| Fog,as a potential freshwater source,can be exploited to address globally freshwater scarcity risk.Unique periodic knots structure of natural spider silk inspires the development of microfibers for collecting freshwater from atmospheric air.Improving water collection ability of microfibers remains an ongoing challenging.Here,bioinspired spindle-knotted microfibers with continuous hollow channel are fabricated via a simple and flexible multiphase-laminar-flow microfluidic method,of which structure and morphology can be precisely manipulated by adjusting flow rates.Noticeably,the compartmental effect of aqueous two-phase system（ATPS）is introduced for forming microfibers with obvious core-shell structure in the capillary channel,and hollow spindle-knotted microfibers（HSFs）are fabricated after drying.Hollow structure is used to improve the water collecting efficiency of microfibers and the mechanism of hollow structure enhancing capillary force,extending the length of three contact lines and strengthening the water collecting performance of fiber is revealed.In the preparation and control part of hollow spindle microfiber,hollow spindle-knotted microfibers are prepared by coaxial annular microfluidic device,and the influence of different flow rate on fiber structure is investigated.The size（height,width）of the spindles decreases and the spacing increases with the increase of the outer phase velocity.With the increase of middle phase velocity,the spindle size increases and the spacing becomes smaller.The inner phase flow rate has little effect on the size spacing of the spindle,but determines the internal structure of the fiber.With the increase of internal flow velocity and the increase of internal channel diameter,solid and hollow spindle-knotted microfibers are generated after drying.The surface roughness difference between the spindle knot and the joint is obvious,and the fiber material has excellent hydrophilicity and good mechanical properties.In the hollow spindle-knotted microfibers are used for fog water collection.The author explores the water collection performance of the fiber,the droplet complete a water collection cycle after condensation,directional transportation,coalescence,and falling.At the fog flow rate of 1 m L·min-1,the suspension ability and durability of the fiber are stable,and the droplet formation rate is positively correlated with the fog flow rate.The water collection performance of hollow spindle-knotted microfibers（HSFs）and solid spindle-knotted microfibers（SSFs）are compared in same condition.The maximum droplet volume of hollow microfibers（9.85μL）is2.04 times that of solid microfibers（4.83μL）.The drop growth rate of hollow microfibers(0.0612μL·s-1)is 1.83 times of that of solid microfibers(0.0335μL·s-1),and the water collecting efficiency of hollow microfibers(2.04 g·h-1)is 2.72 times of that of solid micrefibers(0.75 g·h-1).When the intersection angle of two fibers is 60°,the water collecting effect is the best.20 cm long fiber network collect 35.65μL water in 1 min.Hollow spindle-knotted microfibers with excellent water collection performance provide a new idea for large-scale water collection,drug encapsulation,tissue engineering and other fields.In the part of exploring and verifying the water collecting mechanism of hollow spindle-knotted microfiber,the motion mechanism and force of droplet in the process of water collecting are studied.The directional transport of droplets on the fiber results from the driving force caused by the Laplace pressure difference and the surface energy gradient.The polymerization process converts the surface energy of the droplet into kinetic energy,which increases the droplet volume while maintaining a stable state of low energy level.The droplet increases to a critical volume and falls when the capillary adsorption force fails to support gravity.The hollow structure provides additional capillary adsorption compared to the solid microfibers,lengthening the three phase contact line（TCL）and improving the suspension capacity of the fiber.The maximum droplet volume of hollow spindle-knotted microfiber（9.48μL）is 1663 times that of spindle volume（0.0057μL）,which is much higher than that reported in literature（571 times）.In the process of water collection,the difference of interfacial tension between the two ends of the droplet drives the droplet to move towards the intersection point.When the crossing Angle is 60°,the droplet moves furthest（2.51 mm）.In this study,the mechanism of hollow structure improving the water collecting ability of fiber is proposed,which provides the possibility for preparing fiber materials with high water collecting ability. |
