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Dynamics of high-speed micro-drop impact: numerical simulations and experiments at frame-to-frame times below 100 ns

机译：高速微滴撞击的动力学：低于100 ns的帧间时间的数值模拟和实验

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Technologies including (3D-) (bio-)printing, diesel engines, laser-induced forward transfer, and spray cleaning require optimization and therefore understanding of micrometer-sized droplets impacting at velocities beyond 10 m s−1. However, as yet, this regime has hardly been addressed. Here we present the first time-resolved experimental investigation of microdroplet impact at velocities up to V0 = 50 m s−1, on hydrophilic and -phobic surfaces at frame rates exceeding 107 frames per second. A novel method to determine the 3D-droplet profile at sub-micron resolution at the same frame rates is presented, using the fringe pattern observed from a bottom view. A numerical model, which is validated by the side- and bottom-view measurements, is employed to study the viscous boundary layer inside the droplet and the development of the rim. The spreading dynamics, the maximal spreading diameter, the boundary layer thickness, the rim formation, and the air bubble entrainment are compared to theory and previous experiments. In general, the impact dynamics are equal to millimeter-sized droplet impact for equal Reynolds-, Weber- and Stokes numbers (Re, We, and St, respectively). Using our numerical model, effective scaling laws for the progression of the boundary layer thickness and the rim diameter are provided. The dimensionless boundary layer thickness develops in time (t) according to Image ID:c4sm02474e-t1.gif, and the diameter of the rim develops as Image ID:c4sm02474e-t2.gif, with drop diameter D0 and inertial time scale τ = D0/V0. These scalings differ from previously assumed, but never validated, values. Finally, no splash is observed, at variance with many predictions but in agreement with models including the influence of the surrounding gas. This confirms that the ambient gas properties are key ingredients for splash threshold predictions
机译：包括（3D-）（生物）打印，柴油发动机，激光诱导的前向转移和喷雾清洁在内的技术需要优化，因此需要了解在超过10 m s-1的速度下撞击的微米级液滴。但是，到目前为止，这种制度还没有得到解决。在这里，我们提出了在速度高达V0 = 50 m s-1的情况下，对微滴在亲水和憎水表面上以每秒超过107帧的速度进行冲击的时间分辨实验研究。提出了一种新颖的方法，该方法使用从底视图观察到的条纹图案，以相同的帧速率确定亚微米分辨率下的3D液滴轮廓。通过侧面和底视图测量验证的数值模型被用来研究液滴内部的粘性边界层和边缘的发展。将扩散动力学，最大扩散直径，边界层厚度，边缘形成和气泡夹带与理论和先前的实验进行了比较。通常，对于相等的雷诺数，韦伯数和斯托克斯数（分别为Re，We和St），冲击动力学等于毫米大小的液滴冲击。使用我们的数值模型，为边界层厚度和轮辋直径的发展提供了有效的缩放定律。无量纲边界层厚度根据图像ID：c4sm02474e-t1.gif在时间（t）内发展，轮辋的直径随着图像ID：c4sm02474e-t2.gif增长，液滴直径为D0，惯性时标τ= D0 / V0。这些比例不同于先前假定但从未验证的值。最终，未观察到飞溅，与许多预测不同，但与包括周围气体影响在内的模型一致。这证实了环境气体特性是飞溅阈值预测的关键因素

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