Controlling Liquid Droplet Motion on Solid Surfaces: from Pinning Mechanisms to Driving Forces

Liquid droplets are ubiquitous in both natural and engineered systems, and play a key role in many application scenarios, ranging from painting and cleaning to microfluidics and interfacial engineering. The high mobility and deformability of liquid droplets enable diverse functionalities yet lead to complex physical behaviors (e.g., pinning, motion, coalescence, and dispensing). Inspired by natural phenomena and extensive studies of droplet dynamics, many liquid droplet manipulation strategies (LDMSs) have been developed to control droplet behavior across various contexts. However, most LDMSs remain poorly understood, with their underlying mechanisms still debated in the literature. In this review, we correlate the physics of liquid droplets to the latest advances in the field of surface forces and systematically examine the physical mechanisms underlying representative LDMSs reported over the past decade. Specifically, the latest findings in interactions between liquid droplets and solid surfaces are highlighted, with particular focus on the mechanisms governing droplet pinning and depinning. The origins of pinning forces and the initiation of droplet motion on solid surfaces are discussed. Representative LDMSs are reviewed and categorized into three types based on the sources of the driving forces, and related physical mechanisms for the selected LDMSs are analyzed. Besides, the challenges and perspectives for current LDMSs are discussed, with particular emphasis on potential progress in both fundamental knowledge and practical applications. This work improves the fundamental understanding of the physics governing liquid droplet behaviors and provides valuable insights for developing more efficient LDMSs and related materials.