The Fluid Dynamics of Disease Transmission Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Phys Rev Lett. 2018 May 18;120(20):204503. doi: 10.1103/PhysRevLett.120.204503.
Unsteady fragmentation of a fluid bulk into droplets is important for epidemiology as it governs the transport of pathogens from sneezes and coughs, or from contaminated crops in agriculture. It is also ubiquitous in industrial processes such as paint, coating, and combustion. Unsteady fragmentation is distinct from steady fragmentation on which most theoretical efforts have been focused thus far. We address this gap by studying a canonical unsteady fragmentation process: the breakup from a drop impact on a finite surface where the drop fluid is transferred to a free expanding sheet of time-varying properties and bounded by a rim of time-varying thickness. The continuous rim destabilization selects the final spray droplets, yet this process remains poorly understood. We combine theory with advanced image analysis to study the unsteady rim destabilization. We show that, at all times, the rim thickness is governed by a local instantaneous Bond number equal to unity, defined with the instantaneous, local, unsteady rim acceleration. This criterion is found to be robust and universal for a family of unsteady inviscid fluid sheet fragmentation phenomena, from impacts of drops on various surface geometries to impacts on films. We discuss under which viscous and viscoelastic conditions the criterion continues to govern the unsteady rim thickness.
不稳定的流体整体破碎成液滴对于流行病学很重要，因为它控制着从打喷嚏和咳嗽中病原体的传播，或者从农业中受污染的作物中传播。它在工业过程中也无处不在，如油漆、涂料和燃烧。与迄今为止大多数理论工作都集中在的稳定破碎不同，不稳定破碎是独特的。我们通过研究一个典型的不稳定破碎过程来解决这一差距：液滴冲击有限表面时的破碎，其中液滴流体转移到具有时变特性的自由扩展片上，并由时变厚度的边缘限制。连续的边缘不稳定性选择最终的喷雾液滴，但这一过程仍然知之甚少。我们将理论与先进的图像分析相结合，研究不稳定的边缘不稳定性。我们表明，在任何时刻，边缘厚度都由一个局部瞬时的邦德数控制，邦德数等于 1，用瞬时的、局部的、不稳定的边缘加速度来定义。发现该准则对于一系列不稳定无粘性流体片破碎现象是稳健且通用的，包括液滴冲击各种表面几何形状和冲击薄膜的现象。我们讨论了在哪些粘性和粘弹性条件下，该准则继续控制不稳定的边缘厚度。
