Department of Engineering, University of Bergamo , Viale Marconi 5, 24044 Dalmine (BG), Italy.
Langmuir. 2013 Dec 31;29(52):16045-50. doi: 10.1021/la4012372. Epub 2013 Sep 12.
Data from the literature suggest that the rebound of a drop from a surface can be achieved when the wettability is low, i.e., when contact angles, measured at the triple line (solid-liquid-air), are high. However, no clear criterion exists to predict when a drop will rebound from a surface and which is the key wetting parameter to govern drop rebound (e.g., the "equilibrium" contact angle, θeq, the advancing and the receding contact angles, θA and θR, respectively, the contact angle hysteresis, Δθ, or any combination of these parameters). To clarify the conditions for drop rebound, we conducted experimental tests on different dry solid surfaces with variable wettability, from hydrophobic to superhydrophobic surfaces, with advancing contact angles 108° < θA < 169° and receding contact angles 89° < θR < 161°. It was found that the receding contact angle is the key wetting parameter that influences drop rebound, along with surface hydrophobicity: for the investigated impact conditions (drop diameter 2.4 < D0 < 2.6 mm, impact speed 0.8 < V < 4.1 m/s, Weber number 25 < We < 585), rebound was observed only on surfaces with receding contact angles higher than 100°. Also, the drop rebound time decreased by increasing the receding contact angle. It was also shown that in general care must be taken when using statically defined wetting parameters (such as advancing and receding contact angles) to predict the dynamic behavior of a liquid on a solid surface because the dynamics of the phenomenon may affect surface wetting close to the impact point (e.g., as a result of the transition from the Cassie-Baxter to Wenzel state in the case of the so-called superhydrophobic surfaces) and thus affect the drop rebound.
文献中的数据表明，当润湿性较低时，即固-液-气三相线处的接触角较高时，可以实现从表面弹回的效果。然而，目前还没有明确的标准来预测液滴何时会从表面弹回，以及控制液滴弹回的关键润湿参数是什么（例如，“平衡”接触角θeq、前进接触角θA和后退接触角θR、接触角滞后Δθ，或者这些参数的任意组合）。为了阐明液滴弹回的条件，我们在不同润湿性的干固体表面上进行了实验测试，这些表面从疏水性到超疏水性，前进接触角为 108°<θA<169°，后退接触角为 89°<θR<161°。结果发现，后退接触角是影响液滴弹回的关键润湿参数，同时还与表面疏水性有关：在所研究的冲击条件下（液滴直径 2.4<D0<2.6mm，冲击速度 0.8<V<4.1m/s，韦伯数 25<We<585），只有后退接触角高于 100°的表面才会观察到液滴弹回。此外，随着后退接触角的增加，液滴弹回时间会缩短。还表明，在使用静态定义的润湿参数（如前进和后退接触角）来预测液体在固体表面的动态行为时，通常需要谨慎，因为现象的动力学可能会影响冲击点附近的表面润湿（例如，在所谓的超疏水性表面的情况下，由于从 Cassie-Baxter 状态向 Wenzel 状态的转变），从而影响液滴弹回。
