Effect of geometrical parameters on rebound of impacting droplets on leaky superhydrophobic meshes

Abstract

When a droplet impacts a superhydrophobic sieve, a part of the droplet penetrates through it when the dynamic pressure (ρU) of the impinging droplet exceeds the breakthrough pressure (γΓ/A). At higher impact velocities, the ejected-jet breaks and separates from the main droplet. The remaining part of the droplet bounces off the surface showing different modes (normal bouncing as a vertically elongated drop or pancake bouncing). In this work, we have studied the effect of different geometrical parameters of superhydrophobic copper meshes on different modes of droplet rebound. We observe three different effects in our study. Firstly, we observe pancake like bouncing, which is attributed to the capillary energy of the rebounding interface formed after the breaking of the ejected-jet. Secondly, we observe leakage of the droplet volume and kinetic energy due to the breaking of the ejected-jet, which leads to reduction in the contact times. Finally, we observe that for flexible meshes, the transition to pancake type bouncing is induced at lower Weber numbers. Flexibility also leads to a reduction in the volume loss from the ejected-jet. This study will be helpful in the design of superhydrophobic meshes for use under impact scenarios.

Droplet impact dynamics on textiles

“…Droplet impact was recorded at 64 000 frames per second, a frame rate at least 3 times faster than previous relevant experiments. 15,16,38 The exposure time was 10.0 ms for all experiments. Images were then analysed in ImageJ and Matlab.…”

“…15 In the extreme penetration case, most of the droplet volume shoots out of the pores and forms liquid 'slugs' extending from the opposite surface of the mesh. A further study demonstrated that superhydrophobic meshes 16 could effectively resist incoming droplets and eliminate protrusion. Droplet penetration in a superhydrophobic mesh is suppressed at a critical recoil velocity v r B (s/D) 1/6 (sG/A) 1/3 , indicating that penetration cannot be achieved by surface wettability alone.…”

“…13 Symmetric droplet splitting 2,14 and pancake bouncing 13 have been observed on solid surfaces with superhydrophobic ridges and micro-posts. More recently some authors have studied the impact of droplets on a sieve 15 and on rigid metallic meshes 16,38 concluding that the impact dynamics on these substrates depends on the impact speed and the substrate characteristics. However, the impact of droplets on textiles has received little attention.…”

“…In the context of solid meshes, 20 a scaling d max p DÁWe 1/4 has recently been found to describe the impact dynamics on non-wettable meshes with a solid fraction f no smaller than 70%. 16 Droplet impact on hydrophobic substrates have also been the focus of many recent studies. The reduction of the contact time between a droplet and a solid substrate during the impact is crucial to the design of self-cleaning and liquid repellent surfaces.…”

“…Additionally, 'pancake bouncing' was reported on superhydrophobic meshes. 16,35 In another work, Bordoloi et al 37 studied the penetration of millimetre-sized water droplets (B5 mm diameter) through a millimetric pore while the entire system was submerged in an oil tank. A thin oil film was found to separate the wall of the pore from the droplet for pores with rounded edges.…”