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

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-transmission-containment 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 ≤ Boe ≤ 0.060. In this range, we observe two different interesting behaviors: (a) For 0 ≤ Boe ≤ 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 ≤ Boe ≤ 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.