Arch shaped high curvature edge design for selective microfluidics

Surfaces with fluid-guiding microstructures have attracted growing attention in microfluidics. Conventional studies attribute unidirectional liquid spreading to the sharp-edge effect of micro-cavities, yet the functional significance of their characteristic arched edges remains unexplained. Herein, we establish an energy-based framework grounded in the least action principle to predict three-dimensional liquid shapes and three-phase contact lines, revealing that high curvature arch shaped edges maximize spreading resistance and enable selective contact-angle-dependent liquid guidance. Based on the results, channels with two-side arch shaped convex plates were designed, enabling selective bidirectional fluid steering. Fluid sieving devices, including selective Y-branch channels, a fluidic digital display, and an oil-water separator, were developed. The Y-branch channels steer deionized water, ethanol–water mixtures, and ethanol along different paths. The digital display encodes ethanol–water mixtures as the digits ‘1,’ ‘2,’ and ‘3,’ and the separator achieves 98 % oil–water separation efficiency within minutes without external actuation. These findings reveal the selective liquid guiding mechanism of arched edge structures and establish a general strategy for passive, programmable, and geometry-driven fluid manipulation in microfluidic systems.