Anisotropic liquid transport within fibrous media: spreading and penetration

Fibrous media with special structures enable anisotropic liquid transport (ALT), allowing spontaneous fluid movement along certain directions, e.g., in-plane spreading and/or through-plane penetration. Most research studies focus on creating chemical and/or structural gradients along substrates' vertical axis to achieve through-plane penetration, also known as directional liquid transport (DLT), a topic that has been widely reviewed. In contrast, in-plane liquid spreading within fibrous media has attracted increasing attention, yet it has rarely been summarized. This review aims to provide an in-depth analysis of ALT within fibrous media, including (i) complete in-plane spreading, (ii) spreading & penetration, including penetration followed by in-plane spreading and in-plane spreading-enhanced penetration, (iii) complete through-plane penetration-penetration followed by droplet rolling. It begins with an overview of the basic transport mechanisms of different liquid transport modes, followed by the exploration of various preparation strategies and diversified applications. Finally, key challenges of ALT are concluded, including precise control of liquid absorption, penetration and departure; in-depth research on liquid dynamics within fibrous media; and the integration of ALT and other functions into a single fibrous medium.