Tip Geometry Controls Adhesive States of Superhydrophobic Surfaces

Inspired by biological attachment systems, aligned polystyrene (PS) nanopillars terminating in flat, concave tips and nanotubes were fabricated by a simple and reproducible method. All the obtained surfaces show both the contact angles larger than 150° (superhydrophobicity) and high adhesion of water to it. The tip geometry plays an important role in determining the adhesive property. Surface with the concave tips has the highest adhesion, and then the surface with flat tips, whereas aligned nanotube surface has a relatively lower adhesion. Besides different van der Waals forces between the PS surfaces and water, another important factor, i.e., different negative pressures produced by the different volumes of sealed air, may be the crucial factor for their different adhesions. These findings provide the experimental evidence of the influence of the tip geometry on the adhesion of structured superhydrophobic surfaces, which is helpful for us to further understand the biological attachment systems and to optimum design of artificial analogues

Magnetically Induced Reversible Transition between Cassie and Wenzel States of Superparamagnetic Microdroplets on Highly Hydrophobic Silicon Surface

In this work, we report a magnetic technique for reversible wetting–dewetting transitions of microdroplets on highly hydrophobic surfaces. A superparamagnetic microdroplet can be reversibly switched between the Cassie state and the Wenzel state on a highly hydrophobic microstructured silicon substrate by the application of the magnetic field. The transition can be controlled by both the intensity of the magnetic field and the concentration of the superparamagnetic Fe₃O₄ nanoparticles in the microdroplet. The magnetic force needed during the transition from the Cassie state to the Wenzel state was found to be apparently smaller than that needed in the reverse process. Such asymmetry is ascribed to the higher energy of the Cassie state compared with the Wenzel state, the change of the gravitational potential energy, and the adhesion hysteresis. This report provides a novel method of dynamically controlling liquid/solid interactions, which can not only help us to understand further the transition between the Cassie state and the Wenzel state but also potentially be used in some important applications, such as lab-on-a-chip devices and chemical microreactors

pH-Controllable Water Permeation through a Nanostructured Copper Mesh Film

Water permeation is an important issue in both fundamental research and industrial applications. In this work, we report a novel strategy to realize the controllable water permeation on the mixed thiol (containing both alkyl and carboxylic acid groups) modified nanostructured copper mesh films. For acidic and neutral water, the film is superhydrophobic, and the water cannot permeate the film because of the large negative capillary effect resulting from the nanostructures. For basic water, the film shows superhydrophilic property, and thus the water can permeate the film easily. The permeation process of water can be controlled just by simply altering the water pH. A detailed investigation indicates that nanostructures on the substrate and the appropriate size of the microscale mesh pores can enhance not only the static wettability but also the dynamic properties. The excellent controllability of water permeation is ascribed to the combined effect of the chemical variation of the carboxylic acid group and the microstructures on the substrate. This work may provide interesting insight into the new applications that are relevant to the surface wettability, such as filtration, microfluidic device, and some separation systems

A Compact MXene Film with Folded Structure for Advanced Supercapacitor Electrode Material

A Ti3C2Tx MXene film assembled by using conventional vacuum-assisted filtration has an outstanding volumetric performance due to its excellent pseudocapacitive and high density. However, the severely high energy and time consumption by vacuum-assisted filtration will limit its practical mass production as electrodes; meanwhile, the self-restacking of MXene nanosheets increases ion diffusion limitation, especially for thicker film electrodes. Herein, a simple strategy is employed to fabricate a compact and nanoporous MXene film with folded structure (CN-MX) by mechanically pressing a three-dimensional MXene aerogel, resulting in an increased packing density and electrical conductivity (8681 S m–1) while retaining sufficiently abundant ion-accessible active sites. In addition, the formed highly interconnected nanopore channels can facilitate more rapid ionic and electronic transport. When applied as additive-free electrodes for supercapacitors, the CN-MX delivers a comparable volumetric capacitance and much improved rate capability performance compared to the MXene film fabricated by vacuum-assisted filtration. More impressively, it can still exhibit an attractive volumetric capacitance even at practical levels of mass loading (above 10 mg cm–2). This study opens a feasible avenue forward toward commercial applications of MXene in portable and compact storage devices

Smart Superhydrophobic Shape Memory Adhesive Surface toward Selective Capture/Release of Microdroplets

Controllable manipulation of microdroplets is significant for the microfluidics, biomedical areas, microreactors, and so on; however, until now, reports about no-loss and selective capture/release of different microdroplets are still rare. Herein, we report a new superhydrophobic shape memory adhesive surface that can solve this problem. The surface is prepared by sticking a pillar-structured superhydrophobic polyurethane layer onto a shape memory polyurethane–cellulose nanofiber (PU–CNF) layer. Because of the good shape memory performance of the PU–CNF layer, the obtained surface can memorize and display various microstructure arrangements during the stretching/releasing process. Meanwhile, multiple superhydrophobic adhesive states from low-adhesive rolling performance to high-adhesive pinning performance can be observed on the surface, and all these adhesive states can be reversibly controlled between each other. Based on the smart shape memory ability in surface adhesion, not only traditional in situ capture/release of one microdroplet but also selective capture and release of different microdroplets can be realized. This work reports a new superhydrophobic shape memory adhesive surface; it is envisioned that this smart surface would be a powerful platform for microfluidics systems, complex droplet transportation, biological analysis, and so on

Smart Superhydrophobic Shape Memory Adhesive Surface toward Selective Capture/Release of Microdroplets

Controllable manipulation of microdroplets is significant for the microfluidics, biomedical areas, microreactors, and so on; however, until now, reports about no-loss and selective capture/release of different microdroplets are still rare. Herein, we report a new superhydrophobic shape memory adhesive surface that can solve this problem. The surface is prepared by sticking a pillar-structured superhydrophobic polyurethane layer onto a shape memory polyurethane–cellulose nanofiber (PU–CNF) layer. Because of the good shape memory performance of the PU–CNF layer, the obtained surface can memorize and display various microstructure arrangements during the stretching/releasing process. Meanwhile, multiple superhydrophobic adhesive states from low-adhesive rolling performance to high-adhesive pinning performance can be observed on the surface, and all these adhesive states can be reversibly controlled between each other. Based on the smart shape memory ability in surface adhesion, not only traditional in situ capture/release of one microdroplet but also selective capture and release of different microdroplets can be realized. This work reports a new superhydrophobic shape memory adhesive surface; it is envisioned that this smart surface would be a powerful platform for microfluidics systems, complex droplet transportation, biological analysis, and so on

Underoil Directional Self-Transportation of Water Droplets on a TiO₂‑Coated Conical Spine

Directional self-transportation of tiny droplets is significant in many fields. However, almost all existing studies focus on the phenomenon in air, and to realize similar performance in complex environments, such as oil, is still extremely rare. Here, we report a TiO2-coated conical spine (TCS) and demonstrate underoil directional self-transportation of water droplets on its surface. It is found that high surface hydrophilicity resulting from UV irradiation is necessary to achieve the self-transportation of water in oil. The critical water contact angle in oil is about 57°, and the maximal transport velocity can reach 1.4 mm/s. Mechanism analysis reveals that the excellent self-transportation property is ascribed to the combined effect between the Laplace force (FL) caused by the conical gradient structure and the hysteresis reduction resulting from the high hydrophilicity. Moreover, based on the special underoil self-transportation performance, a droplet-based microreaction and demulsification of water-in-oil emulsions were demonstrated using the TCS. This work reports the self-transportation of water in oil, which could provide some fresh ideas for designing new superwetting self-transportation materials

Smart Superhydrophobic Shape Memory Adhesive Surface toward Selective Capture/Release of Microdroplets

Controllable manipulation of microdroplets is significant for the microfluidics, biomedical areas, microreactors, and so on; however, until now, reports about no-loss and selective capture/release of different microdroplets are still rare. Herein, we report a new superhydrophobic shape memory adhesive surface that can solve this problem. The surface is prepared by sticking a pillar-structured superhydrophobic polyurethane layer onto a shape memory polyurethane–cellulose nanofiber (PU–CNF) layer. Because of the good shape memory performance of the PU–CNF layer, the obtained surface can memorize and display various microstructure arrangements during the stretching/releasing process. Meanwhile, multiple superhydrophobic adhesive states from low-adhesive rolling performance to high-adhesive pinning performance can be observed on the surface, and all these adhesive states can be reversibly controlled between each other. Based on the smart shape memory ability in surface adhesion, not only traditional in situ capture/release of one microdroplet but also selective capture and release of different microdroplets can be realized. This work reports a new superhydrophobic shape memory adhesive surface; it is envisioned that this smart surface would be a powerful platform for microfluidics systems, complex droplet transportation, biological analysis, and so on

Underoil Directional Self-Transportation of Water Droplets on a TiO₂‑Coated Conical Spine

Directional self-transportation of tiny droplets is significant in many fields. However, almost all existing studies focus on the phenomenon in air, and to realize similar performance in complex environments, such as oil, is still extremely rare. Here, we report a TiO2-coated conical spine (TCS) and demonstrate underoil directional self-transportation of water droplets on its surface. It is found that high surface hydrophilicity resulting from UV irradiation is necessary to achieve the self-transportation of water in oil. The critical water contact angle in oil is about 57°, and the maximal transport velocity can reach 1.4 mm/s. Mechanism analysis reveals that the excellent self-transportation property is ascribed to the combined effect between the Laplace force (FL) caused by the conical gradient structure and the hysteresis reduction resulting from the high hydrophilicity. Moreover, based on the special underoil self-transportation performance, a droplet-based microreaction and demulsification of water-in-oil emulsions were demonstrated using the TCS. This work reports the self-transportation of water in oil, which could provide some fresh ideas for designing new superwetting self-transportation materials

Underoil Directional Self-Transportation of Water Droplets on a TiO₂‑Coated Conical Spine

Directional self-transportation of tiny droplets is significant in many fields. However, almost all existing studies focus on the phenomenon in air, and to realize similar performance in complex environments, such as oil, is still extremely rare. Here, we report a TiO2-coated conical spine (TCS) and demonstrate underoil directional self-transportation of water droplets on its surface. It is found that high surface hydrophilicity resulting from UV irradiation is necessary to achieve the self-transportation of water in oil. The critical water contact angle in oil is about 57°, and the maximal transport velocity can reach 1.4 mm/s. Mechanism analysis reveals that the excellent self-transportation property is ascribed to the combined effect between the Laplace force (FL) caused by the conical gradient structure and the hysteresis reduction resulting from the high hydrophilicity. Moreover, based on the special underoil self-transportation performance, a droplet-based microreaction and demulsification of water-in-oil emulsions were demonstrated using the TCS. This work reports the self-transportation of water in oil, which could provide some fresh ideas for designing new superwetting self-transportation materials