Impact Behaviors of a Microdroplet on Superhydrophobic Textured Surfaces

Abstract

The textured surfaces with arrays of microscale pillars have been extensively studied due to their novel water repellency and enormous potential engineering applications. Given that numerous practical applications of water repellent surfaces incorporate the impacting microdroplets, the impact dynamics on textured surfaces are of interest. In this study, therefore, the dynamic behaviors of microdroplets that impact on textured surfaces with various patterns of microscale pillars are experimentally investigated.A piezoelectric inkjet printhead is used to generate the microdroplets that have a diameter of less than 46 μm and a controlled Weber number. The impact and spreading dynamics of an individual droplet are captured by using a high-speed imaging system. The textured surfaces of photoresist are fabricated by using standard photolithography followed by hydrophobic coating.The water repellency of the textured surfaces is evaluated by measuring the wetting state and impact behaviors of the impinging microdroplets on the textured surfaces with isotropic arrays of pillars. The impact behaviors are clearly according to the dimensionless parameter, Weber number (We). The detailed impact behaviors of the microdroplets are observed with varying We. By balancing the wetting pressure (Pwet) of the impacting microdroplet and the antiwetting pressure (Pantiwet) of the textured surface, the impalement transition and the resulting wetting states are evaluated.The textured surfaces with anisotropic arrays of micropillars are prepared to investigate the anisotropic or directional wettability arising from the anisotropic geometry of the micropillars on the textured surfaces. The wetting states of the impacting microdroplets and the impalement transition are evaluated by balancing Pwet and the capillary pressure (PC) even on the textured surfaces with anisotropic arrays of pillars. In addition, We can be used to describe the transition between the bouncing and non-bouncing behaviors. Therefore, the wetting states and impact behaviors on the anisotropic arrays of pillars can be predicted and explained by the pressure range as well as the We of the impacting droplets.The maximum spreading factor (βmax) is measured and compared with the theoretical prediction to elucidate the wettability of the textured surfaces. For a given Weber number, the maximum spreading factor decreases as the texture area fraction of the textured surface decreases.In addition, the anisotropic and directional wettability is observed by measuring different βmax along the two orthogonal directions. The βmax along the direction of longer inter-pillar spacing always have smaller values than those along the direction of shorter inter-pillar spacing when a droplet impacts on the anisotropic arrays of pillars. The difference in βmax along the two orthogonal directions increases as the length difference of the inter-pillar spacings increases. Therefore, the wettability ofthe textured surface can be controlled by adjusting the length of each inter-pillar space.The measured βmax are compared with the prediction models based on the energy conservation condition. The gap between the experimental and theoretical βmax increases as the We of the microdroplet increases. Because the model was established for a smooth surface condition, the relative errors indicate the impalement transition and the dissipation inside the texturing features.