Water entry studies traditionally employ homogeneous projectiles of varying impactor shape, entry speed, and surface roughness. Surface heterogeneity is yet another means to manipulate splash dynamics. In this experimental study, we systematically investigate the water entry of smooth, free-falling, hemispherically coated spheres for Froude numbers in the range of $2.8-6.7$. Hydrophilic spheres are hemispherically coated with a hydrophobic compound and in-turn produce deep seal cavities, provoke changes in super-surface splash features, and alter sphere trajectories. Generally, flow separation is initialized when hydrophobic surfaces make contact with the fluid, leading to air-entrainment across the range of entry speeds and impact orientations on test. Cavity formation induced by the hydrophobic portion of a hemispherically coated sphere promotes flow separation across the hydrophilic surface at impact velocities well below the threshold of 8 m/s required for air-entrainment by completely hydrophilic spheres. Spheres having partially hydrophilic and partially hydrophobic surfaces entering the fluid simultaneously, experience asymmetric cavities and horizontal forces that result in lateral migration from straight-line trajectories. Such observations augur well for water entry applications where the coupled dynamics of flow separation and passive trajectory control are desirable.

中文翻译：

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具有非均质润湿特性的球体的入水动力学

传统上，入水研究采用冲击器形状，进入速度和表面粗糙度各不相同的均质弹丸。表面异质性是控制飞溅动力学的另一种方法。在本实验研究中，我们系统地研究了光滑，自由落体，半球形涂层球体的水进入，其弗劳德数在$2.8-6.7$。亲水性球体在半球面上涂有疏水性化合物，从而产生深层的密封腔，引起超表面飞溅特征的变化，并改变球体的运动轨迹。通常，当疏水性表面与流体接触时会初始化流动分离，从而导致进入速度和测试冲击方向上的空气夹带。由半球形涂层的球的疏水部分引起的空穴形成促进了冲击速度下的亲水表面上的流体分离，该冲击速度远低于完全亲水的球所夹带的空气所要求的8 m / s的阈值。具有部分亲水性和部分疏水性表面的球体同时进入流体，会遇到不对称的空腔和水平力，从而导致从直线轨迹横向迁移。这样的观察对于需要分流与被动轨迹控制相结合的动力学的水进入应用预示着很好的预兆。