Compositional-Asymmetry-Induced Transition of Directional Liquid Transport on Tilted and Janusian Nanohair Arrays

Anisotropic wetting on certain surfaces endowed with structural asymmetry or compositional gradients commonly impedes the directional adjustment of liquid transport. We report here that directional liquid transport (DLT) against the tilt direction of nanohair and in the reverse direction was achieved on tilted-nanohair arrays (TNAs) and tilted-Janusian-nanohair arrays (TJNAs), respectively. Janusian compositional asymmetry on the surface of TJNAs was created by plasma polymer deposition on structurally asymmetric TNAs previously fabricated by Faraday-cage-assisted plasma nanotexturing. The structurally asymmetric TNAs led to DLT against the tilting direction due to the asymmetric wetting under the capillary imbibition between tilted nanohairs and the preferential coalescence of liquid against the tilt direction. The Janusian compositional asymmetry of TJNAs changing the capillarity imbibition condition between tilted nanohairs resulted in the transition of the liquid spreading direction along the tilt direction. The spreading direction along and against the tilt direction is predicted through a comprehensive analysis of the structural and compositional asymmetries of the TNAs and TJNAs.