Microfluidics Laboratory, Department of Mechanical Engineering, IIT Madras, Chennai, 600036, India.
Soft Matter. 2018 Dec 12;14(48):9901-9909. doi: 10.1039/c8sm01915k.
Understanding the formation of different morphological patterns depending on the particle size and surface wettability has great relevance in the separation, mixing and concentration of micro/nano particles and biological entities. We report the evaporation and morphological patterns of evaporating bi-dispersed colloidal droplets on hydrophilic and hydrophobic surfaces. To explain the underlying mechanisms of various particle distribution patterns, we propose a phenomenological model that accounts for the drag force, van der Waals and electrostatic interaction forces, and surface tension force acting on the particles. In the case of the hydrophilic surface (θ ∼ 27°), there is a competition between the frictional force arising due to the van der Waals (∼10-8 N) and electrostatic interaction forces (∼10-10 N) and the surface tension force (∼10-7 N) that depends on the particle size. Consequently, the smaller particles (0.2 and 1.0 μm in diameter) are found to be pinned and form an outer ring at the contact line whereas the larger particles (3.0 and 6.0 μm in diameter) move inward, either forming an inner ring or flocculating depending on the particle size. Interestingly, a completely different morphological pattern of the micro/nano particles is observed on a hydrophobic substrate (θ ∼ 110°): contact line pinning is no longer observed and particles form a centralized deposition pattern. The order of the magnitude of the surface tension force is higher as compared to the frictional force (∼10-8 N); thus the particles are driven radially inward and accumulate at the center of the droplet. Owing to the mixed mode of evaporation toward the end of evaporation, only a fraction of smaller particles travel radially outward due to the coffee-ring effect. Scanning electron microscopy images reveal that smaller particles are present mostly at the center with a small fraction of smaller particles at the edge of the pattern, whereas larger particles are uniformly distributed throughout.
理解不同形态模式的形成取决于颗粒大小和表面润湿性，这在微/纳米颗粒和生物实体的分离、混合和浓缩方面具有重要意义。我们报告了在亲水和疏水表面上蒸发的双分散胶体液滴的蒸发和形态模式。为了解释各种颗粒分布模式的潜在机制，我们提出了一个唯象模型，该模型考虑了作用在颗粒上的阻力、范德华力和静电力以及表面张力。在亲水表面（θ∼27°）的情况下，由于范德华力（∼10-8 N）和静电力（∼10-10 N）产生的摩擦力与表面张力（∼10-7 N）之间存在竞争，因此较小的颗粒（直径为 0.2 和 1.0 μm）被固定并在接触线处形成外环，而较大的颗粒（直径为 3.0 和 6.0 μm）则向内移动，根据颗粒大小形成内环或絮状物。有趣的是，在疏水基底（θ∼110°）上观察到微/纳米颗粒的完全不同的形态模式：接触线固定不再观察到，颗粒形成集中沉积模式。表面张力的量级比摩擦力（∼10-8 N）高，因此颗粒被径向向内驱动并在液滴中心积累。由于蒸发接近尾声时的混合蒸发模式，只有一小部分较小的颗粒由于咖啡环效应而径向向外移动。扫描电子显微镜图像显示，较小的颗粒主要存在于中心，而较小的颗粒在图案的边缘处有一小部分，而较大的颗粒则均匀分布。
