Huawei Chen

Biomimetic Drag Reduction Study on Herringbone Riblets of Bird Feather

Birds have gradually formed various excellent structures such as streamlined shape and hollow shaft of feather to improve their flying performance by millions of years of natural selection. As typical property of bird feather, herringbone riblets align along the shaft of each feather, which is caused by perfect link of barbs, especially for the primary and secondary feathers of wings. Such herringbone riblets of feather are assumed to have great impact on drag reduction. In this paper, microstructures of secondary feathers of adult pigeons are investigated by SEM, and their structural parameters are statistically obtained. Based on quantitative analysis of feather structure, novel biomimetic herringbone riblets with narrow smooth edge are proposed to reduce surface drag. In comparison with traditional microgroove riblets and other drag reduction structures, the drag reduction rate of the proposed biomimetic herringbone riblets is experimentally clarified up to 16%, much higher than others. Moreover, the drag reduction mechanism of herringbone riblets are also confirmed and exploited by CFD.

Stable slippery liquid-infused anti-wetting surface at high temperatures

Recently developed slippery liquid-infused surfaces (LIS) offer a new approach to construct anti-wetting surfaces due to their excellent repellence of various liquids. However, previous studies about LIS are mainly performed at room temperature or low temperature and the LIS with stable anti-wetting at high temperatures are rare. Here we report a facile method to prepare LIS with high-temperature resistance. We directly employed chemically etched stainless steel (CESS) as the substrate structure, which can be applicable to objects regardless of their shapes and sizes. By choosing silicone oil to infuse the silanized CESS, a slippery surface with a very low sliding angle (ca. 2°) was formed, and the successful preparation can be achieved even when the silanized CESS was annealed at a temperature of 600 °C. The as-prepared LIS showed excellent anti-wetting for both room-temperature water and hot water at high temperatures. On the basis of stable LIS with high-temperature resistance, we investigated the influence of temperature on the droplet movement on LIS, and found that there were three movement states for both the sliding and impacting of water droplets on LIS with increasing temperature. We envision that our proposed approach can broaden the applications of LIS in engines, medical instruments, and daily life.

Investigation of the Anisotropic Morphology-Induced Effects of the Slippery Zone in Pitchers of Nepenthes alata

Plant of carnivorous genus Nepenthes alata has evolved specific pitchers to prey insects for survival in the barren habitat, especially its slippery zone. The excellent slippery function has received considerable interest because of its potential application in antifriction surface design. The surface morphologies of intact and de-waxed slippery zones were characterized using scanning electron microscope and scanning white-light interferometer. Hierarchical structures with anisotropic micro- lunate structure and nano- wax crystals were found on the slippery zone. Due to the hierarchical structures, the slippery zone is hydrophobic. It shows a significant anisotropic wettability with static contact angles 153.3° and 140.1° in the directions perpendicular and parallel to the upward direction (toward the peristome), respectively. The sliding angles are ~3° and ~10° in the downward and upward directions, respectively. Crawling experiments indicate that the microscopic surface roughness and the brittleness of the wax crystals may reduce insect attachment in different modes according to the insect mass differences. Moreover, artificial slippery surfaces inspired by the slippery zone of Nepenthes alata were fabricated. Traction experiments quantitatively verified that the friction force of replicated lunate structures with Ra-2.54 μm surface roughness was reduced by about 25% as compared to flat surface with Ra-0.56 μm surface roughness for cricket claws.

Surfaces Inspired by the Nepenthes Peristome for Unidirectional Liquid Transport

The slippery peristome of the pitcher plant Nepenthes has attracted much attention due to its unique function for preying on insects. Recent findings on the peristome surface of Nepenthes alata demonstrate a fast and continuous unidirectional liquid transport, which is enabled by the combination of a pinning effect at the sharp edges and a capillary rise in the wedge, deriving from the multiscale structure, which provides inspiration for designing and fabricating functional surfaces for unidirectional liquid transport. Developments in the fabrication methods of peristome-inspired surfaces and control methods for liquid transport are summarized. Both potential applications in the fields of microfluidic devices, biomedicine, and mechanical engineering and directions for further research in the future are discussed.

Synthetic Effect of Vivid Shark Skin and Polymer Additive on Drag Reduction Reinforcement

Natural shark skin has a well-demonstrated drag reduction function, which is mainly owing to its microscopic structure and mucus on the body surface. In order to improve drag reduction, it is necessary to integrate microscopic drag reduction structure and drag reduction agent. In this study, two hybrid approaches to synthetically combine vivid shark skin and polymer additive, namely, long-chain grafting and controllable polymer diffusion, were proposed and attempted to mimic such hierarchical topography of shark skin without waste of polymer additive. Grafting mechanism and optimization of diffusion port were investigated to improve the efficiency of the polymer additive. Superior drag reduction effects were validated, and the combined effect was also clarified through comparison between drag reduction experiments.

Large-proportional shrunken bio-replication of shark skin based on UV-curing shrinkage

The shark skin effect has attracted worldwide attention because of its superior drag reduction. As the product of natural selection, the maximum drag reduction of shark skin is found in its normal living environment. Large-proportional shrinkage of shark skin morphology is greatly anticipated for its adaptation to faster fluid flow. One novel approach, large-proportional shrunken bio-replication, is proposed as a method to adjust the optimal drag reduction region of shark skin based on the shrinkage of UV-cured material. The shark skin is taken as a replica template to allow large-proportional shrinking in the drag reduction morphology by taking advantage of the shrinkage of UV-curable material. The accuracy of the large-proportional shrunken bio-replication approach is verified by a comparison between original and shrunken bio-replicated shark skin, which shows that the shrinking ratio can reach 23% and the bio-replication accuracy is higher than 95%. In addition, the translation of the optimum drag reduction peak of natural surface function to various applications and environments is proved by drag reduction experiments.

Investigation of controllable self-organized honeycomb micro-structure

Patterned micro- and nanostructure are of great significance in industrial applications such as electronics, optics and sensors. Microporous honeycomb film of polyhpenylene oxide (PPO) was fabricated as the template via evaporation of solution in carbon bisulfide under humid ambience. The effect of fabrication conditions on patterned microstructure was investigated by self-organization experiments to build quantitative relationship between ambient conditions such as humidity, concentration and honeycomb microstructure (diameter and height), through which the honeycomb film formation can be controlled to self-organize desirable PPO patterned microstructure. Especially, the height of honeycomb was derived from the diameter of honeycomb, and its validity was clarified by morphological comparison between PPO template and PDMS molded structure. Moreover, soft mold experiments were conducted to demonstrate its high efficiency and excellence as an alternative to construct regular micro-pattern.

Ultrafast water harvesting and transport in hierarchical microchannels

Various natural materials have hierarchical microscale and nanoscale structures that allow for directional water transport. Here we report an ultrafast water transport process in the surface of a Sarracenia trichome, whose transport velocity is about three orders of magnitude faster than those measured in cactus spine and spider silk. The high velocity of water transport is attributed to the unique hierarchical microchannel organization of the trichome. Two types of ribs with different height regularly distribute around the trichome cone, where two neighbouring high ribs form a large channel that contains 1–5 low ribs that define smaller base channels. This results in two successive but distinct modes of water transport. Initially, a rapid thin film of water is formed inside the base channels (Mode I), which is followed by ultrafast water sliding on top of that thin film (Mode II). This two-step ultrafast water transport mechanism is modelled and experimentally tested in bio-inspired microchannels, which demonstrates the potential of this hierarchal design for microfluidic applications.Ultrafast water transport in the surface of Sarracenia trichome is reported and demonstrated in synthetic bioinspired materials, where nano- and microchannels induce high-speed sliding of droplets on top of a thin water film.

Liquid-Infused Surfaces on Electrosurgical Instruments with Exceptional Antiadhesion and Low-Damage Performances

Electrosurgery based on electrosurgical instruments plays an important role in clinical surgery owing to its advantages of ease of operation, low damage, and less pain to the patient. But soft tissue adhesion on electrosurgical instruments is still a major obstacle to improve the operation efficiency and achieve a better surgical result, regardless of so many developed methods to enhance the antiadhesion performance. In this paper, we successfully demonstrated that liquid-infused surfaces (LISs) can significantly improve the antiadhesion performance of electrosurgical instruments. We developed a microcontact printing method to assist the structure fabrication on the small instrument tip to prepare a firmly liquid-held surface. Soft tissue cutting experiments showed that LIS could not only significantly reduce the adhesion force between the tissue and the instrument tip but also lead to a much smaller charring wound. The underlying mechanism was discussed, and further experiments concluded that LIS have a better duration capability. Our study provides a new insight into the antiadhesion design for electrosurgical instruments.

Breath figure patterns prepared by spraying ultrasonic atomized water droplets

Honeycomb-structured porous films have widely potential applications in chemical sensors, tissue engineering, micro reactors, catalysis and so forth. An enhanced breath figure method with spraying ultrasonic atomized water droplets replacing the water vapor in humidity is proposed to fabricate multi-level and large-area breath figure patterns (BFPs). Well-defined polyphenylene oxide honeycomb films were prepared at a proper spraying time. The pore size and the regularity of the hexagonal arrays could be regulated by changing solution concentration. Especially, honeycomb films with two-level pores were fabricated by spraying atomized water droplets two times. Moreover, large-area BFPs with film area larger than 100 cm2 could be formed on the glass substrate. The large-area BFPs formation capability of the enhanced BFM in this work give new insight into more widely application in surface engineering. For example, slippery surfaces were prepared using the large-area BFPs as the holding structures. And the prepared slippery surfaces showed extremely low critical sliding angles for water, and also demonstrated efficient self-cleaning property.