶段，即使在界面温度梯度消失之后。这基本上是液体惯性的结果。最后，一个基于有限元的二维模型在轴对称的几何形状被发现捕获的测量与合理的保真度和假设，在本研究中考虑证实以及与第一近似定性尺度分析。总体而言，连同一个新的实验条件下，本研究揭示了一个独特的性质Marangoni应力诱导流在干燥液滴和其作用，影响相关的胶体沉积物，这是以前没有探索。从这项研究中获得的见解是有用的，以推进技术应用，如喷雾冷却，喷墨打印，生物测定等。
The present study experimentally and numerically investigates the evaporation and resultant patterns of dried deposits of aqueous colloidal sessile droplets when the droplets are initially elevated to a high temperature before being placed on a substrate held at ambient temperature. The system is then released for natural evaporation without applying any external perturbation. Infrared thermography and optical profilometry are used as essential tools for interfacial temperature measurements and quantification of coffee-ring dimensions, respectively. Initially, a significant temperature gradient exists along the liquid-gas interface as soon as the droplet is deposited on the substrate, which triggers a Marangoni stress-induced recirculation flow directed from the top of the droplet toward the contact line along the liquid-gas interface. Thus, the flow is in the reverse direction to that seen in the conventional substrate heating case. Interestingly, this temperature gradient decays rapidly within the first 10% of the total evaporation time and the droplet-substrate system reaches thermal equilibrium with ambient thereafter. Despite the fast decay of the temperature gradient, the coffee-ring dimensions significantly diminish, leading to an inner deposit. A reduction of 50-70% in the coffee-ring dimensions is recorded by elevating the initial droplet temperature from 25 to 75 degrees C for suspended partide concentration varying between 0.05 and 1.0% v/v. This suppression of the coffee-ring effect is attributed to the fact that the initial Marangoni stress-induced recirculation flow continues until the last stage of evaporation, even after the interfacial temperature gradient vanishes. This is essentially a consequence of liquid inertia. Finally, a finite-element-based two-dimensional modeling in axisymmetric geometry is found to capture the measurements with reasonable fidelity and the hypothesis considered in the present study corroborates well with a first approximation qualitative scaling analysis. Overall, together with a new experimental condition, the present investigation discloses a distinct nature of Marangoni stress-induced flow in a drying droplet and its role in influencing the associated colloidal deposits, which was not explored previously. The insights gained from this study are useful to advance technical applications such as spray cooling, inkjet printing, bioassays, etc.