State Key Laboratory of Engines, Tianjin University, Tianjin, 300072, China.
Soft Matter. 2018 Feb 28;14(9):1540-1551. doi: 10.1039/c7sm02089a.
The impact of droplets on liquid films is a ubiquitous phenomenon not only in nature but also in many industrial applications. Compared to the widely-studied impact of droplets on films of identical fluids, the impact of droplets on immiscible films has received far less attention. In the present work, we show using high-speed imaging that immiscibility has a profound effect on the impact dynamics. The impact of a water droplet on an oil film leads to the formation of a compound crown followed by a central jet, whereas that of an oil droplet on a water film results in rapid spreading on the film surface driven by a large, positive spreading factor. In the former scenario, the central jet occurs due to the severe stretching of the droplet during the formation of the crown and then the retraction of the droplet by capillarity, which leads to the collision of fluid at the impact point. A model for the elongation dynamics of the central jet is proposed based on energy conservation. The effects of key parameters controlling the impact process are analysed, including the droplet Ohnesorge and Weber numbers, the viscosity ratio, and the dimensionless film thickness. Different impact outcomes are discussed, such as bouncing, deposition, and oscillation of the impact droplet, the formation and collapse of the compound crown, and the formation and tip-pinching of the central jet. This study not only provides physical insights into the impact dynamics, but could also facilitate the control and optimisation of the droplet impact process in a number of applications as highlighted herein.
液滴对液膜的冲击不仅在自然界中普遍存在，而且在许多工业应用中也普遍存在。与广泛研究的液滴对同相流体膜的冲击相比，液滴对不混溶膜的冲击受到的关注要少得多。在本工作中，我们通过高速摄像显示，不混溶性对冲击动力学有深远的影响。水滴滴在油膜上会形成复合冠，随后形成中心射流，而油滴滴在水膜上则会因大的正扩展因子驱动而在膜表面迅速扩展。在前一种情况下，中心射流是由于冠形成过程中液滴的严重拉伸以及随后由毛细作用引起的液滴回缩而产生的，这导致了冲击点处的流体碰撞。根据能量守恒，提出了一个用于描述中心射流伸长动力学的模型。分析了控制冲击过程的关键参数的影响，包括液滴的欧内斯特数和韦伯数、粘度比和无量纲膜厚。讨论了不同的冲击结果，如冲击液滴的反弹、沉积和振荡、复合冠的形成和崩塌以及中心射流的形成和尖端压扁。本研究不仅为冲击动力学提供了物理见解，而且还可以为许多应用中的液滴冲击过程的控制和优化提供便利，如文中所述。
