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

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.

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.

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.