Self-directional liquid transport by anisotropic molecular diffusion in 3D nanostructured membranes.

HYPOTHESIS: Asymmetric micro- and nanostructured surfaces have enabled spontaneous directional liquid movement over limited distances, primarily driven by surface tension gradients. However, when structural features approach the nanometer or sub-nanometer scale-comparable to intermolecular spacing in fluid-molecular dynamics begin to dominate transport behavior, leading to a shift in underlying mechanisms.

EXPERIMENTS: In this study, we fabricate oriented-shape nanofiber membranes via electrospinning, featuring fibers with uniformly oriented elliptical cross-sections that form a multilayered anisotropic nanostructure. A systematic investigation is performed to clarify the influences of temperature, droplet dimension, and ambient pressure on the spontaneous directional driving force of droplets on the oriented nanofiber membranes, with a series of controlled comparative experiments conducted on fluids and ambient conditions with distinct evaporation rates.

FINDINGS: We achieve spontaneous directional liquid transport on such membranes, and first confirm the process is co-driven by surface tension and the directional force from directional water vapor-droplet interaction. Under ambient conditions, this yields an equivalent pressure > 1.1 Pa at a low evaporation rate 8.3×10-5kg/m2∙s (far greater than the pressure generated by this vapor flow), scaling positively with liquid evaporation rate and negatively with inclination angle of the shaped fibers. This discovery offers a novel, green, energy-efficient, and low-cost solution for controllable fluid transport-particularly for long-distance manipulation in micro- and nanofluidic systems.