Developing an effective system to clean up large-scale oil spills is of great significance due to their contribution to severe environmental pollution and destruction. Superwetting membranes have been widely studied for oil/water separation. The separation, however, adopts a gravity-driven approach that is inefficient and discontinuous due to quick fouling of the membrane by oil. Herein, inspired by the crossflow filtration behavior in fish gills, we propose a crossflow approach via a hydrophilic, tilted gradient membrane for spilled oil collection. In crossflow collection, as the oil/water flows parallel to the hydrophilic membrane surface, water is gradually filtered through the pores, while oil is repelled, transported, and finally collected for storage. Owing to the selective gating behavior of the water-sealed gradient membrane, the large pores at the bottom with high water flux favor fast water filtration, while the small pores at the top with strong oil repellency allow easy oil transportation. In addition, the gradient membrane exhibits excellent antifouling properties due to the protection of the water layer. Therefore, this bioinspired crossflow approach enables highly efficient and continuous spilled oil collection, which is very promising for the cleanup of large-scale oil spills.Keywords: crossflow; fish gill; gradient; oil spill; superhydrophilic;

AbstractExternal-field-responsive liquid transport has received extensive research interest owing to its important applications in microfluidic devices, biological medical, liquid printing, separation, and so forth. To realize different levels of liquid transport on surfaces, the balance of the dynamic competing processes of gradient wetting and dewetting should be controlled to achieve good directionality, confined range, and selectivity of liquid wetting. Here, the recent progress in external-field-induced gradient wetting is summarized for controllable liquid transport from movement on the surface to penetration into the surface, particularly for liquid motion on, patterned wetting into, and permeation through films on superwetting surfaces with external field cooperation (e.g., light, electric fields, magnetic fields, temperature, pH, gas, solvent, and their combinations). The selected topics of external-field-induced liquid transport on the different levels of surfaces include directional liquid motion on the surface based on the wettability gradient under an external field, partial entry of a liquid into the surface to achieve patterned surface wettability for printing, and liquid-selective permeation of the film for separation. The future prospects of external-field-responsive liquid transport are also discussed.

The transport of fluids at functional interfaces, driven by the external stimuli, is well established. The lossless transport of oil-based fluids under water remains a challenge, however, due to their high stickiness towards the surface. Here, a superhydrophilic and underwater superoleophobic tri-phase water/oil/solid nanoarray surface has been designed and prepared. The unique tri-phase surface exhibits underwater superoleophobic properties with an extremely low stickiness towards oil-based fluids. The magnetic-field-driven manipulation and transport of oil-based magnetic fluids are demonstrated under water, which opens up a new pathway to design flexible and smart devices for the control and transfer of liquid droplets by using tri-phase systems.

Oily wastewater is always a threat to biological and human safety, and it is a worldwide challenge to solve the problem of disposing of it. The development of interface science brings hope of solving this serious problem, however. Inspired by the capacity for capturing water of natural fabrics and by the underwater superoleophobic self-cleaning property of fish scales, a strategy is proposed to design and fabricate micro/nanoscale hierarchical-structured fabric membranes with superhydrophilicity and underwater superoleophobicity, by coating scaly titanium oxide nanostructures onto fabric microstructures, which can separate oil/water mixtures efficiently. The microstructures of the fabrics are beneficial for achieving high water-holding capacity of the membranes. More importantly, the special scaly titanium oxide nanostructures are critical for achieving the desired superwetting property toward water of the membranes, which means that air bubbles cannot exist on them in water and there is ultralow underwater–oil adhesion. The cooperative effects of the microscale and nanoscale structures result in the formation of a stable oil/water/solid triphase interface with a robust underwater superoleophobic self-cleaning property. Furthermore, the fabrics are common, commercially cheap, and environmentally friendly materials with flexible but robust mechanical properties, which make the fabric membranes a good candidate for oil/water separation even under strong water flow. This work would also be helpful for developing new underwater superoleophobic self-cleaning materials and related devices.Keywords: micro/nanoscale hierarchical structure; scaly; superwetting; underwater superoleophobic; water/oil separation

Stimuli-responsive surface wettability has been intensively studied, especially wettability controlled by photoelectric cooperation, which appears to be a trend for more effective surface wetting. In this field, the patterning of controllable surface wettability is still a challenge in the application of liquid-printing techniques because of the high adhesion and high responsive voltage, as well as low mechanical strength, of the substrate. Herein, we have demonstrated the patterning of liquid permeation controlled by photoelectric cooperative wetting on the micro/nano hierarchically structured ZnO mesh film. The special micro/nano hierarchically structured ZnO mesh is beneficial for lowering adhesion force on the mesh surface than those of the TiO2/AAO nanopore array films previously reported for the discontinuous tri-phase contact line, in addition to precisely controlled microscale liquid movement with considerably lower threshold voltage for the hierarchical structure. Moreover, the stainless-steel mesh with different pore sizes as a substrate behaves with higher mechanical strength and lower cost, compared with the anodized Ti mesh. Thus, this work is promising for accelerating the development of patterned liquid permeation and extending the application of micro/nanofluidic system and micronanoelectronic technology.

The surface wettability response has been intensively studied under external stimulus, and the cooperation of different stimuli seems a trend for more effective surface wetting. Despite much progress in this field, the patterning of controllable surface wettability is still a challenge, which is a very important issue for printing techniques. Here, we have developed an approach for the photoelectric cooperative wetting induced liquid permeation through a TiO2 nanotube array coated Ti mesh. The patterned liquid permeation can be realized by patterned light illumination under a voltage which is lower than the electrowetting induced permeation threshold voltage. The permeation process and mechanism are discussed in detail. The results indicate that the microscale movement of a liquid can be controlled precisely by the surface micro/nano hierarchical structure of the device, with a low adhesion and responsive voltage. Therefore, this work is important in the research and application of liquid printing, moreover, it provides a new approach to develop and apply novel devices such as micro/nanofluidic systems, microreactors and micro-nanoelectronic technologies.

Oil contaminated water is a common problem in the world, thus to effectively separate water and oil is an urgent task for us to resolve. By control of surface wettability of a solid substrate, both superhydrophobicity and superoleophilicity on a film can be realized, which is necessary for water and oil separation. Here we report a stable superhydrophobic and superoleophilic ZnO-coated stainless steel mesh film with special hierarchical micro/nanostructures that can be used to separate a water and oil mixture effectively. Namely, the film is superhydrophobic and water cannot penetrate the mesh film because of the large negative capillary effect, while the film is superoleophilic and liquid paraffin oil can spread out quickly and permeate the mesh film spontaneously due to the capillary effect. A detailed investigation indicates that microscale and nanoscale hierarchical structures and the appropriate size of the microscale mesh pores on the mesh films play an important role in obtaining the excellent water and oil separation property. This work provides an alternative to current separation meshes and is promising in various important applications such as separation and filtration, lab-on-a-chip devices and micro/nanofluidic devices.

The transport of fluids at functional interfaces, driven by the external stimuli, is well established. The lossless transport of oil-based fluids under water remains a challenge, however, due to their high stickiness towards the surface. Here, a superhydrophilic and underwater superoleophobic tri-phase water/oil/solid nanoarray surface has been designed and prepared. The unique tri-phase surface exhibits underwater superoleophobic properties with an extremely low stickiness towards oil-based fluids. The magnetic-field-driven manipulation and transport of oil-based magnetic fluids are demonstrated under water, which opens up a new pathway to design flexible and smart devices for the control and transfer of liquid droplets by using tri-phase systems.

Oil contaminated water is a common problem in the world, thus to effectively separate water and oil is an urgent task for us to resolve. By control of surface wettability of a solid substrate, both superhydrophobicity and superoleophilicity on a film can be realized, which is necessary for water and oil separation. Here we report a stable superhydrophobic and superoleophilic ZnO-coated stainless steel mesh film with special hierarchical micro/nanostructures that can be used to separate a water and oil mixture effectively. Namely, the film is superhydrophobic and water cannot penetrate the mesh film because of the large negative capillary effect, while the film is superoleophilic and liquid paraffin oil can spread out quickly and permeate the mesh film spontaneously due to the capillary effect. A detailed investigation indicates that microscale and nanoscale hierarchical structures and the appropriate size of the microscale mesh pores on the mesh films play an important role in obtaining the excellent water and oil separation property. This work provides an alternative to current separation meshes and is promising in various important applications such as separation and filtration, lab-on-a-chip devices and micro/nanofluidic devices.

The surface wettability response has been intensively studied under external stimulus, and the cooperation of different stimuli seems a trend for more effective surface wetting. Despite much progress in this field, the patterning of controllable surface wettability is still a challenge, which is a very important issue for printing techniques. Here, we have developed an approach for the photoelectric cooperative wetting induced liquid permeation through a TiO2 nanotube array coated Ti mesh. The patterned liquid permeation can be realized by patterned light illumination under a voltage which is lower than the electrowetting induced permeation threshold voltage. The permeation process and mechanism are discussed in detail. The results indicate that the microscale movement of a liquid can be controlled precisely by the surface micro/nano hierarchical structure of the device, with a low adhesion and responsive voltage. Therefore, this work is important in the research and application of liquid printing, moreover, it provides a new approach to develop and apply novel devices such as micro/nanofluidic systems, microreactors and micro-nanoelectronic technologies.