Droplet microfluidics generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels Due to its remarkable advantages, droplet microfluidics bears significant value in an extremely wide range of area In this review, we provide a comprehensive and in-depth insight into droplet microfluidics, covering fundamental research from microfluidic chip fabrication and droplet generation to the applications of droplets in bio(chemical) analysis and materials generation The purpose of this review is to convey the fundamentals of droplet microfluidics, a critical analysis on its current status and challenges, and opinions on its future development We believe this review will promote communications among biology, chemistry, physics, and materials science

Emerging Droplet Microfluidics

Bio-inspired superoleophobic and smart materials: Design, fabrication, and application

An oleophobic and hydrophilic surface is a surface that is more wettable to water than to oil. In recent years, this novel surface has attracted significant attention since it is highly desirable in many important applications such as long-term anti-fogging, detergent-free cleaning, and oil and water separation. In this study, we focused on recent advances in oleophobic and hydrophilic surfaces. We briefly summarized three approaches, including simultaneously oleophobic/hydrophilic behavior, oleophobic/hydrophobic to hydrophilic switching behavior, and hydrophilic/underwater superoleophobic behavior, to achieve this surface. Moreover, we discussed the underlying mechanisms and the potential applications of this surface have also been introduced.

Special oleophobic and hydrophilic surfaces: approaches, mechanisms, and applications

Superoleophobic surfaces have drawn wide attention because of their special wetting behaviour. By adjusting surface chemical composition and surface structure, different kinds of superoleophobic surfaces (in air, underwater and others) can be obtained. This account mainly focuses on the recent progress of the fabrication and applications of superoleophobic surfaces. There are various methods to fabricate superoleophobic surfaces, which are generally divided into two ways. One is the top-down method, which includes etching, lithography, anodization and laser processing. The other is the bottom-up method, which contains electrodeposition, the hydrothermal method, sol–gel process and electro-spinning. However, each has its own merits and demerits. Hence, choosing the proper method in different conditions is quite important. These superoleophobic surfaces can be applied in many areas, such as self-cleaning, anti-corrosion, oil transportation, anti-bio-adhesion devices, oil capture, anti-smudge, chemical shielding, micro-droplet manipulation and oil–water separation. In fact, few of them have been put into practice. The development of superoleophobic surfaces is still in the experimental stage. Current and further challenges for superoleophobic surfaces are proposed. Beyond that, some creatures with typical structures are also referred, for instance, the lotus leaf, butterfly wing, rice leaf, desert beetle, rose petal, mosquito eyes, springtail, fish scale, shark skin, snail shell, the lower surface of the lotus leaf and a clam's shell.

Biomimetic superoleophobic surfaces: focusing on their fabrication and applications

Through 3.7 billion years of evolution and natural selection, plants and animals in nature have ingeniously fulfilled a broad range of fascinating functions to achieve optimized performance in responding and adapting to changes in the process of interacting with complex natural environments. It is clear that the hierarchically organized micro/nanostructures of the surfaces of living organisms decisively manage fascinating and amazing functions, regardless of the chemical components of their building blocks. This conclusion now allows us to elucidate the underlying mechanisms whereby these hierarchical structures have a great impact on the properties of the bulk material. In this review, we mainly focus on advances over the last three years in bioinspired multiscale functional materials with specific wettability. Starting from selected naturally occurring surfaces, manmade bioinspired surfaces with specific wettability are introduced, with an emphasis on the cooperation between structural characteristics and macroscopic properties, including lotus leaf-inspired superhydrophobic surfaces, fish scale-inspired superhydrophilic/underwater superoleophobic surfaces, springtail-inspired superoleophobic surfaces, and Nepenthes (pitcher plant)-inspired slippery liquid-infused porous surfaces (SLIPSs), as well as other multifunctional surfaces that combine specific wettability with mechanical properties, optical properties and the unidirectional transport of liquid droplets. Afterwards, various top-down and bottom-up fabrication techniques are presented, as well as emerging cutting-edge applications. Finally, our personal perspectives and conclusions with regard to the transfer of micro- and nanostructures to engineered materials are provided.

Recent advances of bioinspired functional materials with specific wettability: from nature and beyond nature

Composition modification and surface microstructures have been widely utilized in interface science to improve the surface performance. In this paper, we observed a significant improvement of oil contact angle (CA) from 66 ± 2° to 120 ± 4° by introducing a radical silanol group on a flat PDMS surface through oxygen plasma pretreatment. By combining surface microstructures and plasma modification, we produced three kinds of superoleophobic surfaces: 20 μm pitch micropillar arrays, 2.5 μm pitch micropillar arrays and gecko foot-like hierarchical microstructures. Among them, the hierarchical surface with high surface roughness showed extreme underwater superoleophobicity, which featured ultrahigh CA (175 ± 3°) and ultrasmall sliding angle (<1°). Quantitative measurements demonstrated that these superoleophobic surfaces exhibited distinct adhesive behaviors, by which they were interpreted as Wenzel's, Cassie's and the Lotus state, respectively. A microfluidic channel with superoleophobic microstructures was further created by novel curve-assisted imprint lithography, and the characterization based on anti-oil contamination applications was carried out and discussed. We believe that the superoleophobic surfaces will power broad applications in oil microdroplet transportation, anti-oil channels and droplet microfluidic systems.

Jeffrey A. Piersol

Papers

Facile creation of hierarchical PDMS microstructures with extreme underwater superoleophobicity for anti-oil application in microfluidic channels