One-step electrospinning membranes with gradual-transition wettability gradient for directional fluid transport

Gradual-transition wettability gradient is a highly promising wettability profile for driving fluid directionally transport across the thickness of porous membranes. However, the fabrication of such membranes is still challenging, limiting their further application in various fields. Here, we report a one-step strategy to fabricate membranes with gradual-transition wettability gradient (GMs) by programmed in-line-blending electrospinning polymers (poly (ε-caprolactone) (PCL) and poly (ε-caprolactone-co-ethylene glycol) (PCE)) with compatible segments but obviously different wettabilities. The results demonstrate that two opposite gradual-transition wettability gradients across the thickness are successfully fabricated in GMs, and resulting GMs exhibit excellent directional liquid transport property that can only allow liquid drops and continuous flow to permeate across the thickness in the direction from the lyophobicity-rich side to the lyophilicity-rich side while blocking the permeation process in the reverse direction. Additionally, no matter under high or low gravity conditions, the directional fluid transport flux of membranes with different wettability profiles improves as the gradient layers increase from 3 to 78, corresponding to the transition of wettability profile from 3-layered Janus wettability to 78-layered gradual-transition wettability. As a result, GMs achieve much faster directional fluid transport fluxes than membranes with abrupt-change wettability gradient (AMs) and homogeneous wettability, owing to the acceleration of internal driving forces originating from the gradual-transition wettability gradient. Based on this particular property, GMs also show great potentials in ultrafast directional floating oil collection and energy-saving directional separation of oil/water mixtures, offering new insight for designing and fabricating wettability-anisotropic porous membranes with potential and versatility in multiple applications.