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

从雨点打在挡风玻璃上到咖啡溅在桌子上，跌落冲击在我们的日常生活中无处不在，并且与许多重要的自然和工业过程相关。虽然这一现象已经被研究了几十年，但由于典型的快速撞击过程的复杂性，对跌落撞击的理解仍然远远没有完成。近年来，利用高速摄影技术在跌落冲击方面取得了重大进展。依靠直接成像，大多数实验集中在跌落冲击的运动学上，即，撞击水滴的形状对于跌落冲击的动力学特性，如冲击力、压力和剪应力等，研究较少。这些动态量导致撞击事件的最重要结果，并影响到一系列广泛的现象，包括土壤侵蚀、撞击引起的磨损/损坏以及暴露于各种元素的生物体的生存。本研究的目的是对不同条件下跌落冲击的动力学特性进行系统的研究。该研究将开发一种新的实验技术，即3D高速应力显微镜，以高空间和时间分辨率测量液滴冲击的冲击力，压力和剪切应力。该项目将提供一个基本的了解，在未开发的制度和实用的指导方针，以控制冲击力和减轻固体基板上的冲击引起的损害的动态下降。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。通过将牵引力显微镜与高速摄影相结合，该研究将构建一个3D高速应力显微镜，以测量快速冲击过程中冲击液滴下压力和剪切应力分布的时间变化。利用这项新技术，该项目将验证以前的理论预测，包括偏心压力的奇异动力学、阻力的比例关系以及扩散液滴中边界层的传播。因此，这项研究将揭示在时空制度，无法进入传统的运动学测量的动态下降的影响。此外，研究将检查不同的表面特性，如刚度，润湿性和微观纹理的应力分布的影响。该研究旨在寻求最佳的表面设计，这可以大大减少冲击引起的磨损和损坏。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.