Membrane technology plays a significant role in modern separation technology and energy applications [1–5]. Particularly in the application of functional membrane with special wettability in oil-water separation [6–9], fog collection [10, 11], wastewater treatment [12, 13], catalytic contactor [14, 15], osmosis energy harvesting [16–18], aeration [19], and so on [20–23]. Janus membranes are a typical example, which exhibit asymmetric wettability, achieving manipulate liquid wetting and transport behavior by the aid of regulating the gas/liquid/solid multiphase interface [24, 25]. For example, Lin et al. prepared the fabrics with a unidirectional water-transfer effect by a special coating technique to construct a wettability gradient across the fabric thickness [26]. A Janus membrane aerator with superaerophobic-hydrophobic is fabricated by means of single-side polydopamine/polyethyleneimine codeposition, and it significantly promotes fine bubble generation and gas/liquid mass transfer [14]. A Janus copper foam with superhydrophobicity and superhydrophilicity by a novel method for on-demand oil/water separation is reported [27]. Cao et al. reported hydrophobic/hydrophilic cooperative Janus system that achieved reinforced fog collection ability by learning from nature’s strategy [28]. Although the preparation method is simple and operation, but the interfacial stability is relatively weak. Recently, the bubble transport behavior at the superwetting interface has aroused great interest of researchers. Sun et al. constructed various electrodes of nanomaterials with superaerophobicity and applied in energy conversion [29–32]. Moreover, Jiang and co-workers carried out a series of studies about the behaviors of gas bubbles on solid interfaces, including generation, growth, coalescence, release, transport, and collection [33–37]. However, to date, there is still a challenge both simple preparation of single-layer Janus material and directional manipulation of the bubble, improve the practical application in the field of pipe transportation of fluid, corrosion of ocean vessels, and control of foaming process.

Herein, we adopted a water-controlled infiltration strategy to fabricate a single-layer copper mesh (CM) with superhydrophobic and hydrophilic to achieve underwater bubble unidirectional penetration, demonstrating a new underwater bubble “diode”. We show that the bubble can penetrate through the Janus mesh from the superaerophobic side to the aerophilic side, but cannot penetrate from the other way around, owing to the competition of the bubble penetration and sprawl on the mesh. It is the cooperative driving forces of the gradient of Laplace pressure and ambient pressure that overcome the buoyant forces of bubbles to enable the desired unidirectional transport. This work demonstrates a promising methodology on guiding the mechanical behaviors of bubbles in water, which holds promise for applications in microfluidics or microdetectors.

As shown in Figure 1, the fabrication process of the single-layer Janus CM mainly consists of alkaline solution etching and FAS modification. Significantly, the critical step is water wetting-controlled modification. In the fabrication procedure of a single-layer Janus CM, to control the contact time between pre-curing PDMS and lower layer CM by means of adding deionized water. As the contact time is increased, the hydrophobic molecules of the PDMS pre-polymer continuously diffuse to the upper layer of the CM, resulting in the whole CM is superhydrophobicity. Therefore, the single-layer Janus CM can be prepared with water controlling the diffusion of hydrophobic molecules. Compared with the Janus membrane demonstrated in previous work,^[9] the current Janus mesh possesses a smaller mesh thickness, which can heighten the flow rate, conducive to promoting underwater air bubble transportation.

A superhydrophilic mesh was prepared at room temperature by etching the CM in an alkaline corrosive solution consisting of NaOH and (NH₄)₂S₂O₈ (Figure 1B). For the original CM with a smooth surface, as shown in Supplementary Figure S1. After hydrophobic controllable modification, a hydrophilic/superhydrophobic Janus mesh was obtained (Figure 1C). SEM images and contact angles for both the superhydrophobic side and the hydrophilic side of the single-layer Janus CM as shown in Figure 2. Needle-like Cu(OH)₂ nanowires, with a diameter of 300–400 nm and length of 2.5–3.5 um (Figures 2A,B). Moreover, massive hydrophobic substances discontinuously cover the surface of needle-like Cu(OH)₂ nanowires, resulting in the wettability of the hydrophobic side of the Janus CM is enhanced. The water contact angle (WCA) is 151° ± 1.1° (Figure 2C). In contrast, Cu(OH)₂ crystal is still distributed on the surface of the hydrophilic side of the Janus CM (Figures 2D,E), showing hydrophilic characteristics, and the WCA is 88° ± 0.8° (Figure 2F). As the resultant superhydrophobic/hydrophilic Janus CM is placed underwater, the superhydrophobic side is superaerophilic and the hydrophilic side is aerophobic. The superhydrophobic side of the Janus CM can catch an air layer on its surface, and serve as the superaerophilic layer for the underwater bubbles, while the hydrophilic side of the Janus CM can seize a water layer on its surface, and serve as the aerophobic layer for the underwater bubbles. Therefore, the single-layer Janus CM with superaerophilicity/aerophobicity is constructed through a liquid-regulated hydrophobic modification strategy.

As shown in Figure 3, the underwater bubble unidirectional transmission is further examined in conditions where we placed the single-layer Janus mesh horizontally in the water. The air bubble is blocked from the superhydrophobic side (Figure 3A). The small air bubble first contacts the superhydrophobic side of Janus mesh, in which an air layer is captured on the surface of the superhydrophobic side in the water. As the small air bubble approaches the superhydrophobic side, it merges with the trapped air layer and spreads out on the surface of superhydrophobic side, and thus the air bubble cannot pass through the single-layer Janus mesh. However, the superhydrophobic side is up and the hydrophilic side is down (Figure 3C), the air bubble passes through the hydrophilic side partly and touches the superhydrophobic side of the single-layer Janus mesh, achieving the penetration with driven force originating from the bubble buoyant force (F _B ) and Laplace pressure difference (P _L ). From the above experiments of the air bubble unidirectional penetration, it can be concluded that the single-layer Janus CM possesses the diode transport behavior.

As shown in Figure 4, a hypothetical mechanism is suggested to interpret the underwater air bubble unidirectional penetration phenomenon. The single-layer Janus CM is 60^#, the diameter of a copper wire is 0.17 mm and the pore diameter is 253 μm. During the air bubble penetration experiment, the air layer on the superhydrophobic surface plays a decisive role. When the air bubble touches the hydrophilic side, the bubble is intercepted and failed to penetrate the Janus CM (Figure 4A). Because the superhydrophobic side (aerophilic side) is lower and the hydrophilic side (superhydrophobic side) is upper, resulting in a gas film on the lower superhydrophobic side interface. The air bubble first contacts the gas-film and fuses into the gas-film. In contrast, as the hydrophilic side is above and superhydrophobic side is below, the water enters micro/nano binding structure. Meanwhile, the air bubble upon touching the superhydrophobic side of single-layer Janus mesh can through the water layer caught in the gaps of the hydrophilic mesh grid, generating a gas channel between the bubble and the upper-gas layer. As a result, the air bubble firstly moves upward with the action of buoyancy and penetrates the water layer, then merges with the thin gas-film on the surface of the superhydrophobic side. Finally, with the continuous increase of bubble volume, the air bubble penetrates the Janus CM and separates from its surface resulting from the combined action of buoyancy force and Laplace pressure. In this process, buoyancy force and Laplace pressure are both upward forces, forming an upward resultant force. Interestingly, a differential Laplace pressure appears in the gas phase, in which the radius of curvature of the air bubble is importantly smaller than that of the upper superhydrophobic gas-layer. According to Young–Laplace Equation 1: Δ P = γ ( 1 R 1 − 1 R 2 ) (1) where γ is the surface tension of water, R ₁ is the radius of curvature of the air bubble, R ₂ is the radius of curvature of the gas-film, r is the diameter of a copper wire. Obviously, R ₁ is smaller than R ₂ , so the Laplace pressure of the air bubble is greater than that in the lower layer.

Throughout the process, the buoyancy force (F _B ) of air bubble plays a significant role in the process of bubble transmission across the water film, and the Laplace pressure difference plays another critical role in the penetration process when the bubble and the gas-film are combined [38–41]. Based on the above analysis, the minimum critical force (ΔP _min ) of bubble penetration through the single-layer Janus CM can be calculated quantitatively. Suppose the bubble is a regular sphere with radius R ₁ , the bubble volume can be calculated according to the Equations 2, 3: V = 4 3 π R 1 3 (2) F B = ρ g V (3) where V is the volume of air bubble, ρ is the density of water (1 g/cm³), g is the gravity acceleration (9.8 m/s. In the experiment, the air bubble is subject to the resistance (F _R ) of water, but the volume of the air bubble is about 250 μm, which the F _R may be negligible. After calculation, the driving force of 180 Pa can be generated by the Laplace pressure, while the driving force caused by the buoyancy of the bubble itself is only 1.6 × 10^–10 Pa, the former is far greater than the latter. Therefore, the bubble moves vertically upward only under the force of Laplace pressure. Moreover, a critical pressure to penetrate the CM when the small air bubble is transmitted upward through Janus copper also can be calculated by the Equations 4, 5: P = 4 γ | cos ⁡ θ | 4 (4) D = 2 r + d (5) where θ is the contact angle of underwater bubbles, d is the mesh aperture, and D is the width of bubbles passing through CM. In this work, the diameter of a copper wire is 0.17 mm and the pore diameter is 253 μm. According to Eqs. 4, 5, the critical penetration pressure is 420 Pa. Therefore, the difference of Laplace pressure (P _L ) should cause the movement of air bubbles toward the upper layer of superhydrophobic side.

Significantly, the parameter of pore size plays a crucial role in water permeation^[14] Next, we further explore how the pore size of our meshes affects the unidirectional penetration. According to Eqs 4, 5, we calculated 20^#, 40^#, 80^#, and 100^# CM, respectively (Supplementary Figure S2). The results show that the critical penetration pressure increases with the increase of the mesh number of CM (Supplementary Table S1). In other words, the smaller of pore size of the single-layer Janus CM, the greater the resistance of air bubbles through CM. The theoretical results are consistent with the experimental results.

In summary, a novel method was reported for the preparation of the single-layer Janus CMs. Based on the Janus CM with superhydrophobicity and hydrophilicity, we realize the underwater air bubble unidirectional transportation. The experimental results show that the bubble can easily pass through the Janus mesh from the hydrophilic side to superhydrophobic side, while it is blocked from the superhydrophobic side to hydrophilic side, due to the competition of the air bubble penetration and spread on the mesh surface with special infiltrating. This interestingly enables an underwater bubble “diode”. The transporting behavior of underwater bubbles is controllably regulated by surface infiltrating and pore size of the Janus mesh. This work presents a facile method to fabricate a single-layer mesh gas diode Janus membrane, and may serve as enlightening clues for the design and preparation of smart materials and new technologies for practical applications in fluid transportation pipes, engineering vessels, and froth flotation processes.