Regulating Water Adhesion on Superhydrophobic TiO2 Nanotube Arrays

Bioinspired surfaces with special wettability have attracted a significant attention in recent years because of their potential applications for no loss liquid transfer, anti‐icing, and self‐cleaning. Herein, the realization of two extreme superhydrophobic states on 1H, 1H, 2H, 2H–perfluorooctyltriethoxysilane‐modified TiO₂ nanotube arrays (NTAs) is described by changing the structural characteristics of nanotubes while keeping the surface chemical composition constant. The water adhesive force is regulated in a wide range from ≈4.4 to ≈89.6 μN by the nanotubular diameter, length, density, and surface roughness. The cooperation effect between the negative pressures induced by the volume change of sealed air‐pockets and the van der Waals' attraction at solid–liquid interface contributes to the water adhesion. The superhydrophobic TiO₂ NTAs with a high adhesive force is used as a “mechanical hand” to transfer water microdroplets without any loss, and the one with extremely low adhesive force is utilized as a self‐cleaning and anti‐icing surface.     

Trilevel‐Structured Superhydrophobic Pillar Arrays with Tunable Optical Functions

Water‐repelling surfaces inspired by lotus leaves have been developed for their commercial needs in superhydrophobic and self‐cleaning coatings on glasses and windows. The extraordinary properties originate from their multiscale structures with waxy materials. To obtain high transparency as well as superhydrophobicity, microhair arrays are designed with large spacing to reduce optical scattering effects caused by microstructures, but with a trilevel hierarchical structure to compensate for the loss of superhydrophobicity. In this study, a soft molding technique on wet pastes consisting of nanoparticles (NPs) is proposed to create a multilevel hierarchical structure of sub‐100 nm nanoparticles, which demonstrates excellent water repellency. Additionally, full advantage is taken of the TiO₂ NP mesoporous structure for UV protection and for its ability to attach to various kinds of functional (for example, photoresponsive) dyes. Furthermore, the stability of fluorinated surfaces against UV light is enhanced by the passivation of the TiO₂ surface with a thin silica coating.     

Tunable Superparamagnetic Ring (tSPRing) for Droplet Manipulation

The manipulation of droplets via a magnetic field forms the basis of a fascinating technology that is currently in development. Often, the movement of droplets with magnets involves adding magnetic particles in or around the droplet; alternatively, magneto responsive surfaces may also be used. This work, presents and characterizes experimentally the formation and properties of a tunable superparamagnetic ring (tSPRing), which precisely adjusts itself around a water droplet, due to liquid–liquid interaction, and enables the physical manipulation of droplets. The ring is made of an oil-based ferrofluid, a stable suspension of ferromagnetic particles in an oily phase. It appears spontaneously due to the oil–water interfacial interaction under the influence of a magnetic field. The ferrofluid–water interaction resembles a cupcake assembly, with the surrounding ring only at the base of the droplet. The ring is analogous to a soft matter ring magnet, showing dipole repulsive forces, which stabilizes the droplets on a surface. It enables robust, controllable, and programmable manipulation of enclosed water droplets. This work opens the door to new applications in open surface upside or upside-down microfluidics and lays the groundwork for new studies on tunable interfaces between two immiscible liquids.     

Fibrillar Elastomeric Micropatterns Create Tunable Adhesion Even to Rough Surfaces

Biologically inspired, fibrillar dry adhesives continue to attract much attention as they are instrumental for emerging applications and technologies. To date, the adhesion of micropatterned gecko‐inspired surfaces has predominantly been tested on stiff, smooth substrates. However, all natural and almost all artificial surfaces have roughnesses on one or more different length scales. In the present approach, micropillar‐patterned PDMS surfaces with superior adhesion to glass substrates with different roughnesses are designed and analyzed. The results reveal for the first time adhesive and nonadhesive states depending on the micropillar geometry relative to the surface roughness profile. The data obtained further demonstrate that, in the adhesive regime, fibrillar gecko‐inspired adhesive structures can be used with advantage on rough surfaces; this finding may open up new applications in the fields of robotics, biomedicine, and space exploration.     

基于皮秒激光的超疏水镍铝青铜合金表面的制备

利用皮秒激光器在镍铝青铜合金表面制备了具有不同微观形貌的微纳米复合结构,再通过硬脂酸进行表面修饰。采用扫描电镜和X射线衍射仪等表征了所得表面的形貌和化学成分。研究结果表明,经皮秒激光加工和硬脂酸修饰后,表面的接触角都达到150°以上。不同的脉冲能量密度下,试样表面的微观形貌和润湿性不同。随着脉冲能量密度的增大,修饰后的试样表面的滚动角逐渐减小,当脉冲能量密度为6.85 J/cm^2时,滚动角减小到7°,随着脉冲能量密度的进一步增加,滚动角又逐渐增大。耐蚀性测试结果表明:超疏水镍铝青铜合金表面具有更好的耐腐蚀性能。采用优化的工艺参数可以在镍铝青铜合金上加工出超疏水表面,有助于提高其耐腐蚀性能。     

Fabricating Tunable Superhydrophobic Surfaces Enabled by Surface-Initiated Emulsion Polymerization in Water

Fabricating controllable superhydrophobic surfaces remains challenging in various fields ranging from chemical industries to biomedical engineering. Conventional methods commonly require volatile organic solvents and the assistance of special surface deposition and modification equipment, which are detrimental to environment and limit their applications in micro-devices. Herein, an equipment-free method is reported to directly transform fluorinated monomer micro-droplets into hydrophobic polymer particles on flat substrate surfaces in water, simultaneously depositing hydrophobic coatings with tunable surface structures. The as-prepared surfaces show superior superhydrophobicity and great stability in extreme conditions (e.g., varying acidity, basicity, and heating conditions), and excellent anti-fouling property. Meanwhile, surface hydrophobicity can be manipulated by adjusting emulsion droplet number density and reaction time. Hence, superhydrophobic surfaces with tunable hydrophobicity gradients have been successfully fabricated in one pot. This study provides an equipment-free method to facilely fabricate controllable superhydrophobic surfaces, with great potential in the development of smart superhydrophobic materials in various engineering and industrial applications.     

Corrosion‐Resistant Superhydrophobic Coatings on Mg Alloy Surfaces Inspired by Lotus Seedpod

Superhydrophobic surfaces are widely found in nature, inspiring the development of excellent antiwater surfaces with barrier coatings isolating the underlying materials from the external environment. Here, the naturally occurring superhydrophobicity of lotus seedpod surfaces is reported. Protective coatings that mimic the lotus seedpod are fabricated on AZ91D Mg alloy surfaces with the synergistic effect of robust superhydrophobicity and durable corrosion resistance. The predesigned titanium dioxide films are coated on AZ91D by an in situ hydrothermal synthesis technique. Through sonication assisted electroless plating combined with a self‐assembling method, the densely packed Cu‐thiolate layers are uniformly plated with robust adhesion on the Mg alloy substrate, which function as a superhydrophobic barrier that can hold back the transport of water and corrosive ions contained such as Cl^?. Notably, the two extreme wetting behaviors (superhydrophilicity and superhydrophobicity) as well as corrosion resistance and improved corrosion resistance can be easily controlled by removal of the hydrophobic materials (n ‐dodecanethiol) at elevated temperature (350 °C) and modifying them at room temperature for 18 cycles, indicative of exceptional adhesion between the superhydrophobic coating and the underlying AZ91D Mg alloy.     

Bioinspired Superhydrophobic Papillae with Tunable Adhesive Force and Ultralarge Liquid Capacity for Microdroplet Manipulation

Template‐free, highly efficient, and large‐area construction of complex multiscale architectures is still a great challenge for microfabrications. Inspired by the hierarchical micropapillae on the superhydrophobic surface of natural rose petals, here, a facile 3D shrinking method is reported to build a graphene oxide (GO) papillae array. Circular GO speckles with a gradient of thickness are deposited on an inflated latex balloon through the water‐evaporation‐driven assembly of GO nanosheets, which then shrink into hierarchical papillae under compressive stresses upon deflation. The fluoroalkylsilane modified GO papillae array exhibits a combined performance of strong superhydrophobicity (CA > 170°), tunable adhesive force (39.2–129.4 µN), and ultralarge liquid capacity (25 µL). The wetting states (Wenzel, Cassie‐I, and Cassie‐Baxter), the adhesive forces, and the liquid capacities all can be tuned by varying the buckling topography (microwrinkle or microfold), the papillae number (3, 4, 6, or 7), and the array arrangement (triangle, square, or hexagon). For one single papillae, the highest adhesive force and the highest liquid capacity incresed to a record breaking value of 26.5 µN and 4.2 µL, respectively, which are promising for programmable manipulations of microdroplets and relevant for multistep microreactions.     

Directed Self‐Assembly as a Route to Ferromagnetic and Superparamagnetic Nanoparticle Arrays

Block co‐polymer patterns are attractive candidates for nanoparticle assemblies. Directed self‐assembly of block co‐polymers in particular allows for long range ordering of the patterns, making them interesting scaffolds for the organization of magnetic particles. Here, a method to tune the channel width of polymer‐derived trenches via atomic layer deposition (ALD) of alumina is reported. The alumnia coating provides a much more thermally robust pattern that is stable up to 250 °C. Using these patterns, magnetic coupling in both ferromagnetic and superparamagnetic nanocrystal chains is achieved.     

Superparamagnetic Twist‐Type Actuators with Shape‐Independent Magnetic Properties and Surface Functionalization for Advanced Biomedical Applications

Directed nanoparticle self‐organization and two‐photon polymerization are combined to enable three‐dimensional soft‐magnetic microactuators with complex shapes and shape‐independent magnetic properties. Based on the proposed approach, single and double twist‐type swimming microrobots with programmed magnetic anisotropy are demonstrated, and their swimming properties in DI‐water are characterized. The fabricated devices are actuated using weak rotating magnetic fields and are capable of performing wobble‐free corkscrew propulsion. Single twist‐type actuators possess an increase in surface area in excess of 150% over helical actuators with similar feature size without compromising the forward velocity of over one body length per second. A generic and facile combination of glycine grafting and subsequent protein immobilization exploits the actuator's increased surface area, providing for a swimming microrobotic platform with enhanced load capacity desirable for future biomedical applications. Successful surface modification is confirmed by FITC fluorescence.     

Superparamagnetic Nanostructures for Off‐Resonance Magnetic Resonance Spectroscopic Imaging

This study proposes a new method to generate positive contrast in magnetic resonance imaging (MRI) using superparamagnetic contrast agents. Superparamagnetic nanostructures consisting of octahedron manganese ferrite nanoparticles embedded in spherical nanogels are fabricated using a bottom‐up approach. The composite nanoparticles are strongly magnetized in an external magnetic field and produce a unique NMR frequency shift in water protons, which can be demonstrated in MR spectroscopy and imaging to be different from the bulk pool. Moreover, the particles exhibit excellent colloidal stability in aqueous media and good cell biocompatibility. Hence, these particles are potentially useful as biomarkers by taking advantage of the positive contrast effects produced in MRI.     

Fabrication and Size‐Selective Bioseparation of Magnetic Silica Nanospheres with Highly Ordered Periodic Mesostructure

In this paper, we report a novel synthesis and selective bioseparation of the composite of Fe₃O₄ magnetic nanocrystals and highly ordered MCM‐41 type periodic mesoporous silica nanospheres. Monodisperse superparamagnetic Fe₃O₄ nanocrystals were synthesized by thermal decomposition of iron stearate in diol in an autoclave at low temperature. The synthesized nanocrystals were encapsulated in mesoporous silica nanospheres through the packing and self‐assembly of composite nanocrystal–surfactant micelles and surfactant/silica complex. Different from previous studies, the produced magnetic silica nanospheres (MSNs) possess not only uniform nanosize (90 ～ 140 nm) but also a highly ordered mesostructure. More importantly, the pore size and the saturation magnetization values can be controlled by using different alkyltrimethylammonium bromide surfactants and changing the amount of Fe₃O₄ magnetic nanocrystals encapsulated, respectively. Binary adsorption and desorption of proteins cytochrome c (cyt c) and bovine serum albumin (BSA) demonstrate that MSNs are an effective and highly selective adsorbent for proteins with different molecular sizes. Small particle size, high surface area, narrow pore size distribution, and straight pores of MSNs are responsible for the high selective adsorption capacity and fast adsorption rates. High magnetization values and superparamagnetic property of MSNs provide a convenient means to remove nanoparticles from solution and make the re‐dispersion in solution quick following the withdrawal of an external magnetic field.     

In Situ Fully Light‐Driven Switching of Superhydrophobic Adhesion

An in situ fully light‐driven switching of superhydrophobic adhesion is demonstrated based on simply spin‐coating a hydrophobic azo‐polymer on an optimized micro‐nanopost arrayed silicon substrate. Furthermore, the detailed designing principles are discussed, which might shed light on efficient exploitation of superhydrophobic liquid/solid interfaces for smart microfluidic control.     

Electrophoretically‐Deposited Metal‐Decorated CNT Nanoforests with High Thermal/Electric Conductivity and Wettability Tunable from Hydrophilic to Superhydrophobic

A single‐step, room‐temperature, and scalable electrophoretic deposition process is reported to form nanocomposites on any electrically conductive surface with metal nanoparticle decorated carbon nanotubes (CNTs). The contact angles (CAs) can be easily tuned from ≈60° to 168° by varying the deposition voltage, while hydrophobicity and superhydrophobicity surprisingly arise from the hydrophilic CNTs being deposited. The relatively high voltage tends to vertically align CNTs during deposition, leading to architectural micro/nanoscale roughness on the surface. The combination of the multiscale roughness along with the low surface energy of hydrocarbon functional groups on the CNT surface has enabled facile wettability control, including the Petal and Lotus effects. Further, the relatively vertical orientation of the CNTs, without any coating, allows for current and heat transfer along their axis with superior conductivity. Similar behavior in terms of CA control is seen for all three divalent metal ions in the deposition solution (i.e., Cu²⁺, Ni²⁺, and Zn²⁺) that are used to charge the CNTs while eventually getting co‐deposited. This implies that this method could possibly be extended to other metals by selecting appropriate charging salt. A patterning technique is also demonstrated for facile fabrication of superhydrophobic CNT‐metal islands surrounded by hydrophilic CNT coating.     

Adhesion Selectivity Using Rippled Surfaces

Highly selective adhesion can be achieved between surfaces by patterning them with ripples. Materials with such surfaces are fabricated by successive molding of an elastomer, poly(dimethylsiloxane) (PDMS), against a master with a surface rippled by instability of a residually stressed surface thin film. Adhesion of interfaces between both complementary and non‐complementary rippled surfaces was measured. Complementary surfaces showed significantly enhanced interfacial adhesion with increasing ripple amplitude. In contrast, interfaces with mismatched amplitudes had nearly negligible adhesion. Rate‐dependence of adhesion in these surfaces was also studied. For complementary surfaces with low amplitudes we found a multiplicative coupling between the structure and rate enhancement of adhesion. A quantitative model developed for adhesion between complementary surfaces explains these observations.     

Porous Polymer Coatings: a Versatile Approach to Superhydrophobic Surfaces

Here, a facile and inexpensive approach to superhydrophobic polymer coatings is presented. The method involves the in situ polymerization of common monomers in the presence of a porogenic solvent to afford superhydrophobic surfaces with the desired combination of micro‐ and nanoscale roughness. The method is applicable to a variety of substrates and is not limited to small areas or flat surfaces. The polymerized material can be ground into a superhydrophobic powder, which, once applied to a surface, renders it superhydrophobic. The morphology of the porous polymer structure can be efficiently controlled by composition of the polymerization mixture, while surface chemistry can be adjusted by photografting. Morphology control is used to reduce the globule size of the porous architecture from micro down to nanoscale thereby affording a transparent material. The influence of both surface chemistry as well as the length scale of surface roughness on the superhydrophobicity is discussed.     

Active,Programmable Elastomeric Surfaces with Tunable Adhesion for Deterministic Assembly by Transfer Printing

Active, programmable control of interfacial adhesion is an important, desired feature of many existing and envisioned systems, including medical tapes, releasable joints, and stamps for transfer printing. Here a design for an elastomeric surface that offers switchable adhesion strength through a combination of peel‐rate dependent effects and actuation of sub‐surface fluid chambers is presented. Microchannels and open reservoirs positioned under a thin surface membrane can be pressurized in a controlled manner to induce various levels of surface deformation via inflation. These pressurized structures demonstrate utility in controllably decreasing the strength of adhesion of flat, solid objects to the elastomeric surface, particularly in the limit of low peel‐rates. Experimental and theoretical studies of these systems reveal the key mechanisms, and guide optimized geometries for broad control over adhesion, in a programmable and reversible manner. Implementing these concepts in stamps for transfer printing enables new modes for deterministic assembly of micro‐ and nanoscale materials onto diverse types of substrates. Collections of silicon plates delivered onto plastic, paper and other surfaces with single or multiply addressable stamps illustrate some of the capabilities.     

Magneto-Mechanical Metamaterials with Widely Tunable Mechanical Properties and Acoustic Bandgaps

Mechanical metamaterials are architected manmade materials that allow for unique behaviors not observed in nature, making them promising candidates for a wide range of applications. Existing metamaterials lack tunability as their properties can only be changed to a limited extent after the fabrication. Herein, a new magneto-mechanical metamaterial is presented that allows great tunability through a novel concept of deformation mode branching. The architecture of this new metamaterial employs an asymmetric joint design using hard-magnetic soft active materials that permits two distinct actuation modes (bending and folding) under opposite-direction magnetic fields. The subsequent application of mechanical compression leads to the deformation mode branching where the metamaterial architecture transforms into two distinctly different shapes, which exhibit very different deformations and enable great tunability in properties such as mechanical stiffness and acoustic bandgaps. Furthermore, this metamaterial design can be incorporated with magnetic shape memory polymers with global stiffness tunability, which also allows for the global shift of the acoustic behaviors. The combination of magnetic and mechanical actuations, as well as shape memory effects, impart wide tunable properties to a new paradigm of metamaterials.