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

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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.