Directional manipulation of bubble behavior on wettability gradient surfaces: mechanisms, strategies, and applications

Directed transport of bubbles is crucial for achieving precise microfluidic control, improving energy efficiency, and optimizing gas-liquid reaction processes. However, significant challenges still remain in achieving controllable and efficient directed transport of bubbles. Due to surface isotropy (in chemistry and morphology), contact angle hysteresis (CAH), and micro-defects on uniform solids, bubble-directed transport faces significant energy barriers and motion discontinuities. These limitations hinder spontaneous and precise bubble motion. As an innovative passive strategy, wettability gradient surfaces (WGS) induce asymmetric forces through surface energy gradients, directing bubbles along specific paths. Concurrently, the Laplace pressure difference (ΔP) induced by surface geometric gradients further enhances bubble-directed transport efficiency. This approach enables spontaneous, directional bubble motion without external force fields, offering advantages including low energy consumption, structural simplicity, and high controllability. It thus provides efficient, energy-saving solutions for diverse applications. This review initially introduces bubble wettability theory and presents the dynamic theory of bubble behavior through force analysis. It then discusses recent advances in the spontaneous directed transport of bubbles on surfaces with energy or geometric gradients, and offers an in-depth analysis of the synergistic and competitive mechanisms between surface energy and geometric gradients in driving processes. Subsequently, advanced methods for fabricating WGS (e.g., laser processing, electrochemical methods, self-assembly, additive manufacturing) are summarized. Potential applications in bubble collection, microfluidics, and heat transfer are also outlined. By integrating these aspects, this review aims to provide theoretical foundations and practical guidance for developing and optimizing WGS, thereby promoting technological innovation and expanding the applications of bubble-directed transport.