Kim Jin Young, Gonçalves Marta, Jung Narina, Kim Hyoungsoo, Weon Byung Mook
Research Center for Advanced Materials Technology, Sungkyunkwan University, Suwon, 16419, South Korea.
Department of Materials, ETH Zürich, 8093, Zurich, Switzerland.
Sci Rep. 2021 Sep 7;11(1):17784. doi: 10.1038/s41598-021-97256-w.
Colloidal droplets on flat solid substrates commonly leave symmetric ring-like deposits due to coffee-ring flows during evaporation. On inclined substrates, droplet shapes may become asymmetric by gravity. On this basis, it is not clear how their evaporation dynamics and final deposits are changed depending on inclination. Here we explore evaporation and deposition dynamics of colloidal droplets on inclined substrates, mainly by controlling colloidal particle size, substrate inclination, and relative humidity, which are crucial to gravitational intervention and evaporation dynamics. We experimentally investigate two different flows with opposite directions: downward sedimentation flows by gravity ([Formula: see text]) and upward capillary flows by evaporation ([Formula: see text]). We find that the competition of two flows determines the formation of final deposits with a flow speed ratio of [Formula: see text]. Notably, for [Formula: see text] [Formula: see text] 1, evaporation-driven upward flows overwhelm sedimentation-driven downward flows, resulting in accentuated particle movement towards the top ring, which seems to defy gravitational intervention. We suggest a possible explanation for the flow speed dependence of final deposits in evaporating colloidal droplets. This study offers a framework to understand the intervention of inclination to the formation of final deposits and how to overcome the deposit pattern radial asymmetry, achieving symmetric deposit widths from inclined colloidal droplets.
平面固体基底上的胶体液滴在蒸发过程中由于咖啡环流通常会留下对称的环状沉积物。在倾斜基底上，液滴形状可能会因重力而变得不对称。在此基础上，尚不清楚它们的蒸发动力学和最终沉积物如何随倾斜度而变化。在这里，我们主要通过控制对重力干预和蒸发动力学至关重要的胶体颗粒大小、基底倾斜度和相对湿度，来探索倾斜基底上胶体液滴的蒸发和沉积动力学。我们通过实验研究了两种方向相反的不同流动：重力作用下的向下沉降流（[公式：见原文]）和蒸发作用下的向上毛细流（[公式：见原文]）。我们发现两种流动的竞争决定了最终沉积物的形成，其流速比为[公式：见原文]。值得注意的是，当[公式：见原文][公式：见原文]1时，蒸发驱动的向上流动压倒了沉降驱动的向下流动，导致颗粒向顶部环的运动加剧，这似乎无视了重力干预。我们对蒸发胶体液滴中最终沉积物的流速依赖性提出了一种可能的解释。这项研究提供了一个框架，以理解倾斜对最终沉积物形成的干预以及如何克服沉积物图案的径向不对称性，从而实现倾斜胶体液滴的对称沉积宽度。
