Engineering Hierarchical Nanostructures by Elastocapillary Self-Assembly

Response and resilience of carbon nanotube micropillars to shear flow

Interactions between carbon nanotubes (CNTs) and fluid flows are central to the operation of several emerging nanotechnologies. In this paper, we explore the fluid-structure interaction of CNT micropillars in wall-bounded shear flows, relevant to recently developed microscale wall shear stress sensors. We monitor the deformation of CNT micropillars in channel flow as the flow rate and wall shear stress are gradually varied. We quantify how the micropillars bend at low wall shear stress, and then will commonly tilt abruptly from their base above a threshold wall shear stress, which is attributed to the lower density of the micropillars in this region. Some micropillars are observed to flutter rapidly between a vertical and horizontal position around this threshold wall shear stress, before settling to a tilted position as wall shear stress increases further. Tilted micropillars are found to kink sharply near their base, similar to the observed buckling near the base of CNT micropillars in compression. Upon reducing the flow rate, micropillars are found to fully recover from a near horizontal position to a near vertical position, even with repeated on-off cycling. At sufficiently high wall shear stress, the micropillars were found to detach at the catalyst particle-substrate interface. The mechanical response of CNT micropillars in airflow revealed by this study provides a basis for future development efforts and the accurate simulation of CNT micropillar wall shear stress sensors.

Lightweight, Strong, and Transparent Wood Films Produced by Capillary Driven Self-Densification

Wood delignification and densification enable the production of high strength and/or transparent wood materials with exceptional properties. However, processing needs to be more sustainable and besides the chemical delignification treatments, energy intense hot-pressing calls for alternative approaches. Here, this study shows that additional softening of delignified wood via a mild swelling process using an ionic liquid-water mixture enables the densification of tube-line wood cells into layer-by-layer sheet structures without hot-pressing. The natural capillary force induces self-densification in a simple drying process resulting in a transparent wood film. The as-prepared films with ≈150 µm thickness possess an optical transmittance ≈70%, while maintaining optical haze >95%. Due to the densely packed sheet structure with a large interfacial area, the reassembled wood film is fivefold stronger and stiffer than the delignified wood in fiber direction. Owing to a low density, the specific tensile strength and elastic modulus are as high as 282 MPa cm3 g-1 and 31 GPa cm3 g-1. A facile and highly energy efficient wood nanotechnology approach are demonstrated toward more sustainable materials and processes by directly converting delignified wood into transparent wood omitting polymeric matrix infiltration or mechanical pressing.

Artificial-goosebump-driven microactuation

Microactuators provide controllable driving forces for precise positioning, manipulation and operation at the microscale. Development of microactuators using active materials is often hampered by their fabrication complexity and limited motion at small scales. Here we report light-fuelled artificial goosebumps to actuate passive microstructures, inspired by the natural reaction of hair bristling (piloerection) on biological skin. We use light-responsive liquid crystal elastomers as the responsive artificial skin to move three-dimensionally printed passive polymer microstructures. When exposed to a programmable femtosecond laser, the liquid crystal elastomer skin generates localized artificial goosebumps, resulting in precise actuation of the surrounding microstructures. Such microactuation can tilt micro-mirrors for the controlled manipulation of light reflection and disassemble capillary-force-induced self-assembled microstructures globally and locally. We demonstrate the potential application of the proposed microactuation system for information storage. This methodology provides precise, localized and controllable manipulation of microstructures, opening new possibilities for the development of programmable micromachines.

Emergence of an apparent yield phenomenon in the mechanics of stochastic networks with inter-fiber cohesion

In this work we investigate the contribution of inter-fiber cohesion to defining the mechanical behavior of stochastic crosslinked fiber networks. Fibers are athermal and store energy primarily in their bending and axial deformation modes. Cohesion between fibers is defined by an interaction potential. These structures are in equilibrium with the inter-fiber cohesive forces before external load is applied and their mechanical behavior is probed in uniaxial tension. Two types of configurations are considered: a state with high initial free volume in which contacts between fibers are scarce, and a state with low free volume and large number of fiber contacts. While in the absence of cohesion the response is hyperelastic, we observe that a yield point-like phenomenon develops as the strength of cohesion increases in both network types considered; we refer to this as an 'unlocking phenomenon'. The small strain stiffness increases as cohesion becomes more pronounced. The stiffness and unlocking stress are expressed in terms of network parameters and cohesion strength through a product of two functions, one dependent on network parameters only, and the other is a function of the cohesion strength. While the small strain response is controlled by cohesion, the large strain behavior is shown to be largely controlled by the network. Therefore, varying the strength of cohesion has no effect on strain stiffening. These observations provide a physical basis for the unlocking observed in both athermal and thermal network materials and are expected to facilitate the design of soft materials with novel properties.

Very High-Aspect-Ratio Polymeric Micropillars Made by Two-Photon Polymerization

Polymeric micropillars with a high-aspect-ratio (HAR) are of interest for a wide range of applications, including drug delivery and the micro-electro-mechanical field. While molding is the most common method for fabricating HAR microstructures, it is affected by challenges related to demolding the final structure. In this study, we present very HAR micropillars using two-photon polymerization (TPP), an established technique for creating complex 3D microstructures. Polymeric micropillars with HARs fabricated by TPP often shrink and collapse during the development process. This is due to the lack of mechanical stability of micropillars against capillary forces primarily acting during the fabrication process when the solvent evaporates. Here, we report different parameters that have been optimized to overcome the capillary force. These include surface modification of the substrate, fabrication parameters such as laser power, exposure time, the pitch distance between the pillars, and the length of the pillars. On account of adopting these techniques, we were able to fabricate micropillars with a very HAR up to 80.

Plasma-Etched Vertically Aligned CNTs with Enhanced Antibacterial Power

The emergence of multidrug-resistant bacteria represents a growing threat to public health, and it calls for the development of alternative antibacterial approaches not based on antibiotics. Here, we propose vertically aligned carbon nanotubes (VA-CNTs), with a properly designed nanomorphology, as effective platforms to kill bacteria. We show, via a combination of microscopic and spectroscopic techniques, the ability to tailor the topography of VA-CNTs, in a controlled and time-efficient manner, by means of plasma etching processes. Three different varieties of VA-CNTs were investigated, in terms of antibacterial and antibiofilm activity, against Pseudomonas aeruginosa and Staphylococcus aureus: one as-grown variety and two varieties receiving different etching treatments. The highest reduction in cell viability (100% and 97% for P. aeruginosa and S. aureus, respectively) was observed for the VA-CNTs modified using Ar and O₂ as an etching gas, thus identifying the best configuration for a VA-CNT-based surface to inactivate both planktonic and biofilm infections. Additionally, we demonstrate that the powerful antibacterial activity of VA-CNTs is determined by a synergistic effect of both mechanical injuries and ROS production. The possibility of achieving a bacterial inactivation close to 100%, by modulating the physico-chemical features of VA-CNTs, opens up new opportunities for the design of self-cleaning surfaces, preventing the formation of microbial colonies.

Underwater Oleophobic Electrospun Membrane with Spindle-Knotted Structured Fibers for Oil-in-Water Emulsion Separation

The potential of spider silk as an intriguing biological prototype for collecting water from a humid environment has attracted wide attention, and various materials with suitable structures have been engineered. Here, inspired by this phenomenon, a kind of superwetting poly(vinylidene fluoride) (PVDF) membrane with spindle-knotted structured fibers was prepared by the electrospinning method followed by oxygen plasma etching treatment. The prepared membrane presented a satisfactory separation efficiency for various oil-in-water emulsions. The cooperative effect of the special wettability property and the spindle-knot structure stimulated the emulsified oil droplets to accumulate quickly on the membrane surface. A model that explains the accumulation of emulsified oil droplets has also been developed. Furthermore, an artificial fiber comprising a micron-sized spindle-knot structure was prepared by the dip-coating method to clearly illustrate the aggregation process of the emulsified oil droplets and to verify the theoretical explanation. We hope that this study will provide new inspiration for oil/water emulsion separation techniques.

Capillary-force-driven self-assembly of carbon nanotubes: from ab initio calculations to modeling of self-assembly

In elasto-capillary driven self-assembly of arrays of carbon nanotubes (CNTs) different factors play a role, from the mechanical properties of CNTs to the array geometry. In this work, we provide a multi-scale investigation where we first use density functional theory (DFT) to predict fully ab initio relevant mechanical properties such as Young's modulus, Poisson's ratio, and surface energy. To the best of our knowledge, we are the first to report DFT calculations of the surface energy of CNTs. Then, we feed the computed DFT parameters into a model for capillary-force-driven self-assembly of CNTs. By doing so, we are able to derive and predict cross-correlation between material parameters and array architecture.

Elastocapillary deformation of thin elastic ribbons in 2D foam columns

The ability of liquid interfaces to shape slender elastic structures provides powerful strategies to control the architecture of mechanical self assemblies. However, elastocapillarity-driven intelligent design remains unexplored in more complex architected liquids - such as foams. Here we propose a model system which combines an assembly of bubbles and a slender elastic structure. Arrangements of soap bubbles in confined environments form well-defined periodic structures, dictated by Plateau's laws. We consider a 2D foam column formed in a container with square cross-section in which we introduce an elastomer ribbon, leading to architected structures whose geometry is guided by a competition between elasticity and capillarity. In this system, we quantify both experimentally and theoretically the equilibrium shapes, using X-ray micro-tomography and energy minimisation techniques. Beyond the understanding of the amplitude of the wavy elastic ribbon deformation, we provide a detailed analysis of the profile of the ribbon, and show that such a setup can be used to grant a shape to a UV-curable composite slender structure, as a foam-forming technique suitable to miniaturisation. In more general terms, this work provides a stepping stone towards an improved understanding of the interactions between liquid foams and slender structures.

Stereolithography-Derived Three-Dimensional Pyrolytic Carbon/Mn3O4 Nanostructures for Free-Standing Hybrid Supercapacitor Electrodes

The development of permeable three-dimensional (3D) macroporous carbon architectures loaded with active pseudocapacitive nanomaterials offers hybrid supercapacitor (SC) materials with higher energy density, shortened diffusion length for ions, and higher charge-discharge rate capability and thereby is highly relevant for electrical energy storage (EES). Herein, structurally complex and tailorable 3D pyrolytic carbon/Mn₃O₄ hybrid SC electrode materials are synthesized through the self-assembly of MnO₂ nanoflakes and nanoflowers onto the surface of stereolithography 3D-printed architectures via a facile wet chemical deposition route, followed by a single thermal treatment. Thermal annealing of the MnO₂ nanostructures concurrent with carbonization of the polymer precursor leads to the formation of a 3D hybrid SC electrode material with unique structural integrity and uniformity. The microstructural and chemical characterization of the hybrid electrode reveals the predominant formation of crystalline hausmannite-Mn₃O₄ after the pyrolysis/annealing process, which is a favorable pseudocapacitive material for EES. With the combination of the 3D free-standing carbon architecture and self-assembled binder-free Mn₃O₄ nanostructures, electrochemical capacitive charge storage with very good rate capability, gravimetric and areal capacitances (186 F g^-1 and 968 mF cm^-2, respectively), and a long lifespan (>92% after 5000 cycles) is demonstrated. It is worth noting that the gravimetric capacitance value is obtained by considering the full mass of the electrode including the carbon current collector. When only the mass of the pseudocapacitive nanomaterial is considered, a capacitance value of 457 F g^-1 is achieved, which is comparable to state-of-the-art Mn₃O₄-based SC electrode materials.

Preventing the Capillary-Induced Collapse of Vertical Nanostructures

Robust processes to fabricate densely packed high-aspect-ratio (HAR) vertical semiconductor nanostructures are important for applications in microelectronics, energy storage and conversion. One of the main challenges in manufacturing these nanostructures is pattern collapse, which is the damage induced by capillary forces from numerous solution-based processes used during their fabrication. Here, using an array of vertical silicon (Si) nanopillars as test structures, we demonstrate that pattern collapse can be greatly reduced by a solution-phase deposition method to coat the nanopillars with self-assembled monolayers (SAMs). As the main cause for pattern collapse is strong adhesion between the nanopillars, we systematically evaluated SAMs with different surface energy components and identified H-bonding between the surfaces to have the largest contribution to the adhesion. The advantage of the solution-phase deposition method is that it can be implemented before any drying step, which causes patterns to collapse. Moreover, after drying, these SAMs can be easily removed using a gentle air-plasma treatment right before the next fabrication step, leaving a clean nanopillar surface behind. Therefore, our approach provides a facile and effective method to prevent the drying-induced pattern collapse in micro- and nanofabrication processes.

Liquid bridge splitting enhances normal capillary adhesion and resistance to shear on rough surfaces

HYPOTHESIS

'Bridge splitting' is considered in the case of capillary adhesion: a fixed total volume of liquid is split into multiple capillary bridges. Previous studies have shown that bridge splitting only enhances the capillary-induced adhesion force between two planar surfaces in specific circumstances. We hypothesise that bridge splitting significantly enhances the total adhesion force between rough surfaces, since mobile wetting bridges can naturally migrate to narrower gaps. This migration of capillary bridges should also provide a resistance to shear.

NUMERICAL EXPERIMENTS

We theoretically consider an idealized system of many liquid bridges confined between two solid surfaces. By numerically calculating the shape of a single bridge, the total adhesion force is found as the number of bridges and roughness are varied. The resistance to shear is also calculated in the limit of strong surface tension or small shears.

FINDINGS

Bridge splitting on a rough surface significantly enhances the adhesion force, with an enhancement that increases with the amplitude of the roughness; maximising over the number of bridges can increase the total adhesion force by an order of magnitude. Resistance to shear is shown to increase linearly with the translation velocity, and the behaviour of many such shearing bridges is quantified.

Anisotropic Carbon Nanotube Structures with High Aspect Ratio Nanopores for Li-Ion Battery Anodes

Technological advances in membrane technology, catalysis, and electrochemical energy storage require the fabrication of controlled pore structures at ever smaller length scales. It is therefore important to develop processes allowing for the fabrication of materials with controlled submicron porous structures. We propose a combination of colloidal lithography and chemical vapor deposition of carbon nanotubes to create continuous straight pores with diameters down to 100 nm in structures with thicknesses of more than 300 μm. These structures offer unique features, including continuous and parallel pores with aspect ratios in excess of 3000, a low pore tortuosity, good electrical conductivity, and electrochemical stability. We demonstrate that these structures can be used in Li-ion batteries by coating the carbon nanotubes with Si as an active anode material.

Capillary-Force-Driven Self-Assembly of 4D-Printed Microstructures

Capillary-force-driven self-assembly is emerging as a significant approach for the massive manufacture of advanced materials with novel wetting, adhesion, optical, mechanical, or electrical properties. However, academic value and practical applications of the self-assembly are greatly restricted because traditional micropillar self-assembly is always unidirectional. In this work, two-photon-lithography-based 4D microprinting is introduced to realize the reversible and bidirectional self-assembly of microstructures. With asymmetric crosslinking densities, the printed vertical microstructures can switch to a curved state with controlled thickness, curvature, and smooth morphology that are impossible to replicate by traditional 3D-printing technology. In different evaporating solvents, the 4D-printed microstructures can experience three states: (I) coalesce into clusters from original vertical states via traditional self-assembly, (II) remain curved, or (III) arbitrarily self-assemble (4D self-assembly) toward the curving directions. Compared to conventional approaches, this 4D self-assembly is distance-independent, which can generate varieties of assemblies with a yield as high as 100%. More importantly, the three states can be reversibly switched, allowing the development of many promising applications such as reversible micropatterns, switchable wetting, and dynamic actuation of microrobots, origami, and encapsulation.

Onset of Elasto-capillary Bundling of Micropillar Arrays: A Direct Visualization

When a liquid drop gets into contact with a soft array of microstructures, capillary forces at the three-phase contact line can lead to critical deformations. Microstructures may collapse and form bundles or even patterns. So far, viewing the kinetics of bundling at the menisci scale has remained elusive. Here, we use laser scanning confocal microscopy to directly image the menisci between micropillars. We image structural changes in polydimethylsiloxane micropillar arrays during the Cassie-to-Wenzel transitions of a water drop evaporating on top of the array. We demonstrate how the regular pillar array undergoes a spontaneous symmetry breaking as the first step to the formation of pillar bundles. A comparison of the Cassie-to-Wenzel transition in air and FC40 indicates that the local contact angle determines the outcome of the bundling process. Based on these observations, we develop a simple model using the local contact angle, stiffness of the pillars, and interfacial tension of the liquid to predict the onset of the symmetry breaking.

Shape-Programmed Fabrication and Actuation of Magnetically Active Micropost Arrays

Micro- and nanotextured surfaces with reconfigurable textures can enable advancements in the control of wetting and heat transfer, directed assembly of complex materials, and reconfigurable optics, among many applications. However, reliable and programmable directional shape in large scale is significant for prescribed applications. Herein, we demonstrate the self-directed fabrication and actuation of large-area elastomer micropillar arrays, using magnetic fields to both program a shape-directed actuation response and rapidly and reversibly actuate the arrays. Specifically, alignment of magnetic microparticles during casting of micropost arrays with hemicylindrical shapes imparts a deterministic anisotropy that can be exploited to achieve the prescribed, large-deformation bending or twisting of the pillars. The actuation coincides with the finite element method, and we demonstrate reversible, noncontact magnetic actuation of arrays of tens of thousands of pillars over hundreds of cycles, with the bending and twisting angles of up to 72 and 61°, respectively. Moreover, we demonstrate the use of the surfaces to control anisotropic liquid spreading and show that the capillary self-assembly of actuated micropost arrays enables highly complex architectures to be fabricated. The present technique could be scaled to indefinite areas using cost-effective materials and casting techniques, and the principle of shape-directed pillar actuation can be applied to other active material systems.

Nanoscale Elastocapillary Effect Induced by Thin-Liquid-Film Instability

Dense arrays of high-aspect-ratio (HAR) vertical nanostructures are essential elements of microelectronic components, photovoltaics, nanoelectromechanical, and energy storage devices. One of the critical challenges in manufacturing the HAR nanostructures is to prevent their capillary-induced aggregation during solution-based nanofabrication processes. Despite the importance of controlling capillary effects, the detailed mechanisms of how a solution interacts with nanostructures are not well understood. Using in situ liquid cell transmission electron microscopy (TEM), we track the dynamics of nanoscale drying process of HAR silicon (Si) nanopillars in real-time and identify a new mechanism responsible for pattern collapse and nanostructure aggregation. During drying, deflection and aggregation of nanopillars are driven by thin-liquid-film instability, which results in much stronger capillary interactions between the nanopillars than the commonly proposed lateral meniscus interaction forces. The importance of thin-film instability in dewetting has been overlooked in prevalent theories on elastocapillary aggregation. The new dynamic mechanism revealed by in situ visualization is essential for the development of robust nanofabrication processes.

Honeycomb-shaped carbon nanotube supports for BiVO4 based solar water splitting

Advances in the synthesis and assembly of nanomaterials offer a unique opportunity to purposefully design structures according to the requirements of the targeted applications. This paper shows a process to create robust 3D carbon nanotube (CNT) structures, which provide an electrically conductive support for nanoparticle coating. We describe a process to reliably fabricate robust honeycomb structures with walls made out of aligned CNTs. We present a design of experimental analysis of this fabrication process and discuss methods to coat these honeycombs with BiVO₄ for solar fuel applications. The proposed honeycomb structure allows for an efficient transport of electrons through the electrode, as well as an enhanced light-electrode interaction. Finally, we demonstrate that the developed CNT electrodes can survive harsh BiVO₄ synthesis conditions and can subsequently be used as photoelectrodes for solar water splitting.

The importance of design in nanoarchitectonics: multifractality in MACE silicon nanowires

Background: Mechanisms of self-assembly/self-organization are fundamental for the emergence of nanoarchitectonic systems composed by elemental units, and it is important to build a theoretical framework for them. Additionally, because the enhanced functionalities of these systems are related to their spatial morphologies, it is necessary to quantify the self-organized design through suited statistical analysis tools. Results: We have investigated the self-assembly bundling process of nanowires fabricated by metal-assisted chemical etching (MACE). First, we have applied theoretical models in order to obtain a quantitative estimation of the driving forces leading to self-assembly. Then, we have studied the surfaces of the nanoarchitectures by means of multifractal analysis. We have found that these systems are not simple monofractals, but that the more complex paradigm of multifractality (different fractal dimensions across different scales) has to be applied to describe their morphology. Conclusion: The multifractal analysis approach has proven its ability to discriminate among different MACE nanoarchitectures. Additionally, it has demonstrated its capacity to measure the degree of homogeneity of these surfaces. Finally, a correlation between the growth conditions and the capacity dimension of the nanowires was obtained.

Hierarchically Patterned Elastomeric and Thermoplastic Polymer Films through Nanoimprinting and Ultraviolet Light Exposure

The surface relief structure of polymer films over large areas can be controlled by combining nanoscale imprinting and microscale ultraviolet-ozone (UVO) radiation, resulting in hierarchical structured surfaces. First, nanoscale patterns were formed by nanoimprinting elastomer [poly(dimethylsiloxane) (PDMS)] films with a pattern on a digital video disk. Micron-scale patterns were then superimposed on the nanoimprinted PDMS films by exposing them to ultraviolet radiation in oxygen (UVO) through a transmission electron microscopy grid mask having variable microscale patterning. UVO exposure leads to conversion and densification of PDMS to SiO x , leading to micron height relief features that follow a linear scaling relation with pattern dimension. Further, the pattern scopes are shown to collapse into a master curve by normalized feature values. Interestingly, these relief structures preserve the nanoscale features. In this paper, the influence of the self-limiting PDMS densification, wall stress at the boundary of micro-depression, and UVO exposure energy is studied in control of the micro-depression scale. This simple two-step imprinting process involving both nanoimprinting and UV radiation allows for facile fabrication of the dimension adjustable micro-nano hierarchically structures not only on elastomer films but also on thermoplastic polymer films. Coarse-grained molecular dynamics simulations were performed to correlate the surface tension and elastic properties of polymeric materials to the deformation of the pattern structure.

Design rules for a tunable merged-tip microneedle

This publication proposes the use of an elasto-capillarity-driven self-assembly for fabricating a microscale merged-tip structure out of a variety of biocompatible UV-curable polymers for use as a microneedle platform. In addition, the novel merged-tip microstructure constitutes a new class of microneedles, which incorporates the convergence of biocompatible polymer micropillars, leading to the formation of a sharp tip and an open cavity capable of both liquid trapping and volume control. When combined with biocompatible photopolymer micropillar arrays fabricated with photolithography, elasto-capillarity-driven self-assembly provides a means for producing a complex microneedle-like structure without the use of micromolding or micromachining. This publication also explores and defines the design rules by which several fabrication aspects, such as micropillar dimensions, shapes, pattern array configurations, and materials, can be manipulated to produce a customizable microneedle array with controllable cavity volumes, fracture points, and merge profiles. In addition, the incorporation of a modular through-hole micropore membrane base was also investigated as a method for constitutive payload delivery and fluid-sampling functionalities. The flexibility and fabrication simplicity of the merged-tip microneedle platform holds promise in transdermal drug delivery applications.

Clumping Stability of Vertical Nanofibers on Surfaces

The clumping behavior of nanofibers, including nanowires and nanotubes, is a challenge to their fabrication, which may diminish their optical, electrical, and mechanical performance. However, the stability of the clumping status, especially the unstable clumping state, was rarely discussed to give a deep understanding on clumping criteria. In this study, an energy-based analysis of the nanofiber system was introduced to analyze the deformation of the fibers, providing a novel method to define the thermodynamic stability and the kinetic stability of clumping. The clumping stability design map was proposed, further the stability of the clumping status and the criteria of the five states (the stable, the thermodynamic stable, the kinetic stable, unstable, and the nonclumping state) were discussed. The theoretical criteria provide new insights into designing nanofiber arrays on surfaces to achieve desired clumping or nonclumping states.

Filamentary structures that self-organize due to adhesion

We study the self-organization of random collections of elastic filaments that interact adhesively. The evolution from an initial fully random quasi-two-dimensional state is controlled by filament elasticity, adhesion and interfilament friction, and excluded volume. Three outcomes are possible: the system may remain locked in the initial state, may organize into isolated fiber bundles, or may form a stable, connected network of bundles. The range of system parameters leading to each of these states is identified. The network of bundles is subisostatic and is stabilized by prestressed triangular features forming at bundle-to-bundle nodes, similar to the situation in foams. Interfiber friction promotes locking and expands the parametric range of nonevolving systems.

Wettability control of droplet durotaxis

Durotaxis refers to cell motion directed by stiffness gradients of an underlying substrate. Recent work has shown that droplets also move spontaneously along stiffness gradients through a process reminiscent of durotaxis. Wetting droplets, however, move toward softer substrates, an observation seemingly at odds with cell motion. Here, we extend our understanding of this phenomenon, and show that wettability of the substrate plays a critical role: while wetting droplets move in the direction of lower stiffness, nonwetting liquids reverse droplet durotaxis. Our numerical experiments also reveal that Laplace pressure can be used to determine the direction of motion of liquid slugs in confined environments. Our results suggest new ways of controlling droplet dynamics at small scales, which can open the door to enhanced bubble and droplet logic in microfluidic platforms.

Process-morphology scaling relations quantify self-organization in capillary densified nanofiber arrays

Model-informed experiments reveal that cellular pattern formation in capillary-densified aligned carbon nanotube arrays is governed not only by their height, but also by substrate adhesion strength.

Stable Wettability Control of Nanoporous Microstructures by iCVD Coating of Carbon Nanotubes

Scalable manufacturing of structured materials with engineered nanoporosity is critical for applications in energy storage devices (i.e., batteries and supercapacitors) and in the wettability control of surfaces (i.e., superhydrophobic and superomniphobic surfaces). Patterns formed in arrays of vertically aligned carbon nanotubes (VA-CNTs) have been extensively studied for these applications. However, the as-deposited features are often undesirably altered upon liquid infiltration and evaporation because of capillarity-driven aggregation of low density CNT forests. Here, it is shown that an ultrathin, conformal, and low-surface-energy layer of poly perfluorodecyl acrylate, poly(1H,1H,2H,2H-perfluorodecyl acrylate) (pPFDA), makes the VA-CNTs robust against surface-tension-driven aggregation and densification. This single vapor-deposition step allows the fidelity of the as-deposited VA-CNT patterns to be retained during wet processing, such as inking, and subsequent drying. It is demonstrated how to establish omniphobicity or liquid infiltration by controlling the surface morphology. Retaining a crust of entangled CNTs and pPFDA aggregates on top of the patterned VA-CNTs produces micropillars with re-entrant features that prevent the infiltration of low-surface-tension liquids and thus gives rise to stable omniphobicity. Plasma treatments before and after polymer deposition remove the crust of entangled CNTs and pPFDA aggregates and attach hydroxyl groups to the CNT tips, enabling liquid infiltration yet preventing densification of the highly porous CNTs. The latter observation demonstrates the protective character of the pPFDA coating with the potential application of these surfaces for direct contact printing of microelectronic features.

Using evaporation to control capillary instabilities in micro-systems

The instabilities of fluid interfaces represent both a limitation and an opportunity for the fabrication of small-scale devices. Just as non-uniform capillary pressures can destroy micro-electrical mechanical systems (MEMS), so they can guide the assembly of novel solid and fluid structures. In many such applications the interface appears during an evaporation process and is therefore only present temporarily. It is commonly assumed that this evaporation simply guides the interface through a sequence of equilibrium configurations, and that the rate of evaporation only sets the timescale of this sequence. Here, we use Lattice-Boltzmann simulations and a theoretical analysis to show that, in fact, the rate of evaporation can be a factor in determining the onset and form of dynamical capillary instabilities. Our results shed light on the role of evaporation in previous experiments, and open the possibility of exploiting diffusive mass transfer to directly control capillary flows in MEMS applications.

Vertically Aligned Carbon Nanotubes as Platform for Biomimetically Inspired Mechanical Sensing, Bioactive Surfaces, and Electrical Cell Interfacing

Vertically aligned carbon nanotubes (VACNTs) are one dimensional carbon objects anchored atop of a solid substrate. They are geometrically fixed in contrast to their counterparts, randomly oriented carbon nanotubes (CNTs). In this progress report, the breadth in which these one dimensional, mechanically flexible, though robust and electrical conducting carbon nanostructures can be employed as functional material is shown and our research is put in perspective to work in the last five to ten years. The connection between the different areas touched in this report is the biomimetic-materials approach, which rely on the hairy morphology of VACNTs. These properties in connection with their electrical conductivity offer possibilities towards new functional features and applications of VACNTs. To appreciate the possibilities of biomimetic research with VACNTs, first their material characteristics are given to make the reader familiar with specific features of their synthesis, the peculiarities in arranging and controlling the morphology of CNTs in a vertical alignment as well as a current understanding of these properties on a microscopic basis. In doing so, similarities as well as differences, which offer new possibilities for biomimetic studies of VACNTS with respect to multiwalled randomly oriented CNTs, will become clear.

Template-Assisted Preparation of Micrometric Suspended Membrane Lattices of Photoluminescent and Non-Photoluminescent Polymers by Capillarity-Driven Solvent Evaporation: Application to Microtagging

In this work, the bottom-up template-assisted preparation of high-density lattices (up to 11 · 10⁶ membranes/cm²) of suspended polymer membranes with micrometric size (in the order of few μm²) and sub-micrometric thickness (in the order of hundreds of nm) is demonstrated for both photoluminescent and non-photoluminescent polymers by capillarity-driven solvent evaporation. Solvent evaporation of low concentration polymer solutions drop-cast on an array of open-ended micropipes is shown to lead to polymer membrane formation at the inlet of the micropipes thanks to capillarity. The method is proven to be robust with high-yield (>98%) over large areas (1 cm²) and of general validity for both conjugated and non-conjugated polymers, e.g. poly(9,9-di-n-octylfluorene-alt-benzothiadiazole (F8BT), poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), polystyrene (PS), thus breaking a new ground on the controlled preparation of polymer micro and nanostructures. Angle dependence and thermal stability of photoluminescence emission arising from F8BT membrane lattices was thorough investigated, highlighting a non-Lambertian photoluminescence emission of membrane lattices with respect to F8BT films. The method is eventually successfully applied to the preparation of both photoluminescent and non-photoluminescent micro Quick Response (μQR) codes using different polymers, i.e. F8BT, MDMO-PPV, PS, thus providing micrometric-sized taggants suitable for anti-counterfeiting applications.

CVD Polymers for Devices and Device Fabrication

Chemical vapor deposition (CVD) polymerization directly synthesizes organic thin films on a substrate from vapor phase reactants. Dielectric, semiconducting, electrically conducting, and ionically conducting CVD polymers have all been readily integrated into devices. The absence of solvent in the CVD process enables the growth of high-purity layers and avoids the potential of dewetting phenomena, which lead to pinhole defects. By limiting contaminants and defects, ultrathin (<10 nm) CVD polymeric device layers have been fabricated in multiple laboratories. The CVD method is particularly suitable for synthesizing insoluble conductive polymers, layers with high densities of organic functional groups, and robust crosslinked networks. Additionally, CVD polymers are prized for the ability to conformally cover rough surfaces, like those of paper and textile substrates, as well as the complex geometries of micro- and nanostructured devices. By employing low processing temperatures, CVD polymerization avoids damaging substrates and underlying device layers. This report discusses the mechanisms of the major CVD polymerization techniques and the recent progress of their applications in devices and device fabrication, with emphasis on initiated CVD (iCVD) and oxidative CVD (oCVD) polymerization.

Precise regulation of tilt angle of Si nanostructures via metal-assisted chemical etching

The ability to regulate the tilt angle of Si nanostructures is important for their applications in photoelectric devices. Herein we demonstrate a facile method to precisely regulate the tilt angle of nanocones with metal-assisted chemical etching (MaCE) in a one-step process based on the systematic investigation of the formation mechanism of the tilt angle. With Au nanohole arrays as templates, the tilt angles of Si nanocone arrays can be tuned from 69.2° to 88.6° by varying the composition of the etchant. When the Si nanocone arrays are the same height (2.2 μm), the reflectivity decreases with the decreasing of the tilt angle. When the tilt angle is 83.0°, the average reflectivity is lowered to 1.37% in the 250-1000 nm range. This method can be applied for fabrication over a large area (as large as 2 cm × 2 cm). This chemical method should be applicable to other Si nanostructures, which may promote the applications of MaCE in semiconductor manufacturing.

Drop morphologies on flexible fibers: influence of elastocapillary effects

Various materials are made of long thin fibers that are randomly oriented to form a complex network in which drops of wetting liquid tend to accumulate at the nodes. The capillary force exerted by the liquid can bend flexible fibers, which in turn influences the morphology adopted by the liquid. In this paper, we investigate through a model situation the role of the fiber flexibility on the shape of a small volume of liquid on a pair of crossed flexible fibers. We characterize the liquid morphologies as we vary the volume of liquid, the angle between the fibers, and the length of the fibers. The drop morphologies previously reported for rigid crossed fibers, i.e., a drop, a column and a mixed morphology, are also observed on flexible crossed fibers with modified domains of existence. In addition, at small tilt angles between the fibers, a new behavior is observed: the fibers bend and collapse. Depending on the volume of liquid, a thin column with or without a drop is reported on the collapsed fibers. Our study suggests that the fiber flexibility adds a rich variety of behaviors that may be important for some applications.

Ultrathin high-resolution flexographic printing using nanoporous stamps

Since its invention in ancient times, relief printing, commonly called flexography, has been used to mass-produce artifacts ranging from decorative graphics to printed media. Now, higher-resolution flexography is essential to manufacturing low-cost, large-area printed electronics. However, because of contact-mediated liquid instabilities and spreading, the resolution of flexographic printing using elastomeric stamps is limited to tens of micrometers. We introduce engineered nanoporous microstructures, comprising polymer-coated aligned carbon nanotubes (CNTs), as a next-generation stamp material. We design and engineer the highly porous microstructures to be wetted by colloidal inks and to transfer a thin layer to a target substrate upon brief contact. We demonstrate printing of diverse micrometer-scale patterns of a variety of functional nanoparticle inks, including Ag, ZnO, WO₃, and CdSe/ZnS, onto both rigid and compliant substrates. The printed patterns have highly uniform nanoscale thickness (5 to 50 nm) and match the stamp features with high fidelity (edge roughness, ~0.2 μm). We derive conditions for uniform printing based on nanoscale contact mechanics, characterize printed Ag lines and transparent conductors, and achieve continuous printing at a speed of 0.2 m/s. The latter represents a combination of resolution and throughput that far surpasses industrial printing technologies.

Carbon Nanotube (CNT) Honeycomb Cell Area-Dependent Optical Reflectance

The relationship between the physical structure of carbon nanotube (CNT) honeycomb structures and their total, diffuse, and specular reflectance is investigated for the first time. It is found that CNT honeycomb structures with average cell areas of smaller than 30 μm² show a higher total reflectance. Particularly, a thinner, highly packed CNT (buckypaper) film, along with a larger wall height and higher ratio of wall height to cell area, markedly increase the total reflectance for cell areas smaller than 30 μm², which means that a higher total area of buckypapers in CNT walls and bottom areas increases the total reflectance, including the diffuse reflectance. It is also found that the total reflection of non-absorbed light in CNT honeycomb structures consists primarily of diffuse reflectance.

Let's twist again: elasto-capillary assembly of parallel ribbons

We show the self-assembly through twisting and bending of side by side ribbons under the action of capillary forces. Micro-ribbons made of silicon nitride are batch assembled at the wafer scale. We study their assembly as a function of their dimensions and separating distance. Model experiments are carried out at the macroscopic scale where the tension in ribbons can easily be tuned. The process is modeled considering the competition between capillary, elastic and tension forces. Theory shows a good agreement for macroscale assemblies, while the accuracy is within 30% at the micrometer scale. This simple self-assembly technique yields highly symmetric and controllable structures which could be used for batch fabrication of functional 3D micro-structures.

Hierarchical Assemblies of Carbon Nanotubes for Ultraflexible Li-Ion Batteries

The flexible batteries that are needed to power flexible circuits and displays remain challenging, despite considerable progress in the fabrication of such devices. Here, it is shown that flexible batteries can be fabricated using arrays of carbon nanotube microstructures, which decouple stress from the energy-storage material. It is found that this battery architecture imparts exceptional flexibility (radius ≈ 300 μm), high rate (20 A g(-1) ), and excellent cycling stability.

Directional Clustering of Slanted Nanopillars by Elastocapillarity

The unidirectional clustering induced by capillary force of drying liquids between pillars is investigated and a theoretical model to set a criterion of the unidirectional clustering of the slanted nanopillars is proposed.

Formulating an Ideal Protein Photoresist for Fabricating Dynamic Microstructures with High Aspect Ratios and Uniform Responsiveness

The physical properties of aqueous-based stimuli-responsive photoresists are crucial in fabricating microstructures with high structural integrity and uniform responsiveness during two-photon lithography. Here, we quantitatively investigate how various components within bovine serum albumin (BSA) photoresists affect our ability to achieve BSA microstructures with consistent stimuli-responsive properties over areas exceeding 10(4) μm(2). We unveil a relationship between BSA concentration and dynamic viscosity, establishing a threshold viscosity to achieve robust BSA microstructures. We also demonstrate the addition of an inert polymer to the photoresist as viscosity enhancer. A set of systematically optimized processing parameters is derived for the construction of dynamic BSA microstructures. The optimized BSA photoresists and processing parameters enable us to extend the two-dimensional (2D) microstructures to three-dimensional (3D) ones, culminating in arrays of micropillars with aspect ratio > 10. Our findings foster the development of liquid stimuli-responsive photoresists to build multifunctional complex 3D geometries for applications such as bioimplantable devices or adaptive photonic systems.

Micro-total envelope system with silicon nanowire separator for safe carcinogenic chemistry

Exploration and expansion of the chemistries involving toxic or carcinogenic reagents are severely limited by the health hazards their presence poses. Here, we present a micro-total envelope system (μ-TES) and an automated total process for the generation of the carcinogenic reagent, its purification and its utilization for a desired synthesis that is totally enveloped from being exposed to the carcinogen. A unique microseparator is developed on the basis of SiNWs structure to replace the usual exposure-prone distillation in separating the generated reagent. Chloromethyl methyl ether chemistry is explored as a carcinogenic model in demonstrating the efficiency of the μ-TES that is fully automated so that feeding the ingredients for the generation is all it takes to produce the desired product. Syntheses taking days can be accomplished safely in minutes with excellent yields, which bodes well for elevating the carcinogenic chemistry to new unexplored dimensions.

Carcinogenic compounds present difficulties in synthesis not just during reaction but perhaps especially during purification. Here, the authors report a microfluidic device for the use of carcinogenic chloromethyl methyl ether that includes a silicon nanowire separator allowing isolated, automated purification.

Superamphiphobic Silicon-Nanowire-Embedded Microsystem and In-Contact Flow Performance of Gas and Liquid Streams

Gas and liquid streams are invariably separated either by a solid wall or by a membrane for heat or mass transfer between the gas and liquid streams. Without the separating wall, the gas phase is present as bubbles in liquid or, in a microsystem, as gas plugs between slugs of liquid. Continuous and direct contact between the two moving streams of gas and liquid is quite an efficient way of achieving heat or mass transfer between the two phases. Here, we report a silicon nanowire built-in microsystem in which a liquid stream flows in contact with an underlying gas stream. The upper liquid stream does not penetrate into the lower gas stream due to the superamphiphobic nature of the silicon nanowires built into the bottom wall, thereby preserving the integrity of continuous gas and liquid streams, although they are flowing in contact. Due to the superamphiphobic nature of silicon nanowires, the microsystem provides the best possible interfacial mass transfer known to date between flowing gas and liquid phases, which can achieve excellent chemical performance in two-phase organic syntheses.

Hierarchical 3-dimensional nickel-iron nanosheet arrays on carbon fiber paper as a novel electrode for non-enzymatic glucose sensing

Three-dimensional nickel-iron (3-D/Ni-Fe) nanostructures are exciting candidates for various applications because they produce more reaction-active sites than 1-D and 2-D nanostructured materials and exhibit attractive optical, electrical and catalytic properties. In this work, freestanding 3-D/Ni-Fe interconnected hierarchical nanosheets, hierarchical nanospheres, and porous nanospheres are directly grown on a flexible carbon fiber paper (CFP) substrate by a single-step hydrothermal process. Among the nanostructures, 3-D/Ni-Fe interconnected hierarchical nanosheets show excellent electrochemical properties because of its high conductivity, large specific active surface area, and mesopores on its walls (vide infra). The 3-D/Ni-Fe hierarchical nanosheet array modified CFP substrate is further explored as a novel electrode for electrochemical non-enzymatic glucose sensor application. The 3-D/Ni-Fe hierarchical nanosheet arrays exhibit significant catalytic activity towards the electrochemical oxidation of glucose, as compared to the 3-D/Ni-Fe hierarchical nanospheres, and porous nanospheres. The 3-D/Ni-Fe hierarchical nanosheet arrays can access a large amount of glucose molecules on their surface (mesopore walls) for an efficient electrocatalytic oxidation process. Moreover, 3-D/Ni-Fe hierarchical nanosheet arrays showed higher sensitivity (7.90 μA μM(-1) cm(-2)) with wide linear glucose concentration ranging from 0.05 μM to 0.2 mM, and the low detection limit (LOD) of 0.031 μM (S/N = 3) is achieved by the amperometry method. Further, the 3-D/Ni-Fe hierarchical nanosheet array modified CFP electrode can be demonstrated to have excellent selectivity towards the detection of glucose in the presence of 500-fold excess of major important interferents. All these results indicate that 3-D/Ni-Fe hierarchical nanosheet arrays are promising candidates for non-enzymatic glucose sensing.

Double-walled carbon nanotube array for CO2 and SO2 adsorption

Grand-canonical Monte Carlo simulations and adsorption experiments are combined to find the optimized carbon nanotube (CNT) arrays for gas adsorption at low pressures and 303 K. Bundles of 3D aligned double-walled carbon nanotube (DWCNT) with inner diameter of 8 nm and different intertube distances were made experimentally. The experimental results show that decreasing intertube distance leads to a significant enhancement in carbon-dioxide (CO2) adsorption capacity at 1 bar. The molecular simulation study on CO2 adsorption onto bundles of 3D aligned DWCNT with inner diameters of 1, 3, and 8 nm and intertube distance of 0-15 nm shows that the intertube distance plays a more important role than the CNT diameter. The simulation results show that decreasing the intertube distance up to 1 nm increases the excess adsorption generally in all the studied systems at pressures 0 < p < 14 bars (the increase can be up to ∼40% depending on the system and pressure). This is in agreement with the experimental result. Further reduction in intertube distance leads to a decrease in the excess adsorption in the pressure range 9 < p < 14 bars. However, at lower pressure, 0 < p < 9 bars, intertube distance of 0.5 nm is found to have the highest excess adsorption. This result is indifferent to tube diameter. Furthermore, molecular simulations are conducted to obtain the optimal parameters, for the DWCNT bundle, for SO2 adsorption, which are similar to those observed for CO2 in the pressure range 0 < p < 3 bars.

Nanocapillarity-mediated magnetic assembly of nanoparticles into ultraflexible filaments and reconfigurable networks

The fabrication of multifunctional materials with tunable structure and properties requires programmed binding of their building blocks. For example, particles organized in long-ranged structures by external fields can be bound permanently into stiff chains through electrostatic or van der Waals attraction, or into flexible chains through soft molecular linkers such as surface-grafted DNA or polymers. Here, we show that capillarity-mediated binding between magnetic nanoparticles coated with a liquid lipid shell can be used for the assembly of ultraflexible microfilaments and network structures. These filaments can be magnetically regenerated on mechanical damage, owing to the fluidity of the capillary bridges between nanoparticles and their reversible binding on contact. Nanocapillary forces offer opportunities for assembling dynamically reconfigurable multifunctional materials that could find applications as micromanipulators, microbots with ultrasoft joints, or magnetically self-repairing gels.

Bio-inspired Hierarchical Nanowebs for Green Catalysis

Bio-inspired 3D hierarchical nanowebs are fabricated using silicon micropillars, carbon nanotubes (CNT), and manganese oxide. The Si pillars act as artificial branches for growing CNTs and the secondary metal coating strengthens the structures. The simple but effective structure provides both chemical and mechanical stability to be used as a green catalyst for recycling waste polymers into raw materials.