Smart Directional Liquid Manipulation on Curvature-Ratchet Surfaces

Structured surfaces leverage interfacial energy for directional liquid manipulation without external power, showing tremendous potential in microfluidics, green energy and biomedical applications. While the interplay of interfacial energy between solid surfaces and liquids is crucial for liquid manipulation, a systematic understanding of how the balance in liquid-solid interfacial energy affects liquid behaviors remains lacking. Here, using the curvature-ratchet surface as a generic example, we reveal the complex directional liquid dynamics inherent in the subtle regulation of liquid-solid interfacial energy. We show that curvature and tilt features regulate Laplace pressure asymmetry to enable directional, bidirectional and reverse liquid manipulation. These processes can be modulated by surface free energy and liquid surface tension, and we define their ratio as a new dimensionless number ζ to characterize the liquid-solid interfacial energy relationship. The balanced liquid control happens when ζ ∼ 1, which facilitates versatile liquid behaviors, e.g., fan-shaped spreading, gradient-induced redirection, and back-and-forth transport on various surface array arrangements, all resulting from matching structural designs with the proper ζ. Inspired by this, we showcase an innovative liquid-based information encryption technique, where the liquid displays correct information on preprogrammed surfaces only at designated ζ values. This study lays the groundwork for smart directional liquid manipulation and broadens its application domains.