Polymer Engineering and Colloid Science Laboratory, Department of Chemical Engineering , Indian Institute of Technology Madras , Chennai 600036 , Tamil Nadu , India.
Langmuir. 2018 Sep 25;34(38):11473-11483. doi: 10.1021/acs.langmuir.8b02051. Epub 2018 Sep 11.
Patterns generated by controlled evaporation of droplets containing colloids are dictated by internally generated flows. This advective particle transport is crucial to the efficacy of printing and coating processes and is also an elegant route to the self-assembly of particles. We propose a novel particle transport route, which involves adsorption of particles to the interface and subsequent curvature-driven migration of the particles along the interface. This interface-mediated transport can be exploited to control the distribution of particles in the dried patterns, which we experimentally elucidate by achieving gravity-induced drop shape changes. Our experiments demonstrate that the interplay between the bulk and the interfacial transport leads to strikingly different patterns: while dried aqueous sessile drops of colloidal dispersions produce well known "coffee-rings", dried pendant drops lead to "coffee-eyes". We support our experimental findings using scaling arguments. In previous studies, the effect of gravity-induced change in drop shape on the patterns formed in drying drops has been neglected. However, we show that the structure of the patterns formed by the colloidal particles after solvent evaporation is markedly different when the drops are deformed by gravity.
受胶体控制的液滴蒸发模式由内部生成流决定。这种对流颗粒输运对印刷和涂层工艺的效率至关重要，也是颗粒自组装的一种优雅途径。我们提出了一种新的颗粒输运途径，涉及到颗粒吸附到界面上，以及随后颗粒在界面上沿着曲率驱动的迁移。这种界面介导的输运可以用来控制颗粒在干燥图案中的分布，我们通过实现重力诱导的液滴形状变化来实验阐明这一点。我们的实验表明，体相和界面输运之间的相互作用导致了截然不同的图案：尽管胶体分散体的干燥固着液滴产生了众所周知的“咖啡环”，但干燥的悬垂液滴会导致“咖啡眼”。我们使用标度分析来支持我们的实验发现。在之前的研究中，重力引起的液滴形状变化对干燥液滴中形成的图案的影响被忽略了。然而，我们表明，当液滴受到重力变形时，胶体颗粒在溶剂蒸发后形成的图案的结构明显不同。
