Water droplet bouncing on a non-superhydrophobic Si nanospring array

Nanowall Textured Hydrophobic Surfaces and Liquid Droplet Impact

Water droplet impact on nanowires/nanowalls' textured hydrophobic silicon surfaces was examined by assessing the influence of texture on the droplet impact dynamics. Silicon wafer surfaces were treated, resulting in closely packed nanowire/nanowall textures with an average spacing and height of 130 nm and 10.45 μm, respectively. The top surfaces of the nanowires/nanowalls were hydrophobized through the deposition of functionalized silica nanoparticles, resulting in a droplet contact angle of 158° ± 2° with a hysteresis of 4° ± 1°. A high-speed camera was utilized to monitor the impacting droplets on hydrophobized nanowires/nanowalls' textured surfaces. The nanowires/nanowalls texturing of the surface enhances the pinning of the droplet on the impacted surface and lowers the droplet spreading. The maximum spreading diameter of the impacting droplet on the hydrophobized nanowires/nanowalls surfaces becomes smaller than that of the hydrophobized as-received silicon, hydrophobized graphite, micro-grooved, and nano-springs surfaces. Penetration of the impacted droplet fluid into the nanowall-cell structures increases trapped air pressure in the cells, acting as an air cushion at the interface of the droplet fluid and nanowalls' top surface. This lowers the droplet pinning and reduces the work of droplet volume deformation while enhancing the droplet rebound height.