Splash: Understanding the Dynamics of High-Speed Drop Impact

毛细管驱动的自由表面流动动力学，以及湍流行为，可以说是流体动力学中最具挑战性的话题之一。它们涉及跨越多个长度和时间尺度的复杂流。液滴形成和解体过程中涉及的动态流体过程是迷人的，但非常复杂，具有时间依赖性的流体界面破坏。水滴撞击表面（固体、液体或颗粒状）会导致简单的扩散、弹跳或飞溅。所产生的动力学不仅取决于液体的性质和液滴的速度，还取决于各种其他参数，如表面的粗糙度、刚度、化学性质、温度和周围条件。虽然在某些工业应用中需要溅水（例如冷却和燃烧），但在其他应用中（例如喷墨打印或预防Covid-19大流行所显示的传染病传播），应不惜一切代价避免溅水。对于我的研究，我目前正在使用先进的数学技术，结合最先进的计算能力，以及实验成像技术的技术进步，这使我能够以前所未有的细节和前所未有的速度观察和模拟动力学。在水滴领域，溅水是最吸引人的话题之一，尽管具有挑战性。然而，引发水花的确切机制仍然难以捉摸。借助尖端的超高速摄影，现代超高分辨率数值模拟和渐近理论，提出的研究的主要目的是揭示水花背后和触发的动力学。特别是，我们的目标是：1-确定当液滴撞击混相和非混相流体池时导致飞溅的参数。这包括开发能够在这些剧烈条件下求解三相流的模型。2-显示碰撞时接触线（落/池）的（垂直）速度。初步结果表明，液滴和池中液体的密度和粘度之比之间的复杂关系在这里起着重要作用。3-了解由于液滴和目标的纯粘性和粘弹性效应的贡献。探索目标的曲率对产生的动力学的影响。在此基础上，探索抑制溅水的技巧。本研究的第一部分（1和2）已经进行了一系列关于液滴对不同粘度（不混溶）基质影响的系统实验。目前正在进行流体体积模拟，它提供了实验无法提供的细节（例如内部速度场），以便更好地理解潜在的动力学以及这是如何导致飞溅的。虽然上述研究领域主要关注液体与液体碰撞时液滴的运动，但池本身的运动也是一个非常有趣的运动，但经常被忽视，特别是在碰撞的早期。虽然存在一些用于池运动的模型，但这些模型在很大程度上仅限于液滴和池流体是相同流体的情况，并且通常在不合适的情况下使用。因此，另一个需要研究的领域是撞击前后池的运动，以及池的运动如何受到相关流体性质的影响，以及池的运动如何影响飞溅。这些目标是主要的事业，并与华威大学的R. Cimpeanu博士密切合作。EPSRC的研究领域包括流体动力学和空气动力学、复杂流体和流变学，以及在喷墨打印等工业应用中具有重要意义的未来制造。

The dynamics of capillary-driven free surface flows, together with turbulent behaviour are, arguably, some of the most challenging topics in fluid dynamics. They involve complex flows evolving across several length and time scales. The dynamic fluid processes involved during drop formation and disintegration are fascinating but extremely complicated, with time-dependent fluid interface disruptions. A drop impacting on to a surface (solid, liquid, or granular) can lead to simple spreading, bouncing, or splashing. The resulting dynamics depends not only on the liquid properties and speed of the drop, but on a variety of other parameters, such as the surface's roughness, stiffness, chemistry, and temperature, and surrounding conditions. While in some industrial applications splashing is desired (e.g. cooling and combustion), it is to be avoided at all cost in others (e.g. inkjet printing or in the prevention of the spreading of infectious diseases as the Covid-19 pandemic has shown). For my research, I am currently using advanced mathematical techniques combined with state-of-the-art computational power, and technological advances in experimental imaging techniques, which allow me to observe and model the dynamics in unprecedented detail and at unprecedented speeds.Splashing is one of the most fascinating, albeit challenging, topics in the field of drops. However, the exact mechanisms triggering a splash have remained elusive. With the help of cutting-edge ultrahigh speed photography, modern ultrahigh resolution numerical simulations and asymptotic theory the main objective of the proposed research is to reveal the dynamics underlying and triggering a splash. In particular, we aim at: 1- Identifying the parameters leading to a splash in when a droplet impacts a pool of miscible and immiscible fluids. This includes developing models capable of resolving three phase flows under these violent conditions.2- Reveal the (vertical) speed of the contact line (drop/pool) upon impact. Preliminary results demonstrate that a complex relationship between the ratio of densities and viscosities between the liquids of the drop and the pool play an important role here.3- Understand the contributions due to pure viscous and viscoelastic effects of both the drop and target. 4- Explore the influence that the curvature of the target has on the resulting dynamics.5- Based on the above, explore techniques to suppress splashing. The first part (1 and 2) of this this research has already been performed carrying out a series of systematic experiments of the impact of drops onto (immiscible) substrates of varying viscosity. Volume of Fluid simulations are currently being carried out which are providing detail not available from experiments (e.g. internal velocity fields) to enable greater understanding of the underlying dynamics and how this can lead to splashing. Whilst the aforementioned area of research is largely concerned with the motion of the droplet in the case of liquid on liquid impact the motion of the pool itself is also one of great interest but is often overlooked, especially in the early times upon impact. Whilst some models exist for the pool motion, these are largely limited to when the droplet and pool fluid are the same fluid and are often used when they are often not appropriate. For this reason, another area to be researched is the motion of the pool both before and after impact and how this pool motion is affected by the relevant fluid properties and how this pool motion can affect splashing. These objectives are major undertakings, and a close collaboration with Dr. R. Cimpeanu at the University of Warwick.The proposed research falls within the EPSRC areas of Fluid Dynamics and Aerodynamics, Complex Fluids and Rheology, as well as Manufacturing the Future for the importance identified above in industrial applications such as inkjet printing.