Xiaofang Zhang

Photo-induced water–oil separation based on switchable superhydrophobicity–superhydrophilicity and underwater superoleophobicity of the aligned ZnO nanorod array-coated mesh films

Stimulus-responsive surface wettability, especially photoresponsive surface wettability, has been intensively studied. Meanwhile, multifunctional surfaces, especially for the treatment of oil contaminated water, have aroused worldwide attention. Recently, pH-responsive surfaces with controllable oil–water separation have also been reported. However, photoresponsive water–oil separation is still a challenge. Here we report photo-induced water–oil separation based on the switchable superhydrophobicity–superhydrophilicity and underwater superoleophobicity of aligned ZnO nanorod array-coated mesh films, which shows excellent controllability and high separation efficiency of different types of water–oil mixtures in an oil–water–solid three-phase system. The underwater superoleophobicity of the aligned ZnO nanorod array-coated stainless steel mesh film can effectively prevent the mesh film from being polluted by oils. This work is promising in photo-induced water–oil mixture treatments such as water-removal from a micro-reaction system and controllable filtration, and may also provide interesting insight into the design of novel functional devices based on controllable surface wettability.

Underwater Self-Cleaning Scaly Fabric Membrane for Oily Water Separation

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.

Photocontrollable water permeation on the micro/nanoscale hierarchical structured ZnO mesh films.

Most research of responsive surfaces mainly focus on the wettability transition on different solid substrate surfaces, but the dynamic properties of the micro/nanostructure-enhanced responsive wettability on microscale pore arrays are lacking and still remain a challenge. Here we report the photocontrollable water permeation on micro/nanoscale hierarchical structured ZnO-coated stainless steel mesh films. Especially, for aligned ZnO nanorod array-coated stainless steel mesh film, the film shows good water permeability under irradiation, while it is impermeable to water after dark storage. A detailed investigation indicates that the special nanostructure and the appropriate size of the microscale mesh pores play a crucial role in the excellent controllability over water permeation. The excellent controllability of water permeation on this film is promising in various important applications such as filtration, microreactor, and micro/nano fluidic devices. This work may provide interesting insight into the design of novel functional devices that are relevant to surface wettability.

External-Field-Induced Gradient Wetting for Controllable Liquid Transport: From Movement on the Surface to Penetration into the Surface

External-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.

Patterned liquid permeation through the TiO2 nanotube array coated Ti mesh by photoelectric cooperation for liquid printing

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.

Closed Pore Structured NiCo2O4-Coated Nickel Foams for Stable and Effective Oil/Water Separation

To solve the serious problem caused by oily wastewater pollution, unique interface designs, for example, membranes with superwetting properties such as superhydrophobicity/superoleophilicity and superhydrophilicity/underwater superoleophobicity, provide a good way to achieve oil/water separation. Here, inspired by the liquid storage property of the honeycomb structure, we propose a strategy to fabricate NiCo2O4-coated nickel foams for stable and efficient oil/water separation. NiCo2O4 with a closed-pore structure was formed by assembling nanoflakes with a micro/nanoscale hierarchical structure. Compared with nickel foam coated by NiCo2O4 with an open-pore structure (NiCo2O4 nanowires), the enclosed nanostructure of NiCo2O4 nanoflakes can firmly hold water for a more stable superhydrophilic/underwater superoleophobic interface. As a consequence, the NiCo2O4-nanoflake-coated nickel foam has a larger oil breakthrough pressure than the NiCo2O4-nanowire-coated nickel foam because of a slightly larger oil advancing angle and a lower underwater oil adhesion force, which makes it more stable and efficient for oil/water separation. Moreover, the NiCo2O4-coated nickel foams have excellent chemical and mechanical stability, and they are reusable for oil-water separation. This work will be beneficial for the design and development of stable underwater superoleophobic self-cleaning materials and related device applications, such as oil/water separation.