Numerous natural systems contain surfaces or threads that enable directional water transport1,2,3,4,5,6,7. This behaviour is usually ascribed to hierarchical structural features at the microscale and nanoscale, with gradients in surface energy8,9 and gradients in Laplace pressure10 thought to be the main driving forces. Here we study the prey-trapping pitcher organs of the carnivorous plant Nepenthes alata. We find that continuous, directional water transport occurs on the surface of the ‘peristome’—the rim of the pitcher—because of its multiscale structure, which optimizes and enhances capillary rise11,12 in the transport direction, and prevents backflow by pinning in place any water front that is moving in the reverse direction. This results not only in unidirectional flow despite the absence of any surface-energy gradient, but also in a transport speed that is much higher than previously thought. We anticipate that the basic ‘design’ principles underlying this behaviour could be used to develop artificial fluid-transport systems with practical applications.
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We thank the National Natural Science Foundation of China (grant nos 51290292, 51175020, 51475029 and 21431009) and the Innovation Foundation of the Beijing University of Aeronautics and Astronautics (BUAA; now Beihang University) for PhD Graduates for providing financial support. We also thank G. Wang and Y. Lai from the National Natural Science Foundation of China, and S. Wang from Tianjin University, for support and discussions.
The authors declare no competing financial interests.
Extended data figures and tables
a, SEM image of a cross-section of the peristome, showing that two-order, regular radial ridges align perfectly to form regular two-order parallel hierarchical microgrooves. b, SEM image showing a side view of the vertical section of a microcavity. The image reveals the microcavity’s geometry in the plane perpendicular to the peristome surface, and shows that the microcavity has a sharp edge with an angle, ϕ, of approximately 2° to 8°.
a–h, Sequential cross-sectional images of the microcavity (moving deeper and deeper into the microcavity) show that it has a gradient wedge corner, with an opening angle that varies from roughly 90.5° to 28.2°.
a, High-speed digital images of water filling a single duck-billed microcavity show that the water rapidly rises along the wedge corner and squeezes air out of the microcavity, rising into the cavity to fill it completely. b, The water-filling process; blue lines correspond to the water boundary change with time.
a, A water droplet (about 4 μl; white circle) is dropped from a microsyringe and spreads quickly without a significant contact angle, indicating that the peristome is superhydrophilic. The water spreading also shows substantial directionality from the peristome’s inner side to the outer side. b, EDS measuring points along the second-order microgrooves, and associated carbon/oxygen (C/O) values. These six values are almost the same as each other, indicating no notable chemical gradient along the microgrooves.
Like the natural peristome surface, the surfaces of the replicas also possess two-order microgrooves (left); moreover, duck-billed microcavities (right) are distributed along the second-order microgrooves.
a, b, The natural peristome. a, Water transport occurred when the water droplet made contact with the inner side. b, Water could not be transported when it made contact with the outer side. Thus, the natural peristome has a capacity for directional water transport. c, d, A hydrophobic replica. Water could not be transported from either the inner (c) or the outer (d) side. e, f, A superhydrophilic replica. e, Water was transported from the inner side to the outer side. f, Water could not be transported from the outer side to the inner side.
a, Capillary rise in a fixed wedge, in which the bottom opening angle, α1, equals the top opening angle, α2. The water rise decreases as the opening angle increases. b, Capillary rise in a gradient wedge, with α2 = 5°, and α1 > α2; here, water rise is higher than that in the fixed wedge. c, Capillary rise in a wedge with the opposite gradient to that in panel b; α2 = 90°, α1 < α2. Here, the water rise was the lowest of that seen in these four experiments. The red triangles in a–c show the height of the water rise. d, Capillary rise in a top-closed wedge, with α2 = 0°. The water can rise along the vertical wedge corner to fill the horizontally top-closed wedge independently of α1. Diagrams showing the water rise are at the right of a–d. The heights were limited to 100 mm because of the restricted sizes of the available material. e, The experimental water-rise results show that the water rise generally decreases with increasing α1. Water rise in the gradient wedge (red line) is higher than that in the fixed wedge (black line); the top-closed wedge (yellow line) shows additional horizontal water transport. Water rise was the lowest in the opposite gradient wedge (green line). Error bars indicate standard deviations from at least five independent measurements.
Extended Data Figure 8 Volume of rise water and water-retention time for the four types of wedge corner.
a, Volume of rise water in the four types of wedge—fixed wedge, gradient wedge, opposite gradient wedge, and top-closed wedge. The volume of rise water for the top-closed wedge is the largest, and is about 40% greater than that for the fixed wedge. b, Water-retention time in the four types of wedge. Water-retention time for the top-closed wedge is the longest, and is about twice that for the fixed wedge. Error bars indicate standard deviations from at least five independent measurements.
a, Liquid rise in the gradient wedge corner. b, Section view of the liquid rise in panel a. α1, bottom opening angle of a wedge; α2, top opening angle; g, gravitational constant; β1 and β2, angle between the plate and the horizontal plane (β1 = β2); h, height of the intersecting plates; wup, up width of selected small liquid element; wdown, down width; dz, thickness of the small liquid element.
A small amount of water (approximately 0.1 μL) was deposited on the inner surface of the peristome. Water was transported along the channel in the direction from the inner side to the outer side of the peristome. The starting boundary line of the deposited water was fixed indicating a water pinning phenomenon in the direction from the outer side to the inner side. (MP4 13267 kb)
Approximately 0.2 μL water was deposited on the peristome surface and water was transported along two neighbouring large channels. Water transport was confined within the separate large channels. (MP4 5010 kb)
Approximately 0.01 μL water was deposited on the peristome surface and the water top boundary was monitored by the high-speed camera. Water initially spread along the wedge corner of the micro-cavity and then extruded air out to fully fill the micro-cavity. Water filling of the next micro-cavity started before the fully filling of the prior micro-cavity and the continuous filling of a single micro-cavity accomplished the water transport from the inner side to the outer side of the peristome. (MP4 2722 kb)
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Chen, H., Zhang, P., Zhang, L. et al. Continuous directional water transport on the peristome surface of Nepenthes alata. Nature 532, 85–89 (2016). https://doi.org/10.1038/nature17189
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