Numerical Simulation Of Drop Dynamics On Hybrid Surface With Hydrophilic Dots Populated On Super-hydrophobic Surface

Drop impacting on a hybrid surface opens new functionalities, but impacting drops should not be mutual interfered. To achieve ”One cell, one well" it is necessary to keep drop completeness and ensure limited spreading diameter. Here, the problem is newly described by non-dimensional conservation equations. A set of parameters such as impacting velocity, drop size etc. are combined to form three key non-dimensional parameters of We, Oh and ?_i (size ratio of hydrophilic dot to drop), providing general guidance to droplet array system design.Regarding the drop dynamics on hybrid surface, three regimes of complete-drop,single-drop-pinching-off and multi-drops-pinching-off were found, covering a wide range of We=1-100, Oh=0.001-1 and ?_i=0.5-10. The increased ?_i enlarges the complete drop regime. The single-drop-pinching-off mode involves combined wall adhesion and surface tension induced short wave mechanism, while the multi-drops-pinching-off mode is caused by the propagation and interference of capillary waves from both ends of an elongated liquid column. We found the superposition principle for the first time: drop patterns include an adhesion part on the wall, similar to that on a hydrophilic surface,plus a rebounding part, similar to that on a super-hydrophobic surface.Attention was also paid to the maximum spreading diameter. At small We, the hydrophilic dot does not influence the maximum spreading diameter for smaller ?_i, but it confines the drop within the hydrophilic dot for larger ?_i. Such relationship reflects the competition between hydrophilic-dot/liquid and hydrophobic-wall/liquid force interactions. At larger We, the maximum spreading diameter is not influenced by the hydrophilic dot, under which the inertia force thoroughly suppresses the surface tension effect.After the drop dissipates all the kinetic energy and reach the minimum surface energy, the drop is stabilized on the wall. When the hydrophilic dot is small such as?_i=0.5, the stabilized drop is out of the hydrophilic dot boundary. The stabilized drop at moderate ?_i=1.0 and 1.5 is successfully confined by the hydrophilic dot at its margin.When the hydrophilic dot is large such as ?_i=2.0, the drop is inside the hydrophilic dot.