Han Jeonghoon, Kim Wonjung, Bae Changwoo, Lee Dongwook, Shin Seungwon, Nam Youngsuk, Lee Choongyeop
Department of Mechanical Engineering, Kyung Hee University, Yongin 17104, Republic of Korea.
Department of Mechanical Engineering, Sogang University, Seoul 04107, Republic of Korea.
Phys Rev E. 2020 Apr;101(4-1):043108. doi: 10.1103/PhysRevE.101.043108.
When a water drop impinges on a flat superhydrophobic surface, it bounces off the surface after a certain dwelling time, which is determined by the Rayleigh inertial-capillary timescale. Recent works have demonstrated that this dwelling time (i.e., contact time) is modified on curved superhydrophobic surfaces, as the drop asymmetrically spreads over the surface. However, the contact time on the curved surfaces still remains poorly understood, while no successful physical model for the contact time has been proposed. Here, we propose that the asymmetric spreading on the curved surface is driven by either the Coanda effect or inertia depending on the ratio of the drop diameter to the curvature diameter. Then, based on scaling analysis, we develop the contact time model that successfully predicts the contact time measured under a wide range of experiment conditions such as different impact velocities and curvature diameters. We believe that our results illuminate the underlying mechanism for the asymmetric spreading over the curved surface, while the proposed contact time model can be utilized for the design of superhydrophobic surfaces for various thermal applications, where the thermal exchange between the surface and the water drop occurs via a direct physical contact.
当水滴撞击平坦的超疏水表面时，它会在一定的停留时间后从表面反弹，该停留时间由瑞利惯性 - 毛细管时间尺度决定。最近的研究表明，由于水滴在曲面上不对称地扩散，这种停留时间（即接触时间）在弯曲的超疏水表面上会发生改变。然而，曲面上的接触时间仍然知之甚少，同时尚未提出成功的接触时间物理模型。在这里，我们提出曲面上的不对称扩散是由康达效应或惯性驱动的，这取决于液滴直径与曲率直径的比值。然后，基于尺度分析，我们开发了接触时间模型，该模型成功地预测了在各种实验条件下（如不同的撞击速度和曲率直径）测量的接触时间。我们相信，我们的结果揭示了在曲面上不对称扩散的潜在机制，而所提出的接触时间模型可用于设计用于各种热应用的超疏水表面，其中表面与水滴之间的热交换通过直接物理接触发生。
