Directional water droplet manipulation on functional surfaces: A critical review of passive gradient-driven and...

The directional manipulation of water droplets is a core research topic in the fields of surface science, biomimetic engineering, and microfluidics. It has broad application prospects in areas such as nuclear power steam collection, seawater desalination, and biomedical systems. Conventional droplet manipulation strategies are usually classified into passive gradient-driven transport and active field-controlled manipulation. However, this binary classification obscures the fact that both strategies are governed by the same fundamental competition between driving and resistive forces, making it difficult to establish a unified understanding of their operational behaviors, performance limits, and application boundaries. In this review, passive and active droplet manipulation strategies are systematically reinterpreted within a unified drive-resistance ratio (DRR) framework. Based on this framework, the droplet manipulation space is divided into four representative regimes: autonomous transport, precise controllable motion, hysteresis-dominated pinning, and dynamic dissipative actuation. Representative passive and active strategies are critically compared in terms of transport efficiency, controllability, actuation requirement, and application adaptability. To improve cross-platform evaluation, semi-quantitative metrics, including the transport efficiency coefficient (EVF) and operational flexibility factor (ROF), are further introduced. Finally, key scientific challenges and future directions are discussed, including passive-active hybrid interfaces, durable antifouling surfaces, standardized performance evaluation, and intelligent closed-loop droplet manipulation systems.