## 1 Introduction  
In recent years, superhydrophobic surfaces have attracted considerable attention due to their unique surface wetting properties, which have been widely applied in various fields, including self-cleaning surfaces, waterproof and anti-corrosion coatings, anti-icing surfaces, inkjet printing, and energy harvesting [1, 2, 3, 4, 5]. Due to the development of high precision manufacturing technology, many artificial superhydrophobic surfaces have been developed by chemical or mechanical processes, such as etching, anodic oxidation, galvanic deposition, electro-spinning, and so on [6, 7, 8, 9, 10]. One of them is regular structured surfaces with an array of micro- or nanoscale pillars [11, 12, 13]. By increasing the roughness, the pillar-arrayed surfaces may reduce the contact angle hysteresis of droplets to make them move easily [14]. These configurations enable droplets to remain in a Cassie-Baxter state, wherein the droplets are suspended on a layer of air and make contact only with the tips of the structures [15]. The spaces between pillars create air pockets that offer a shear-free interface to decrease the viscous dissipation of droplets, enhancing the surface's water-repellent properties [16, 17]. Generally, the impact of a droplet on the micro- or nanoscale pillar-array superhydrophobic surfaces shows a process of spreading, partial or complete rebound, and then shattering probably. Besides the factors that determine the Weber number of droplet, such as the impact velocity and liquid properties, the geometric parameters of the micro- or nanoscale pillars also play vital roles in the droplet impact dynamics. [PERSON] et al. [18] experimentally investigated the effects of microscale pillars with various patterns on the dynamic behaviors of microdroplet impact. They mentioned that a short inter-pillar spacing results in a larger maximum spreading factor, which means that the surface wettability can be controlled by adjusting the arrangement of the pillars. A similar conclusion has also been reported by [PERSON] and [PERSON] [19] and [PERSON] et al. [20] that the fraction of the pillars affects the impact processes. [PERSON] and [PERSON] [12] studied the impact of droplets on different microstructured superhydrophobic surfaces by Computational Fluid Dynamics (CFD). Six kinds of pillars were tested, namely, triangle, square, pentagon, cylinder, cross and sphere. They found that the surface with crisscross pillars has the best bouncing ability. Moreover, a reduced size of the pillars can shorten the contact time when the geometric shape is determined. [PERSON] et al. [21] reported an experiment of droplet impact on the surface with millimeter-scale tapered post arrays. Comparing to straight post arrays, the truncated pyramidal post arrays can reduce the contact time significantly and make the pancake bounce. They also confirmed that the apex angle is a critical parameter to control the drop bouncing behavior. In the study by [PERSON] et al. [22], the regular micro-pillar surfaces were coated further with nanoparticles and organic reagents to produce two-tier roughness. These modified surfaces have good performance in droplet detachment with a high Weber number. [PERSON] et al. [23] systematically studied the effects of the spacing and height of micro-pillars on the splitting patterns of droplet impact process, and a flow pattern map was proposed. According to their experimental results, the dense, tall pillars suppress the droplet breakup because the gas exhausts from below. To understand the mechanism of droplet impact, molecular dynamics simulation was employed by [PERSON] et al. [24] to investigate the effects of nano-pillar surfaces. Different from the macroscale pillars, they found an exponential relationship between the maximum spreading factor and the Reynold number of the nanoscale pillars. The material of pillars is also a variable factor that leads to different contact angles. For example, [PERSON] et al.