Low-Voltage Reversibly Switchable Wettability through Electrochemical Manipulation of Oxidation State

Various biological organisms in nature exhibit unique surface wettability in order to adapt to their living environment. Diverse range of wettability can be observed, ranging from the low-adhesion superhydrophobic lotus leaf with self-cleaning property, high-adhesion superhydrophobic gecko foot, to the in-air superhydrophilic and underwater superoleophobic fish scale. Inspired by these natural systems with superwetting/antiwetting properties, significant efforts have been devoted to fabricate artificial surfaces with different wettabilities by engineering the surface morphology and chemical composition. Of particular interest is the stimuli-responsive surface which integrates two extreme wetting states of water-attracting and water-repelling properties. External stimuli such as temperature, light, pH, and electrical potential could induce reversible changes in the wetting behavior of the smart surface through transformation in the topological structure and/or surface chemistry. Here we present a novel approach for reversible wettability cycling on dendritic core-shell copper nanostructure surface through electrochemical modulation of the oxidation state. Application of low voltage in the range of regular alkaline battery (<1.5 V) converts the as-prepared copper-based surface from roll-off superhydrophobic, to sticky superhydrophobic, and superhydrophilic wetting state. Precise control over the rate and extent of the wetting switching is achieved by tuning the magnitude and period of the applied voltage. Air drying at room temperature for 1 hour or mild heat drying at 100°C for 30 min reverses the wettability transition to initial superhydrophobic state with low adhesion easy roll-off property. We describe the underlying mechanism for the reversible adhesion and wettability switching from the physical and electrochemical perspectives, as well as practical applicability of this method with specific demonstration for on-demand oil-water separation. The in-situ adhesion and wettability control reported in this work provide a platform for design of oxidation state-mediated wetting transition on different metal oxides for application in remote water filtration, atmospheric water harvesting, droplet manipulation, and microfluidic lab-on-a-chip application.