A millimetre-size superhydrophobic sphere impacting on the free surface of a quiescent bath can be propelled back into the air by capillary effects and dynamic fluid forces, whilst transferring part of its energy to the fluid. We report the findings of a thorough investigation of this phenomenon, involving different approaches. Over the range from minimum impact velocities required to produce rebounds to impact velocities that cause the sinking of the solid sphere, we focus on the dependence of the coefficient of restitution, contact time and maximum surface deflection on the different physical parameters of the problem. Experiments, simulations and asymptotic analysis reveal trends in the rebound metrics, uncover new phenomena at both ends of the Weber number spectrum, and collapse the data. Direct numerical simulations using a pseudo-solid sphere successfully reproduce experimental data whilst also providing insight into flow quantities that are challenging to determine from experiments. A model based on matching the motion of a perfectly hydrophobic impactor to a linearised fluid free surface is validated against direct numerical simulations and used in the low Weber number regime. The hierarchical and cross-validated models in this study allow us to explore the entirety of our target parameter space within a challenging multi-scale system.
Droplet impact of emulsions on inclined microstructured surfaces
Facultad de Ingeniería, Universidad Nacional Autónoma de México（墨西哥国立自治大学工程学院）；Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México（墨西哥国立自治大学材料研究所）
Constitutive Modelling of Korteweg Fluids Using Liu's Method
Faculty of Technical Sciences, University of Novi Sad（诺维萨德大学技术科学学院）；Faculty of Sciences, University of Novi Sad（诺维萨德大学理学院）；HUN-REN Wigner Research Centre for Physics（匈牙利科学院维格纳物理研究中心）；Budapest University of Technology and Economics（布达佩斯技术与经济大学）
The characteristic rupture height of the mediating air film beneath an impacting drop on atomically smooth mica
在撞击过程中，液滴在接触表面之前，必须先置换其下方的空气。在很宽的撞击速度范围内，空气被压缩成一层薄膜，减缓液体向表面的前进。低于临界撞击速度时，空气膜保持完整;液滴从空气膜上反弹而不接触。超过这个临界冲击速度，液滴总是接触。接触形成的开始经由拓扑转变发生，由此最初连接的气体域破裂，并且形成液体毛细管桥，将液滴结合到表面。在这里，我们探测这种过渡的详细周围的临界冲击速度，使用校准的全内反射（TIR）显微镜，以监测高速在冲击过程中的气膜轮廓。观察到两种空气膜破裂模式：成核接触，这是孤立的，不对应于全球最小的空气膜厚度，和自发接触，总是发生在一个环上的影响轴上的空气膜达到其全球最小值为中心。我们的测量结果表明，对于超过临界速度的接触引发的冲击速度，空气膜破裂在几乎相同的高度h min ~ 20 nm，两种流体-硅油和水甘油混合物。接触前的空气膜的高度和持续时间，超过180液滴冲击实验分析，是一致的线性不稳定性驱动的范德华力。水溶液液滴的冲击事件显示出不同的统计接触成核比硅油，然而，一个临界的冲击速度以上的接触总是发生确定为两种液体。
Experimentally Resolving Gravity-Capillary Wave Evolution in Vessels of Unknown Boundary Conditions
School of Mathematical Sciences, University of Nottingham（诺丁汉大学数学科学学院）；School of Physics and Astronomy, University of Nottingham（诺丁汉大学物理与天文学院）；Photon Science Institute, University of Manchester（曼彻斯特大学光子科学研究所）
