Bouncing of cloud-sized microdroplets on superhydrophobic surfaces

Abstract

The control of microdroplet impact on superhydrophobic surfaces (SHSs) is becoming imperative owing to its effect on several industrial applications, such as corrosion protection, self-cleaning, ice resisting, and de-icing. While most of the experimental studies on the impact dynamics of droplets are based on macrodroplets, it is unclear how the obtained results can be applied to microdroplet impact on SHSs. In this work, a comprehensive experimental analysis ranging from millimeter- to micrometer-sized droplets using a novel drop on demand microdispensing system is performed. Several SHSs were synthesized to control droplet impact by enforcing bouncing on the surface during the impingement process. The current analysis focuses on experimentally capturing and analyzing the impact behavior of cloud-sized microdroplets and macrodroplets (D0 = 10 μm–2500 μm) upon SHS impact, with hysteresis, under controlled environmental conditions. Different droplet impact parameters, such as droplet contact time, maximum spreading diameter, and restitution coefficient, were experimentally obtained. Interestingly, this investigation highlighted a contrast in the behavior of microdroplets and macrodroplets upon impact on rough SHSs. It was found that critical parameters controlling droplet dynamics, such as the maximum spreading diameter and coefficient of restitution, cannot be described by current models in the literature. A preliminary theoretical model based on energy balance and accounting for the substrate hysteresis is proposed to explain some of these findings. Finally, the effect of SHS roughness on the bouncing of cloud-sized microdroplets (D0 = 10 μm–100 μm) was examined in the context of synthesizing SHSs.