Heterogeneously engineered porous media for directional and asymmetric liquid transport

Passive regulation of liquid transport in porous media in a directional or asymmetric manner has become increasingly critical to emerging applications, such as personal moisture management, water harvesting, and liquid separation. Over the past decade, heterogeneously engineering porous materials and structures for tunable liquid transport has triggered technological revolutions in those areas based on nature-inspired design, metamaterial development, and model-driven optimization. Herein, we discuss the latest developments in directional and asymmetric liquid movement created by material and structural heterogeneity, with a focus on mechanistic models, physical mechanisms, and engineering strategies to provide an improved understanding of the controllability of the directed liquid motion. We also explore the diverse applications of enhanced fluid directionality and asymmetry, from overviewing fabrication methods to analyzing significant affecting factors, including surface wettability, pore size, and flow path profile. Current challenges and research gaps are summarized to provide a road map for potential research opportunities.