Chatterjee Sanghamitro, Murallidharan Janani Srree, Bhardwaj Rajneesh
Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India.
Langmuir. 2022 May 17;38(19):6128-6147. doi: 10.1021/acs.langmuir.2c00492. Epub 2022 May 4.
We experimentally and theoretically investigate a distinct problem of spreading, evaporation, and the associated dried deposits of a colloidal particle-laden aqueous sessile droplet on a surface in a saturated alcohol vapor environment. In particular, the effect of particle size on monodispersed suspensions and efficient self-sorting of bidispersed particles have been investigated. The alcohol vapor diffuses toward the droplet's curved liquid-vapor interface from the far field. The incoming vapor mass flux profile assumes a nonuniform pattern across the interface. The alcohol vapor molecules are adsorbed at the liquid-vapor interface, which eventually leads to absorption into the droplet's liquid phase due to the miscibility. This phenomenon triggers a liquid-vapor interfacial tension gradient and causes a reduction in the global surface tension of the droplet. This results in a solutal Marangoni flow recirculation and spontaneous droplet spreading. The interplay between these phenomena gives rise to a complex internal fluid flow within the droplet, resulting in a significantly modified and strongly particle-size-dependent dried colloidal deposit. While the smaller particles form a multiple ring pattern, larger particles form a single ring, and additional "patchwise" deposits emerge. High-speed visualization of the internal liquid-flow revealed that initially, a ring forms at the first location of the contact line. Concurrently, the Marangoni flow recirculation drives a collection of particles at the liquid-vapor interface to form clusters. Thereafter, as the droplet spreads, the smaller particles in the cluster exhibit a "jetlike" outward flow, forming multiple ring patterns. In contrast, the larger particles tend to coalesce together in the cluster, forming the "patchwise" deposits. The widely different response of the different-sized particles to the internal fluid flow enables an efficient sorting of the smaller particles at the contact line from bidispersed suspensions. We corroborate the measurements with theoretical and numerical models wherever possible.
我们通过实验和理论研究了一个独特的问题，即在饱和酒精蒸汽环境中，含胶体颗粒的水性 sessile 液滴在表面上的扩散、蒸发以及相关的干燥沉积物。特别研究了粒径对单分散悬浮液的影响以及双分散颗粒的有效自分选。酒精蒸汽从远场向液滴的弯曲液 - 气界面扩散。进入的蒸汽质量通量分布在整个界面上呈现出不均匀的模式。酒精蒸汽分子吸附在液 - 气界面，由于互溶性，最终导致被吸收到液滴的液相中。这种现象引发了液 - 气界面张力梯度，并导致液滴的整体表面张力降低。这导致了溶质马兰戈尼流再循环和液滴的自发扩散。这些现象之间的相互作用在液滴内产生了复杂的内部流体流动，导致干燥的胶体沉积物发生显著改变且强烈依赖于粒径。较小的颗粒形成多个环形图案，较大的颗粒形成单个环形，并且出现额外的“斑块状”沉积物。内部液体流动的高速可视化显示，最初，在接触线的第一个位置形成一个环。同时，马兰戈尼流再循环驱使液 - 气界面处的颗粒聚集形成团簇。此后，随着液滴扩散，团簇中的较小颗粒表现出“喷射状”向外流动，形成多个环形图案。相比之下，较大的颗粒倾向于在团簇中聚结在一起，形成“斑块状”沉积物。不同大小的颗粒对内部流体流动的广泛不同响应使得能够从双分散悬浮液中在接触线处有效地分选较小的颗粒。我们尽可能用理论和数值模型来证实这些测量结果。
