The unique structural features of the cactus, such as the spines and the barbs with aligned gradient grooves and the cooperation between the spines and the trichomes, contributed to this system’s excellent functioning. To characterize the structure–function relationship in this efficient fog collection process, we propose a potential mechanism. Figure 4a shows an overview of the entire process of fog collection. ‘Deposition’ initially occurs on the barb and the spine, with the water drops moving directionally along them. As the deposition proceeds and the water drops coalesce, these drops increase in size, leaving from the tip side of the spine (‘Collection’). The bigger drops are then further transported along the gradient grooves (‘Transportation’) and absorbed through the trichomes at the base of the spines (‘Absorption’). The gradient of the Laplace pressure arising from the conical shape of the spine and the gradient of the surface-free energy arising from the gradient of the surface roughness along the spine are the two forces that drive the directional movement of the water drops.

Figure 4: Mechanism of the fog collection on the cactus. (a) An overview of the efficient fog collection system of O. microdasys progressing from ‘1 Deposition’ on the barbs and the spine to ‘2 Collection’ on the tip of the spine, ‘3 Transportation’ on the gradient grooves, and ‘4 Absorption’ upon contact with the trichomes. (b,c) Analysis of the driving forces arising from the gradient of the Laplace pressure and the gradient of the surface-free energy. A water drop on a conical spine should move towards the base side with the larger radius (R 2 ) due to the relatively smaller Laplace pressure (b). In addition to the conical shape, the surface of the spine was covered with multi-level grooves. The gradient of the microgrooves was sparser near the base than near the tip of the spine (as indicated by the black arrows). The aligned submicrogrooves were similar along the spine as indicated by the red arrows (c). This gradient of the microgrooves produced a gradient of roughness, contributing to a gradient of the surface-free energy along the spine, driving the water drops towards the base side. (d) Cooperation among the multiple spines, the multiple trichomes and the spines-trichomes. The water drops that progress through the ‘1 Deposition’, ‘2 Collection’ and ‘3 Transportation’ processes are quickly absorbed into the cactus stem upon contact with the trichomes (‘4 Absorption’). Full size image

A water drop can be driven by chemical7,8, thermal24,25 and shape9,26 gradients. Specifically, a drop on a conical-shaped surface is often driven to the side with the larger radius due to the gradient of the Laplace pressure9. As illustrated in Fig. 4b, a cactus spine can be considered as a conical object with aligned grooves. This type of conical shape generates a Laplace pressure difference (ΔP curvature ) between the two opposite sides of the drop5,9 as follows:

where R is the local radius of the spine (R 1 and R 2 are the local radii of the spine at the two opposite sides of the drop), γ is the surface tension of water, R 0 is the drop radius, α is the half-apex angle of the conical spine, and dz is the incremental radius of the spine (Fig. 4b). The Laplace pressure on the region near the spine’s tip (small radius R 1 ) is larger than that near the base (large radius R 2 ). This difference (ΔP curvature ) within the water drop initiates a driving force that makes the drop move from the tip to the base side along the cactus spine.

In addition to the gradient of the Laplace pressure, the gradient of the surface-free energy is another driving force. Specifically, the microgrooves on the cactus spines have a gradient in width. The microgrooves are sparser near the base (less rough) than near the tip (rougher) of the spine (Fig. 4c and Supplementary Fig. S2). This roughness can be described using Wenzel’s equation27 as follows:

where r is the roughness factor defined as the ratio of the actual surface area to the geometric projected area of a rough surface, and θ and θ w are the intrinsic and apparent contact angles, respectively. The gradient of roughness generates a gradient of wettability, (that is, a gradient of surface-free energy)28,29,30. For the surface of the cactus spines covered with vegetable wax19, the tip is rougher and more hydrophobic; whereas the base is less rough and less hydrophobic (see Supplementary Fig. S6). In other words, the tip of the spine has a lower surface-free energy than the base. This gradient of the surface-free energy produces a driving force F, driving the water drops collected on the tip directionally towards the base5 as described as follows:

where θ A and θ R are the advancing and receding contact angles of water drops on the middle of the spine, respectively, and dl is the integral variable along the length of the middle of the spine from the region near the tip (l tip ) to the region near the base (l base ). The roughness arising from the microgrooves on the cactus spine enhances the gradient of the Laplace pressure (see Supplementary Fig. S7), contributing to the movement of the water drops along the cactus spines.

The aligned grooves can also generate an anisotropic contact angle hysteresis (CAH)31,32 in the direction parallel or perpendicular to the grooves, enhancing the directional movement of the water drops along the grooves on the barbs and spines (See Supplementary Fig. S8). Specifically, a water drop moves more readily in the direction that is parallel to the aligned structures than in other directions33. The drop has a continuous, three-phase contact line along the grooves that reduces the energy barrier and therefore facilitates the spreading and moving of the drop34. Without a gradient in width along the cactus spine, the submicrogrooves further enhance this anisotropic CAH (Supplementary Fig. S3), facilitating the directional movement of the water drops along the barbs and the spines.

In addition to the aligned grooves, the oriented barbs reduce the drop’s ability to spread or move towards the tip side with the barbs, facilitating movement towards the base side lacking barbs. With an anisotropic CAH35, a water drop on a surface with asymmetrical structures has a preferred direction to spread or move2,21,22. Because the barbs orient in the direction towards the spine’s base (Fig. 1e), the water drops prefer to spread and move along the oriented direction of the barbs. This CAH difference further aids the growth and movement of the water drops to the base side of the spine under deposition.

These fog collection abilities have been examined using the mechanisms of a single spine. To extend this approach to a more complete system with tens of spines and trichomes in a single cluster (Fig. 1b), the cooperation among spines-trichomes, multiple spines and multiple trichomes in the collection process (Fig. 4d) must also be considered. In addition, the array of multiple clusters on the stem surface (Fig. 1a) may further enhance the fog collection ability. The integration of the multi-level structures and the consequent integration of the multi-functional abilities, including the deposition, collection, transportation and absorption of the water drops, may provide O. microdasys with an efficient fog collection system. The investigation into the structure–function relationship within this system may offer systematic opinions that can be used to design novel materials and devices to efficiently collect fog. By mimicking the primary characters of the areole system of O. microdasys and considering the existing artificial fog collectors36,37,38, an artificial system with similar mechanisms to cactus is currently being developed, as shown in Supplementary Fig. S9 and Supplementary Note 1. Owing to its high efficiency and portability, this artificial system may find applications in the self water supply for plants and human beings that live in arid areas (Supplementary Fig. S9).