LAUSR

DOI: 10.1002/admi.201902128 particles,[4–6] collecting useful gas like methane or harmful gas like toxic sulfide gas in water.[7,8] Meanwhile, the bubbles also do harm in microfluidic field, which increase the fluid frictional and cause cultured cell death,[9–12] and in gas-generating electrochemical reaction process in which the generated bubbles stick on the electrode and account for the reduction of the reaction efficiency.[13,14] Hence, bubble manipulation could foster strengths and circumvent weaknesses of underwater bubbles’ unbridled behaviors and have received fevered attention.[15–19] The underwater bubble manipulation in the aspect of superwettability transition[20–29] and unidirectional transportation relying on the substrate with extreme wettability attracts a plenty of researches.[30–33] Yong et al. prepared Cu(OH)2 nanoneedles on a copper mesh by the immersion of the copper mesh in NaOH/(NH4)S2O8 solution.[20] The nanoneedle-structured mesh is underwater superaerophobic and has the ability of gas interception in water. After modified with fluoroalkylsilane, the resultant mesh exhibits underwater superaerophobicity and could absorb underwater gas bubbles. Huo et al. realized the reversible transition between underwater superaerophilicity and superaerophobicity on the femtosecond laser-induced rough polytetrafluoroethylene (PTFE) surface by vacuum-pumping treatment.[21] The structured superaerophilic PTFE sheet could switch to be superaerophobic in water derived from its structures being wetted and filled by the water under external pressure produced by vacuum pumping. Yong et al. reported the superwettability transition of underwater bubbles on the femtosecond laser-structured polydimethylsiloxane (PDMS) polymer sheet through oxygen plasma irradiation.[22] By inducing hydrophilic group, silanol radical groups (SiOH), onto the laser-induced rough PDMS surface, the original underwater superaerophilicity of structured PDMS sheet transformed into underwater superaerophobicity. Although these reports could control the bubble behavior in water on the chosen substrates, the controls of bubble were homogeneous in cross-sectional direction that limits the applications of the resulted sample. Janus substrates that described the asymmetric bubble behavior on upper and lower surfaces made improvements to those symmetrical substrates.[30–32] Jiang et al. fabricated To control the behavior of underwater bubbles, stainless steel meshes are treated through femtosecond laser processing, and the bubble absorption, bubble interception, and unidirectional bubble passage are realized by using structured meshes. The surface of the mesh presents a micro–sub-micro– nano trinary-scale structure (microscale mesh wires, sub-microripples, and nanoparticles) after one-step laser ablation on both sides. The surface shows superhydrophilic in air and superaerophobic once immersed in water. After further modified with fluoroalkylsilane, the wettability of the sample surface is switched to be superaerophilic in water with bubble being absorbed by the sample. When a plenty of underwater bubbles arrive at the structured stainless steel mesh surface, they can be blocked by the underwater superaerophobic mesh but pass through the underwater superaerophilic mesh. In addition, after the mesh being treated only one side and further modified, it is to be Janus mesh and presents asymmetrical wettability of aerophilicity/superaerophilicity. The Janus mesh shows the unidirectional passage of underwater bubbles. Bubbles can only penetrate from aerophilic side to superaerophilic side, but be blocked from the other direction. The mesh is verified to be used to eliminate the stuck bubbles in the container.