Ultra-Fast, Unidirectional Water Absorption on Wood Ear.

Materials exhibiting rapid, unidirectional liquid absorption are desirable for comfort textiles and wound dressings. Implementing chemical or structural gradient along the vertical axis of substrates is an effective way to achieve such properties. Liquid's lateral spreading across the substrate affects area occurring vertical imbibition. However, the influence of lateral spreading on liquid's overall absorption remains unexplored. Findings on ultra-fast, unidirectional water absorption on the abhymenium of wood ear fungi are presented, featuring dense micro-sized hairs atop a porous sublayer containing smaller micro-/nano-pores. These hairs facilitate lateral spreading, and the gradient-sized structures from the surface hairs to the internal pores enhance capillary force, promoting efficient vertical imbibition. The synergistic wicking mechanism in both directions shortens absorption time of a 1-µL droplet by two orders of magnitude compared to a solely porous surface (35 ms vs 5.2 s). An artificial micro-pillar array on a porous substrate also exhibits ultra-fast, unidirectional water absorption. This study advances the understanding of liquid dynamics in porous media and provides a blueprint for engineering materials with superior liquid management.