The macroscopic dynamics of a droplet impacting a solid is crucially determined by the intricate air dynamics occurring at the vanishingly small length scale between droplet and substrate prior to direct contact. Here we investigate the inverse problem, namely, the role of air for the impact of a horizontal flat disk onto a liquid surface, and find an equally significant effect. Using an in-house experimental technique, we measure the free surface deflections just before impact, with a precision of a few micrometers. Whereas stagnation pressure pushes down the surface in the center, we observe a liftup under the edge of the disk, which sets in at a later stage, and which we show to be consistent with a Kelvin-Helmholtz instability of the water-air interface.

中文翻译：

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光盘撞击水面之前的气垫效应和Kelvin-Helmholtz不稳定性

液滴撞击固体的宏观动力学至关重要地取决于在直接接触之前，液滴与基材之间的微小长度尺度逐渐消失时所发生的复杂的空气动力学。在这里，我们研究了反问题，即空气对水平平板撞击液体表面的作用，并发现了同样重要的作用。使用内部实验技术，我们可以测量撞击前的自由表面挠度，精度为几微米。停滞压力将中心的表面向下推，我们观察到圆盘边缘下方的隆起，该隆起在稍后阶段出现，并且表明与水-空气界面的Kelvin-Helmholtz不稳定性相符。