Abstract
The ability of hydrophobic surfaces to repel impinging liquid droplets is important in applications ranging from self-cleaning of solar panels to avoiding ice formation in freezing rain environments. In quest of maximizing water repellency, modification of droplet dynamics and subsequent reduction of contact time have been achieved by incorporating macrotexture on the superhydrophobic surfaces. However, the dynamics of low temperature water, and other viscous liquid droplets impacting anti-wetting surfaces with macrotextures is not well explored. Here, we investigate the effect of viscosity on the bouncing dynamics of liquid droplets impacting macrotextured superamphiphobic surfaces using various glycerol-water mixtures as model liquids at different impacting conditions. We demonstrate that the changes of reduction in contact times by macrotextures due to the increasing viscosity are in opposite trends at low and at high impact velocities. Since macrotexture executes substantial contact time reduction for the droplets which exhibit splitting after the impact, a preliminary model for predicting the minimum impact velocity to observe droplet splitting by macrotexture is proposed considering the important parameters of an impinging droplet along with the surface characteristics and the macrotexture size. This work aims to provide an insight on several possible outcomes of viscous droplets impacting on the macrotextured surfaces and a model that will help to design the desired superamphiphobic surfaces capable of exhibiting reduced contact time and enhanced repellency of low-temperature water droplets (such as freezing rain) and other viscous liquids (such as oils) under different impacting conditions.
Highlights
Liquid droplets impacting onto solid surfaces is a common phenomena encountered in nature and is important for many industrial applications such as pesticide deposition, spray cooling of hot surfaces, spray painting and coating, ink-jet printing, microfabrication, and impact erosion[1,2,3,4,5,6,7,8]
If the impact velocity is below vc, the impacting droplet has contact only with the highest parts of the surface texture, so the air pockets remain trapped between the droplet and the substrate, avoiding the impalement[34]
It has previously been reported that the contact time of a bouncing water droplet is not dependent on the impact velocity over a wide range of Weber number (We = ρv2R0/σ, where ρ, v, R0 and σ are the liquid droplet density, impact velocity, initial droplet radius and surface tension of the liquid, respectively) though the details of the intermediate stage deformation of droplets significantly depend on it[21,41]
Summary
Liquid droplets impacting onto solid surfaces is a common phenomena encountered in nature and is important for many industrial applications such as pesticide deposition, spray cooling of hot surfaces, spray painting and coating, ink-jet printing, microfabrication, and impact erosion[1,2,3,4,5,6,7,8]. Three important forces affecting these outcomes are inertial, viscous, and capillary forces These forces are dependent on factors associated with the droplet (size, density, surface tension, impact velocity, viscosity, and temperature) and the surface (chemistry, roughness, and temperature). The impacting droplet does not fully rebound if the impact velocity (v) is higher than a certain value, which is known as critical velocity (vc)[37] In such a case, wetting pressure is higher than the resistive capillary pressure and the impacting drop remains partially or entirely attached to the surface, which is known as an impalement or sticking phenomena[37]. We noticed a significant reduction in contact time of water droplet due to splashing
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