Superhydrophobic surfaces created by a one-step solution-immersion process and their drag-reduction effect on water

Abstract

A simple one-step process was developed to fabricate a superhydrophobic surface on copper alloy substrates, which was applied by immersing the sheets in a solution containing sodium hydroxide (NaOH), ammonium persulphate ((NH₄)₂S₂O₈) and fluoroalkyl-silane (FAS-17) molecules. The consequent surface was characterized by contact angle measurements, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The water contact angle (CA) and sliding angle (SA) on the resulting surface were ∼158° and ∼4°, respectively. Here, the stability of the obtained surfaces was measured by a wearing test. The damaged coatings can be quickly repaired by a one-step solution-immersion process. Subsequently, the liquid/solid friction on the distinct wetting surface was studied by using a water spraying system. By comparing the friction on a normal surface and the as-prepared surface at different flow velocities, we found that superhydrophobic coatings exhibited an excellent drag reduction of ∼40% at a low wall shear rate and ∼20% at high wall shear rate. Based on these results, we attribute the superhydrophobic drag-reducing results to the plastron effect, which is characterized by surface wetting and roughness. This study not only presented a simple one-step method to fabricate a superhydrophobic surface on the copper alloy substrates but also showed the drag-reducing effect on the obtained surfaces for engineering applications.