The engulfment of aqueous droplets on perfectly wetting oil layers
The engulfment of aqueous droplets on perfectly wetting oil layers
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DOI：
10.1017/jfm.2021.90
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发表时间：
2020-09
影响因子：
3.7
通讯作者：
Callum Cuttle;A. Thompson;D. Pihler-Puzović;A. Juel
中科院分区：
文献类型：
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作者：
Callum Cuttle;A. Thompson;D. Pihler-Puzović;A. Juel
将一滴矿物油滴在水面上，油就会在水面上形成一层薄膜。在本文中，我们研究了everted问题：一个水滴沉积在一层深层硅油上。由于在能量上有利于油相完全覆盖液滴表面，液滴最终被油层吞没。我们提出了一个详细的吞没动力学研究，从液滴撞击油表面的那一刻起，直到它最终沉积到密度较小的油中。我们研究了广泛的液滴尺寸（微米到毫米）和油的运动粘度（$10^2$ - $10^5$ cSt），对应于粘度主导的参数体系。我们发现，液滴的吞噬过程包括在油表面毛细力的驱动下，液滴迅速浸入油表面，随后液滴从界面上缓慢剥离，液滴通过早期形成的一层薄薄的油覆盖层粘附在界面上。后期剥离阶段是由表观接触线处的几何约束和液滴的重力牵引共同驱动的。因此，引力效应对于完全吞噬是必不可少的，即使对于微小的液滴也是如此。此外，几何和重力的相反作用导致中等大小的液滴吞没时间最长。在固定液滴尺寸下的实验表明，吞噬时间与油的运动粘度呈幂律关系，我们认为这反映了油遮蔽层的动力学形成。
Abstract Place a droplet of mineral oil on water and the oil will spread to cover the water surface in a thin film. In this paper we study the everted problem: an aqueous droplet deposited onto a deep layer of silicone oil. As it is energetically favourable for the oil phase to spread to cover the droplet surface completely, the droplet is ultimately engulfed in the oil layer. We present a detailed study of engulfment dynamics, from the instant the droplet impacts the oil surface until it finally sediments into the less dense oil. We study a broad range of droplet sizes (micrometric to millimetric) and oil kinematic viscosities ($10^2$–$10^5$ cSt), corresponding to a viscosity-dominated parameter regime. We find that droplet engulfment involves the rapid submersion of the droplet driven by capillary forces in the oil surface, followed by the much slower peeling of the droplet from the interface, to which it is adhered via a thin cloaking layer of oil formed during the earlier stage. The later peeling stage is driven by a combination of geometric constraints at the apparent contact line and gravity pulling on the droplet. Gravitational effects are therefore essential to complete engulfment, even for micrometric droplets. Furthermore, the opposing effects of geometry and gravity result in the longest engulfment times for droplets of intermediate size. Experiments at fixed droplet size reveal a power law dependence of engulfment time on oil kinematic viscosity, which we argue reflects the dynamical formation of the oil cloaking layer.