Capillary spreading of ethanol-water on hierarchical nanowire surfaces with interconnected V-groove

Compared to micro/nanostructured surfaces, hierarchical structured surfaces have shown outstanding wicking capability due to the excellent combination of capillary pressure and liquid transport channel. However, the capillary wicking process of different working fluids on structured surfaces, especially hierarchical structured surfaces, was not adequately studied. In this work, a group of hierarchical nanowire surfaces with interconnected V-groove were fabricated and their capillary spreading characteristics were studied with semi-theoretical analyses and experimental tests using ethanol-water droplets with different ethanol volume fractions. The effects of surface structural parameters (nanowire diameter and nanowire height) and the ratio of surface tension to viscosity on the wicking coefficient were discussed with dimensionless wicking coefficient, overall structural contribution, Laplace pressure difference, and average volume flowrate. The wicking coefficient is proportional to the 0.5 power of the ratio of surface tension to viscosity. The hierarchical nanowire surfaces with smaller nanowire diameters possess larger Laplace pressure differences in the nanowire clusters to serve as the major driving pressure difference for capillary spreading. The surfaces with larger nanowire height transport larger volume flowrate in the V-groove.