title: Controlled directional water-droplet spreading on a high-adhesion surface.
abstract: Controlled directional spreading of a droplet on a smart high-adhesion surface was made possible by simply controlling anodic oxidation. The wettability gradient of the surface was controlled from 0.14 to 3.38° mm(-1) by adjusting the anodic oxidation conditions. When a water droplet made contact with the substrate, the droplet immediately spread in the direction of the wettability gradient but did not move in other directions, such as those perpendicular to the gradient direction, even when the surface was turned upside down. The spreading behavior was mainly controlled by the wettability gradient. Surfaces with a V- or inverse-V-shaped wettability gradient were also formed by the same method, and two droplets on these surfaces spread either toward or away from one another as designed. This method could be used to oxidize many conductive substrates (e.g., copper, aluminum) to form surfaces with variously shaped wettability gradients. It has potential for application in microfluidic devices.
authors.affiliation: Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of the Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, 100191 (P. R. China).
authors.forename: Shile
authors.initials: S
authors.lastname: Feng
authors.forename: Sijie
authors.initials: S
authors.lastname: Wang
authors.forename: Longcheng
authors.initials: L
authors.lastname: Gao
authors.forename: Guangjun
authors.initials: G
authors.lastname: Li
authors.forename: Yongping
authors.initials: Y
authors.lastname: Hou
authors.forename: Yongmei
authors.initials: Y
authors.lastname: Zheng
mesh_terms: D057927:Hydrophobic and Hydrophilic Interactions; D014867:Water / Q000737:chemistry*; D017155:Wettability
chemical_list: D014867:Water
keywords: anodic oxidation; directional spreading; high adhesion; materials science; wettability gradients