The influence of nanostructure on the wetting transition of polyvinylidene fluoride nanoweb: from the petal effect to the lotus effect

Abstract

In this study, the effects of the surface structure of electrospun polyvinylidene fluoride (PVDF) nanoweb on surface wettability were analyzed. The conditions of the surface structure representing the lotus and petal effects were derived, and the difference in the dynamic behavior of the water droplets on the surfaces was investigated. To this end, a PVDF nanoweb was fabricated by electrospinning various concentrations of PVDF solutions. The nanoscale roughness was adjusted by varying the CF4 plasma etching time. It was seen that when the concentration of the electrospun PVDF solution was 15 or 20 wt%, a hierarchical structure of microbeads and nanofibers was formed. In the 20 wt% nanoweb, droplets formed an apparent contact angle of 149.5 ± 2.2°, and the petal effect was observed in which the droplets were pinned on the surface and did not roll off even when the nanoweb was tilted by 180°. As a result of introducing fine nanostructures with CF4 plasma etching on the 20 wt% nanoweb, the apparent contact angle increased to 162.8–164.4°, and the shedding angle decreased to 5.3–8.1°, showing a wetting transition to the lotus effect, regardless of the plasma etching time. In addition, the lotus effect was observed when 15 wt% nanoweb was treated with CF4 plasma etching for more than 10 min. We confirmed that the lotus effect was exhibited when the three-phase contact line of the PVDF nanoweb/water/air was discontinuous, and the contact area between the surface and the water droplets was reduced with increased air pockets at this interface, which led to a decrease in the adhesive force and the impact of negative pressure.