A study of the dynamics of drop impact: Impact forces, pressure and shear stress distributions

从打在挡风玻璃上的雨滴到溅在桌子上的咖啡，水滴的影响在我们的日常生活中无处不在，与许多重要的自然和工业过程有关。虽然这种现象已经研究了几十年，但由于典型的快速冲击过程的复杂性，对跌落冲击的理解仍然远远不够。近年来，利用高速摄影技术对跌落冲击的研究取得了重大进展。依靠直接成像，大多数实验都集中在液滴撞击的运动学上，即撞击液滴的形状。很少有研究探究跌落冲击的动力特性，如冲击力、压力和剪应力。这些动态量导致了撞击事件的最重要结果，并影响了广泛的现象，包括土壤侵蚀、撞击引起的磨损/损坏以及暴露在这些元素中的生物的生存。本研究的目的是对不同条件下跌落冲击的动力学方面进行系统的研究。该研究将开发一种新的实验技术——三维高速应力显微镜，以高空间和时间分辨率测量跌落冲击的冲击力、压力和剪应力。该项目将提供对未开发地区的跌落冲击动力学的基本理解，并提供控制冲击力和减轻冲击对固体基材造成的损伤的实用指南。该项目还将提供一个独特的机会，在本科课程中引入前沿研究，并与涂料公司建立合作关系。本研究将采用牵引力显微镜与高速摄影相结合的方法，构建三维高速应力显微镜，测量快速冲击过程中冲击落点下压力和剪应力分布的时间变化。利用这项新技术，该项目将验证之前的理论预测，包括偏心压力的奇异动力学、阻力的标度关系以及扩散液滴中边界层的传播。因此，该研究将揭示时空机制中跌落冲击的动力学，而传统的运动学测量是无法实现的。此外，研究还将考察不同表面特性（如刚度、润湿性和微观纹理）对应力分布的影响。该研究旨在寻求最优的表面设计，从而大大减少冲击引起的磨损和损伤。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

From raindrops battering on a windshield to coffee splashing on a table, drop impact is ubiquitous in our daily life and relevant to many important natural and industrial processes. Although the phenomenon has been studied for many decades, the understanding of drop impact is still far from complete due to the complexity associated with typical fast impact processes. Significant progress has been made on drop impact in recent years using high-speed photographic techniques. Relying on direct imaging, most experiments focused on the kinematics of drop impact, i.e., the shape of impacting drops. Few studies have probed the dynamic properties of drop impact such as its impact force, pressure and shear stress. These dynamic quantities lead to the most important outcome of an impact event and affect a wide range of phenomena, including soil erosion, impact-induced wear/damage and the survival of living organisms exposed to the elements. The goal of this research is to conduct a systematic study on mapping the dynamic aspects of drop impact under different conditions. The research will develop a new experimental technique, 3D high-speed stress microscopy, to measure the impact force, pressure, and shear stress of drop impact with high spatial and temporal resolutions. The project will provide a fundamental understanding of the dynamics of drop impact in unexplored regimes and a practical guideline to control impact forces and mitigate impact-induced damage on solid substrates. The project will also provide a unique opportunity to introduce frontier research in undergraduate curriculum and forge collaborations with coating companies. By combining traction force microscopy with high-speed photography, the research will construct a 3D high-speed stress microscope to measure the temporal variation of pressure and shear stress distributions beneath an impacting drop during a fast impact process. With the new technique, the project will verify previous theoretical predictions, including the singular dynamics of off-center pressures, the scaling relation of drag forces, and the propagation of boundary layers in spreading drops. As such, the study will reveal the dynamics of drop impact in spatiotemporal regimes, inaccessible to conventional kinematic measurements. Moreover, the research will examine the influence of different surface properties such as stiffness, wettability, and micro-textures on the stress distributions. The study aims to seek optimal surface designs, which can substantially reduce impact-induced wear and damage.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.