Electric Field Mediated Contact Time Reduction of Impacting Drops on Cu(OH)₂ Nanoneedle Clusters: Limitations and Implications for Anti-Icing and Pathogen-Containment Applications

Abstract

In this study, the water drop impact on a copper-based nanotextured superhydrophobic surface inside a uniform electric field is investigated. Because of the wider attention that drop impact draws in the scientific community, this study gives emphasis on the effect of the electric field on the droplet's residence time, a quantity that plays a key role in processes that involve heat and/or mass transport between the surface and impacting droplet. The reduction of the residence time is of vital importance especially for anti-icing and pathogen-transmissioncontainment applications. Shorter residence times enable droplets to rebound at supercooled surfaces before the occurrence of ice nucleation. Moreover, they restrict the likelihood of the deposition of viruses and bacteria for the case of pathogen-laden impacting droplets. Reduction of the residence time is achieved by a twofold strategy. The surface is textured in the nanoscale with the growth of a Cu(OH) 2 nanoneedle cluster so that the nanoroughness topography in combination with the hydrophobic coating imparts to the surface an extreme water-repellent behavior and impalement resistance. Moreover, we introduce an additional external force exerted on the droplet, which originates from an electric field. We focus on the range of the electric Bond number 0 ≤ Bo e ≤ 0.060. In this range, we observe two different interesting behaviors: (a) For 0 ≤ Bo e ≤ 0.020, the contact time reduces with the applied electric field. We also conduct simulations to support our experimental findings concerning the effect of the electric field on the contact time. (b) For 0.025 ≤ Bo e ≤ 0.060, the contact time increases. We demonstrate that this happens because of partial discharges that induce electrowetting, resulting in altering the wetting behavior of the droplet during retraction. Even though limitations exist, the application of electric fields can be considered to be a promising and flexible strategy for reducing the residence time because it can be applied on a wide range of superhydrophobic surfaces.

Bouncing Regimes of Supercooled Water Droplets Impacting Superhydrophobic Surfaces with Controlled Temperature and Humidity

“…Inspired by the water-repellent surfaces of natural plants and animals, such as lotus leaves and duck feathers, researchers proposed that superhydrophobic surfaces (SHSs) have considerable prospect in the anti-icing area. − Combining micro/nanostructures with low surface energy, SHSs show high contact angles of water droplets and low contact angle hysteresis. − An impacting droplet may also bounce off the SHSs quickly with a contact time in the millisecond range. Furthermore, SHSs retain air pockets between the droplet and the surface structures, resulting in the Cassie–Baxter wetting state. , This reduces the solid–liquid contact area, which weakens the interfacial heat transfer and significantly delays the droplet freezing process. − Besides the contact area, the contact time also plays a critical role in the icing of an impacting droplet because it governs the momentum, mass, and energy exchange between the droplet and the underlying surface. − One can imagine that icing might be avoided if the droplet bounces off the superhydrophobic surface before icing takes place upon impingement.…”