Drying of Droplets of Colloidal Suspensions on Rough Substrates
Drying of Droplets of Colloidal Suspensions on Rough Substrates
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DOI：
10.1021/acs.langmuir.7b02341
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发表时间：
2017-09-26
期刊：
影响因子：
3.9
通讯作者：
Kumar, Satish
中科院分区：
文献类型：
--
作者：
Truong Pham;Kumar, Satish
在许多技术应用中，必须从液滴中除去过量的溶剂以将溶质存款到基底上。通常，液滴停留在其上的基底可能具有一些粗糙度，无论是有意的还是无意的。出于这些观察，我们提出了一个润滑理论为基础的模型来研究干燥的胶体悬浮液滴在含有地形缺陷的基板上。该模型由一个系统的一维偏微分方程占液滴的形状和深度平均浓度的胶体颗粒。的前体膜和分离压力被用来描述的接触线区域，和蒸发包括使用众所周知的单侧模型。微差解表明，当胶体颗粒不存在时，由于毛细压力梯度和分离压力梯度之间的平衡，液滴接触线可以在干燥时间的很大一部分固定在缺陷上。液滴半径和接触角的时间演化表现出在先前的实验中已经观察到的恒定半径和恒定接触角阶段。当胶体颗粒存在且缺陷不存在时，该模型预测颗粒将以锥形图案沉积在液滴中心附近。然而，当存在缺陷时，接触线的钉扎加速液滴固化，导致颗粒在液滴边缘附近以咖啡环图案沉积。这些预测与以前的实验观察是一致的，并说明了关键作用的接触线钉扎在控制干燥液滴的动力学。
In many technological applications, excess solvent must be removed from liquid droplets to deposit solutes onto substrates. Often, the substrates on which the droplets rest may possess some roughness, either intended or unintended. Motivated by these observations, we present a lubrication-theory-based model to study the drying of droplets of colloidal suspensions on a substrate containing a topographical defect. The model consists of a system of one-dimensional partial differential equations accounting for the shape of the droplet and depth-averaged concentration of colloidal particles. A precursor film and disjoining pressure are used to describe the contact-line region, and evaporation is included using the well-known one-sided model. Finite-difference solutions reveal that when colloidal particles are absent, the droplet contact line can pin to a defect for a significant portion of the drying time due to a balance between capillary-pressure gradients and disjoining-pressure gradients. The time-evolution of the droplet radius and contact angle exhibits the constant-radius and constant-contact-angle stages that have been observed in prior experiments. When colloidal particles are present and the defect is absent, the model predicts that particles will be deposited near the center of the droplet in a cone-like pattern. However, when a defect is present, pinning of the contact-line accelerates droplet solidification, leading to particle deposition near the droplet edge in a coffee-ring pattern. These predictions are consistent with prior experimental observations, and illustrate the critical role contact-line pinning plays in controlling the dynamics of drying droplets.