Self-Motion of Water Droplets along a Spacing Gradient ofMicropillar Arrays on Copper

Authors：Su, Sheng-Ying; Li, Er-Mei; Li, Chao-Xiong; Li, Bing; Li, Fang; He, Jian-Bo^*

Source：Langmuir : the ACS journal of surfaces and colloids , 2022, 38(13): 4111-4120.

DOI：10.1021/acs.langmuir.2c00185

• POTENTIAL DEPENDENCE
• SURFACE
• WETTABILITY
• MOVEMENT
• DIFFERENTIATION
• NANOSTRUCTURES
• GENERATION
• EVOLUTION
• OXIDES
• FILM

Self-driven droplet transport along an open gradient surface is increasingly becoming popular for various microfluidicsapplications. In this work, a gradient copper oxide layer is formed on a copper sheet (as a bipolar electrode, BPE) in a KOH solutionby bipolar electrochemistry. The deposits at different positions present a rich variety of colors, compositions, and microstructuresalong the longitudinal axis of the BPE. More than half the length of the anodic pole is covered by a Cu(OH)2/CuO composite layerof several micrometers thick, which is composed of dense micropillars with a decreasing spacing gradient to the anodic direction.The micropillar arrays are superhydrophilic, and after modified with 1-dodecanethiol, the tops of the dense micropillars constitute ahydrophobic and microscopically discontinuous surface with a wettability gradient. On such a gradient surface water droplets canmove spontaneously to more hydrophilic direction at a velocity of about 16 mm s-1. The superhydrophobicity of the modifiedmicropillar arrays is discussed through a comparison with the wax tubules on a lotus leaf. Theoretical analysis of the driving forcereveals that the concave surface effect of water at the spacings between the micropillars is the critical factor for driving the rollingmotion of the droplets along the gradient micropillar arrays.