Oil/water separation is a field of high significance as it has direct practical implications for resolving the problem of industrial oily wastewater and other oil/water pollution. Therefore, the development of functional materials for efficient treatment of oil-polluted water is imperative. In this feature article, we have reviewed the recent progress of oil/water separation technologies based on filtration and absorption methods using various materials that possess surface superwetting properties. In each section, we present in detail representative work and describe the concepts, employed materials, fabrication methods, and the effects of their wetting/dewetting behaviors on oil/water separation. Finally, the challenges and future research directions of this promising research field are briefly discussed.

Oil/water separation techniques: a review of recent progresses and future directions

Microfluidic devices have attracted increasing attention in the fields of biomedical diagnostics, food safety control, environmental protection, and animal epidemic prevention. Micromixing has a considerable impact on the efficiency and sensitivity of microfluidic devices. This work reviews recent advances on the passive and active micromixers for the development of various microfluidic chips. Recently reported active micromixers driven by pressure fields, electrical fields, sound fields, magnetic fields, and thermal fields, etc. and passive micromixers, which owned two-dimensional obstacles, unbalanced collisions, spiral and convergence-divergence structures or three-dimensional lamination and spiral structures, were summarized and discussed. The future trends for micromixers to combine with 3D printing and paper channel were brought forth as well.

A Review on Micromixers

Separation of oil/water mixtures has become an increasingly important subject worldwide because of frequent oil spill incidents and industrial oily wastewater. The filter materials for the mixtures separation need to collect first and then filter, which is energy-intensive and cumbersome. Utilization of adsorbents appears to be an economical and direct way for mixture disposal. In this study, the superhydrophobic polyurethane (PU) sponge is fabricated by coating superhydrophobic attapulgite (APT) onto its skeleton surface. The coated PU sponges exhibit robust superhydrophobicity and high adsorption capability under a series of harsh conditions, which are used for the separation of mixtures of oil and various corrosive solutions and hot water. Moreover, the coated PU sponges can selectively adsorb oils from mixtures under extreme and harsh turbulent conditions. Furthermore, the coated PU sponge connected to a vacuum system could remove up to 3200 times its self-weight in kerosene within 20 s under a vacuum degree of about 30 kPa. More importantly, the coated PU sponges can separate tiny oil droplets from surfactant-stabilized oil-in-water emulsions with a separation efficiency of over 99.87% by employing a compression and agitation procedure. All of these characteristics make the coated PU sponge a promising adsorbent for realizing oil and organic pollutant removal in realistic aquatic environments.

Robust superhydrophobic attapulgite coated polyurethane sponge for efficient immiscible oil/water mixture and emulsion separation

Although the construction of superwettability materials for oil/water separation has been developed rapidly, the postprocess of the used separation materials themselves is still a thorny problem due to their nondegradation in the natural environment. In this work, we reported the functionalization of polylactic acid (PLA) nonwoven fabric as superoleophilic and superhydrophobic material for efficient treatment of oily wastewater with eco-friendly post-treatment due to the well-known biodegradable nature of PLA matrix.

Functionalization of Biodegradable PLA Nonwoven Fabric as Superoleophilic and Superhydrophobic Material for Efficient Oil Absorption and Oil/Water Separation

Efficient and rapid separation of emulsified oil/water mixtures is urgently needed and still remains a worldwide challenge. Even though traditional superhydrophobic/superoleophilic filtration membranes have demonstrated to be effective for separation of water-in-oil emulsions, they still suffer from complicated fabrication procedures and lower flux, resulting from their nanoscale pore size. Herein, green desert sands (50 μm to 1 mm) with under-oil superhydrophilicity were introduced, for the first time, to develop into a layer for efficient gravity-directed separation of various water-in-oil emulsions, which could avoid not only sophisticated filtration membranes fabrication process but also the use of expensive low energy materials of fluorosilane involved in traditional superhydrophobic materials. It is worth mentioning that the sand layer could serve as an adsorbent material with under-oil superhydrophilicity, achieving ultrafast gravity-driven separation of tiny water droplets from various water-in-oil emulsions with flux as high as 2342 L m−2 h even though the interspacing between the sand particles is greater than the size of emulsified droplets. Moreover, the sand can be abundantly obtained from deserts, which is another advantage that the current filtrate materials do not possess. In summary, this study provides a general avenue to design under-oil superhydrophilic materials for rapid separation of water-in-oil emulsions. Such an approach can provide some new perspectives for fabrication of novel emulsion-separating materials.

Underoil superhydrophilic desert sand layer for efficient gravity-directed water-in-oil emulsions separation with high flux

Dr. J. Yong, Prof. F. Chen, Prof. Q. Yang, Dr. H. Bian, Dr. G. Du, Dr. C. Shan, J. Huo, Y. Fang, Prof. X. Hou State Key Laboratory for Manufacturing System Engineering & Key Laboratory of Photonics Technology for Information of Shaanxi Province School of Electronics & Information Engineering Xi’an Jiaotong University Xi’an 710049 , P. R. China E-mail: chenfeng@mail.xjtu.edu.cn; yangqing@mail.xjtu.edu.cn

Oil-Water Separation: A Gift from the Desert

The natural compound eye is a striking imaging device with a wealth of fascinating optical features such as a wide field of view (FOV), low aberration, and high sensitivity. Dragonflies in particular possess large, sophisticated compound eyes that exhibit high resolving power and information-processing capacity. Here, a large-scale artificial compound eye inspired by the unique designs of natural counterparts is presented. The artificial compound eye is created by a high-efficiency strategy that combines single-pulse femtosecond laser wet etching with thermal embossing. These eyes have a macrobase diameter of 5 mm and ≈30 000 close-packed ommatidia with an average diameter of 24.5 μm. Moreover, the optical properties of the artificial compound eyes are investigated; the results confirm that the eye demonstrates advanced imaging quality, an exceptionally wide FOV of up to 140°, and low aberration.

Dragonfly-Eye-Inspired Artificial Compound Eyes with Sophisticated Imaging

Reported here is a bioinspired fabrication of transparent underwater superoleophobic and anti-oil surfaces using a femtosecond laser treatment. Rough nanoscale structures were readily created on silica glass surfaces by femtosecond laser-induced ablation. Underwater superoleophobicity and ultralow oil-adhesion were obtained by the rough nanostructures with a wide variation of processing parameters, and the as-prepared surfaces exhibited a high transparency in water. This phenomenon is attributed to the presence of the water environment because scattering and refraction are effectively weakened. As a maskless and cost-effective method, the femtosecond laser processing of transparent materials (glass) may provide a new method to create biomimetic transparent underwater surfaces, allowing for the development of novel underwater anti-oil optical devices.

Bioinspired transparent underwater superoleophobic and anti-oil surfaces

In this Letter, a novel fabrication of large-area concave microlens array (MLA) on silicon is demonstrated by combination of high-speed laser scanning, which would result in single femtosecond laser pulse ablation on surface of silicon, and subsequent wet etching. Microscale concave microlenses with tunable dimensions and accessional aspherical profile are readily obtained on the 1  cm×1  cm silicon film, which are useful as optical elements for infrared (IR) applications. The aperture diameter and height of the microlens were characterized and the results reveal that they are both proportional to the laser scanning speed. Moreover, the optical property of high-performance silicon MLAs as a reflective homogenizer was investigated for the visible wavelength, and it can be easily extended to IR light.

Fabrication of large-area concave microlens array on silicon by femtosecond laser micromachining

We demonstrate an improved femtosecond laser irradiation followed by chemical etching process to create complex three-dimensional (3D) microchannels with arbitrary length and uniform diameter inside fused silica. A segmented chemical etching method of introducing extra access ports and a secondary power compensation is presented, which enables the fabrication of uniform 3D helical microchannels with length of 1.140 cm and aspect-ratio of 522. Based on this method, a micromixer which consists of a long helical microchannel and a y-tape microchannel was created inside the fused silica. We measured the mixing properties of the micromixer by injecting the phenolphthalein and NaOH solution through the two inlets of the y-tape microchannel. A rapid and efficient mixing was achieved in the 3D micromixer at a low Reynolds number.

Chao Shan

Papers

Oil-Water Separation: A Gift from the Desert

Dragonfly-Eye-Inspired Artificial Compound Eyes with Sophisticated Imaging

Bioinspired transparent underwater superoleophobic and anti-oil surfaces

Fabrication of large-area concave microlens array on silicon by femtosecond laser micromachining

Fabrication of three-dimensional helical microchannels with arbitrary length and uniform diameter inside fused silica.