Electrostatic self-assembly of macroscopic crystals using contact electrification

The significance of multipole interactions for the stability of regular structures composed from charged particles

Identifying the forces responsible for stabilising binary particle lattices is key to the controlled fabrication of many new materials. Experiments have shown that the presence of charge can be integral to the formation of ordered arrays; however, a complete analysis of the forces responsible has not included many of the significant lattice types that may form during fabrication. A theory of many-body electrostatic interactions has been applied to six lattice stoichiometries, AB, AB2, AB3, AB4, AB5 and AB6, to show that induced multipole interactions can make a very significant (>80 %) contribution to the total lattice energy of arrays of charged particles. Particle radii ratios which favour global minima in electrostatic energy are found to be the same or a close match to those observed by experiment. Although certain lattice types exhibit local energy minima, the calculations show that many-body rather than two-body interactions are ultimately responsible for the structures observed by experiment. For a lattice isostructural with CFe4, a particle size ratio not previously observed is found to be particularly stable due to many-body effects.

Driving macro-scale transformations in three-dimensional-printed biopolymers through controlled induction of molecular anisotropy at the nanoscale

Motivated by the need to harness the properties of renewable and biodegradable polymers for the design and manufacturing of multi-scale structures with complex geometries, we have employed our additive manufacturing platform that leverages molecular self-assembly for the production of metre-scale structures characterized by complex geometries and heterogeneous material composition. As a precursor material, we used chitosan, a chemically modified form of chitin, an abundant and sustainable structural polysaccharide. We demonstrate the ability to control concentration-dependent crystallization as well as the induction of the preferred orientation of the polymer chains through the combination of extrusion-based robotic fabrication and directional toolpathing. Anisotropy is demonstrated and assessed through high-resolution micro-X-ray diffraction in conjunction with finite element simulations. Using this approach, we can leverage controlled and user-defined small-scale propagation of residual stresses to induce large-scale folding of the resulting structures.

Van der Waals Colloidal Crystals

A general guiding principle for colloidal crystallization is to tame the attractive enthalpy such that it slightly overwhelms the repulsive interaction. As-synthesized colloids are generally designed to retain a strong repulsive potential for the high stability of suspensions, encoding appropriate attractive potentials into colloids has been key to their crystallization. Despite the myriad of interparticle attractions for colloidal crystallization, the van der Waals (vdW) force remains unexplored. Here, it is shown that the implementation of gold cores into silica colloids and the resulting vdW force can reconfigure the pair potential well depth to the optimal range between -1 and -4 kB T at tens of nanometer-scale colloidal distances. As such, colloidal crystals with a distinct liquid gap can be formed, which is evidenced by photonic bandgap-based diffractive colorization.

Toward the Assembly of 2D Tunable Crystal Patterns of Spherical Colloids on a Wafer-Scale

Entering an era of miniaturization prompted scientists to explore strategies to assemble colloidal crystals for numerous applications, including photonics. However, wet methods are intrinsically less versatile than dry methods, whereas the manual rubbing method of dry powders has been demonstrated only on sticky elastomeric layers, hindering particle transfer in printing applications and applicability in analytical screening. To address this clear impetus of broad applicability, we explore here the assembly on nonelastomeric, rigid substrates by utilizing the manual rubbing method to rapidly (≈20 s) attain monolayers comprising hexagonal closely packed (HCP) crystals of monodisperse dry powder spherical particles with a diameter ranging from 500 nm to 10 μm using a PDMS stamp. Our findings elucidate that the tribocharging-induced electrostatic attraction, particularly on relatively stiff substrates, and contact mechanics force between particles and substrates are critical contributors to attain large-scale HCP structures on conductive and insulating substrates. The best performance was obtained with polystyrene and PMMA powder, while silica was assembled only in HCP structures on fluorocarbon-coated substrates under zero-humidity conditions. Finally, we successfully demonstrated the assembly of tunable crystal patterns on a wafer-scale with great control on fluorocarbon-coated wafers, which is promising in microelectronics, bead-based assays, sensing, and anticounterfeiting applications.

Static charge is an ionic molecular fragment

What is static charge? Despite the long history of research, the identity of static charge and mechanism by which static is generated by contact electrification are still unknown. Investigations are challenging due to the complexity of surfaces. This study involves the molecular-scale analysis of contact electrification using highly well-defined surfaces functionalized with a self-assembled monolayer of alkylsilanes. Analyses show the elementary molecular steps of contact electrification: the exact location of heterolytic cleavage of covalent bonds (i.e., Si-C bond), exact charged species generated (i.e., alkyl carbocation), and transfer of molecular fragments. The strong correlation between charge generation and molecular fragments due to their signature odd-even effects further shows that contact electrification is based on cleavage of covalent bonds and transfer of ionic molecular fragments. Static charge is thus an alkyl carbocation; in general, it is an ionic molecular fragment. This mechanism based on cleavage of covalent bonds is applicable to general types of insulating materials, such as covalently bonded polymers. The odd-even effect of charging caused by the difference of only one atom explains the highly sensitive nature of contact electrification.

Construction of Triboelectric Series and Chirality Detection of Amino Acids Using Triboelectric Nanogenerator

Triboelectrification necessitates a frictional interaction between two materials, and their contact electrification is characteristically based on the polarity variance in the triboelectric series. Utilizing this fundamental advantage of the triboelectric phenomenon, different materials can be identified according to their contact electrification capability. Herein, an in-depth analysis of the amino acids present in the stratum corneum of human skin is performed and these are quantified regarding triboelectric polarization. The principal focus of this study lies in analyzing and identifying the amino acids present in copious amounts in the stratum corneum to explain their positive behavior during the contact electrification process. Thus, an augmented triboelectric series of amino acids with quantified triboelectric charging polarity by scrutinizing the transfer charge, work function, and atomic percentage is presented. Furthermore, the chirality of aspartic acid as it is most susceptible to racemization with clear consequences on the human skin is detected. The study is expected to accelerate research exploiting triboelectrification and provide valuable information on the surface properties and biological activities of these important biomolecules.

Enhanced Triboelectric Charge Stability by Air-Stable Radicals

This paper demonstrates that air-stable radicals enhance the stability of triboelectric charge on surfaces. While charge on surfaces is often undesirable (e.g., static discharge), improved charge retention can benefit specific applications such as air filtration. Here, it is shown that self-assembled monolayers (SAMs) containing air-stable radicals, 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO), hold the charge longer than those without TEMPO. Charging and retention are monitored by Kelvin Probe Force Microscopy (KPFM) as a function of time. Without the radicals on the surface, charge retention increases with the water contact angle (hydrophobicity), consistent with the understanding that surface water molecules can accelerate charge dissipation. Yet, the most prolonged charge retention is observed in surfaces treated with TEMPO, which are more hydrophilic than untreated control surfaces. The charge retention decreases with reducing radical density by etching the TEMPO-silane with tetrabutylammonium fluoride (TBAF) or scavenging the radicals with ascorbic acid. These results suggest a pathway toward increasing the lifetime of triboelectric charges, which may enhance air filtration, improve tribocharging for patterning charges on surfaces, or boost triboelectric energy harvesting.

Recent Developments in the Methods and Applications of Electrostatic Theory

ConspectusThe review improves our understanding of how electrostatic interactions in the electrolyte, gas phase, and on surfaces can drive the fragmentation and assembly of particles. This is achieved through the overview of our advanced theoretical and computational modeling toolbox suitable for interpretation of experimental observations and discovery of novel, tunable assemblies and architectures. In the past decade, we have produced a significant, fundamental body of work on the development of comprehensive theories based on a rigorous mathematical foundation. These solutions are capable of accurate predictions of electrostatic interactions between dielectric particles of arbitrary size, anisotropy, composition, and charge, interacting in solvents, ionized medium, and on surfaces. We have applied the developed electrostatic approaches to describe physical and chemical phenomena in dusty plasma and planetary environments, in Coulomb fission and electrospray ionization processes, and in soft matter, including a counterintuitive but widespread attraction between like-charged particles.Despite its long history, the search for accurate methods to provide a deeper understanding of electrostatic interactions remains a subject of significant interest, as manifested by a constant stream of theoretical and experimental publications. While major international effort in this area has focused predominantly on the computational modeling of biocatalytic and biochemical performance, we have expanded the boundaries of accuracy, generality, and applicability of underlying theories. Simple solvation models, often used in calculating the electrostatic component of molecular solvation energy and polarization effects of solvent, rarely go beyond the induced dipole approximation because of computational costs. These approximations are generally adequate at larger separation distances; however, as particles approach the touching point, more advanced charged-induced multipolar descriptions of the electrostatic interactions are required to describe accurately a collective behavior of polarizable neutral and charged particles. At short separations, the electrostatic forces involving polarizable dielectric and conducting particles become nonadditive which necessitates further developments of quantitatively accurate many-body approaches. In applications, the electrostatic response of materials is commonly controlled by externally applied electric fields, an additional complex many-body problem that we have addressed most recently, both theoretically and numerically.This review reports on the most significant results and conclusions underpinning these recent advances in electrostatic theory and its applications. We first discuss the limitations of classical approaches to interpreting electrostatic phenomena in electrolytes and complex plasmas, leading to an extended analytical theory suitable for accurate estimation of the electrostatic forces in a dilute solution of a strong electrolyte. We then introduce the concept and numerical realization of many-body electrostatic theory focusing on its performance in selected experimental cases. These experiments underpin, among other applications, electrostatic self-assembly of two-dimensional lattice structures, melting of ionic colloidal crystals in an external electric field, and coalescence of charged clusters.

Melting of a macroscale binary Coulombic crystal

The question of melting has been addressed theoretically and experimentally for two-dimensional crystals in thermal equilibrium. However, as it pertains to out-of-equilibrium systems, the question is unresolved. Here, we present a platform to study the melting of a two-dimensional, binary Coulombic crystal composed of equal numbers of nylon and polytetrafluoroethylene (PTFE) beads that measure a couple of millimeters in diameter. The beads are tribocharged-nylon positively and PTFE negatively-and they experience long-range electrostatic interactions. They form a square crystal in which nylon and PTFE beads sit at alternating sites on a checkerboard lattice. We melt the crystal by agitating the dish in which it resides using an orbital shaker. We compare the melting behavior of the crystal without impurities to that of the crystal with impurities, where we use gold-coated nylon beads as impurities because they tribocharge negligibly. Our results reveal that impurities do not influence the melting of the crystal. Instead, the crystal undergoes shear-induced melting, beginning from its edges, due to its collisions with the dish. As a result of repetitive collisions, the beads acquire kinetic energy, undergo rearrangements, and become disordered. Unlike most examples of shear-induced melting, portions of the crystal remain locally ordered given the persistence of electrostatic interactions and the occurrence of some collisions that are favorable to ordering clusters of beads. Our work clarifies the melting behavior of sheared crystals whose constituents have persistent long-range interactions. It may prove valuable in determining the conditions under which such materials are immune to disorder.

Triboelectric Charging of Particles, an Ongoing Matter: From the Early Onset of Planet Formation to Assembling Crystals

Triboelectrification is the spontaneous charging of two bodies when released from contact. Even though its manifestation is commonplace, in for instance triboelectric nanogenerators, scientists find the tribocharging mechanism a mystery. The primary aim of this mini-review is to provide an overview of different tribocharging concepts that have been applied to study and realize the formation of ordered stable structures using different objects on various length scales. Relevance spans from materials to planet formations. Especially, dry assembly methods of particles of different shapes based on tribocharging to obtain crystal structures or monolayers are considered. In addition, the current technology employed to examine tribocharging in (semi)dry environments is discussed as well as the relevant forces playing a role in the assembly process. In brief, this mini-review is expected to provide a better understanding of tribocharging in assembling objects on the nano- and micrometer scales.

Effect of electrostatic interaction on impact breakage of agglomerates formed by charged dielectric particles

In this paper, the prototypical process of the normal impact of dense agglomerates is investigated using the discrete element method-boundary element method coupled simulations. The agglomerate consists of 50 charged particles with the surface energy equal to 10mJ/m^{2}. The particles are assumed to be tribocharged and follow an exponential charge distribution, while the varying levels of coupled polarization are also considered. Simulation results reveal that the presence of the electrostatic interactions due to particle charging and polarization could drive more pronounced re-agglomeration after the collision, which effectively reduces the degree of agglomerate fragmentation. Moreover, when quantifying the collision outcomes using the fragmentation ratio, the influence of the electrostatic force is most significant at a moderate incident velocity. This is because, at such incident velocities, the impact is violent enough to break the agglomerate, but many ejected fragments are usually at low velocities and are attracted back by the long-range electrostatic force. Furthermore, the electrostatic force between same-sign particles even becomes attractive when particles are strongly polarized, leading to qualitative changes in particle dynamics. Finally, by comparing the collision outcomes under different incident velocities, the contact interactions are found to prevail when particles are still bounded in the agglomerate, while the electrostatic interaction becomes dominant after particles detach from each other.

Self-organization of agitated microspheres on various substrates

The vibration dynamics of relatively large granular grains is extensively treated in the literature, but comparable studies on the self-assembly of smaller agitated beads are lacking. In this work, we investigate how the particle properties and the properties of the underlying substrate surface affect the dynamics and self-organization of horizontally agitated monodisperse microspheres with diameters between 3 and 10 μm. Upon agitation, the agglomerated hydrophilic silica particles locally leave traces of particle monolayers as they move across the flat uncoated and fluorocarbon-coated silicon substrates. However, on the micromachined silicon tray with relatively large surface roughness, the agitated silica agglomerates form segregated bands reminiscent of earlier studies on granular suspensions or Faraday heaps. On the other hand, the less agglomerated hydrophobic polystyrene particles form densely occupied monolayer arrangements regardless of the underlying substrate. We explain the observations by considering the relevant adhesion and friction forces between particles and underlying substrates as well as those among the particles themselves. Interestingly, for both types of microspheres, large areas of the fluorocarbon-coated substrates are covered with densely occupied particle monolayers. By qualitatively examining the morphology of the self-organized particle monolayers using the Voronoi approach, it is understood that these monolayers are highly disordered, i.e., multiple symmetries coexist in the self-organized monolayers. However, more structured symmetries are identified in the monolayers of the agitated polystyrene microspheres on all the substrates, albeit not all precisely positioned on a hexagonal lattice. On the other hand, both the silica and polystyrene monolayers on the bare silicon substrates transition into less disordered structures as time progresses. Using Kelvin probe force microscopy measurements, we show that due to the tribocharging phenomenon, the formation of particle monolayers is promoted on the fluorocarbon surface, i.e., a local electrostatic attraction exists between the particle and the substrate.

Photo-Controlled Macroscopic Self-Assembly Based on Photo-Switchable Hetero-Complementary Quadruple Hydrogen Bonds

A photo-switchable hetero-complementary quadruple H-bonding array, which consists of an azobenzene-derived ureidopyrimidinone (UPy) module (Azo-UPy) and a nonphotoactive diamidonaphthyridine (DAN) derivative (Napy-1), is constructed based on a reversible photo-locking approach. Upon UV (390 nm)/Vis (460 nm) light irradiations, photo-switchable quadruple H-bonded dimerization between Azo-UPy and Napy-1 can be achieved with exhibiting 4.8×10⁴ -fold differences in binding strength (ON/OFF ratios). Furthermore, smart polymeric gels with unique photo-controlled macroscopic self-assembly behavior can be fabricated by introducing such quadruple H-bonding array as photo-regulable noncovalent interfacial connections.

Stimuli-responsive granular crystals assembled by dipolar and multipolar interactions

This work describes granular crystals held together by unusual, multipolar interactions and, under the application of an external bias, undergoing reversible structural transitions between closed and open forms. The system comprises two types of polymeric beads agitated on one or between two conductive plates and gradually acquiring charges by contact electrification. The charges thus developed induce a series of electrostatic images in the conductive supports and, in effect, the beads interact via dipolar or multipolar interactions, enabling the stabilization of non-electroneutral crystals. Furthermore, under an applied bias, the beads become polarized and their complex interactions (due to the series of image charges as well as series of image dipoles) result in open-pore crystals which return to compact forms upon bias removal. These effects are rationalized by analytical calculations, and the crystal structures observed in the experiments are reproduced by molecular dynamics simulations.

Mixed Triboelectric and Flexoelectric Charge Transfer at the Nanoscale

The triboelectric effect is a ubiquitous phenomenon in which the surfaces of two materials are easily charged during the contact-separation process. Despite the widespread consequences and applications, the charging mechanisms are not sufficiently understood. Here, the authors report that, in the presence of a strain gradient, the charge transfer is a result of competition between flexoelectricity and triboelectricity, which could enhance charge transfer during triboelectric measurements when the charge transfers of both effects are in the same direction. When they are in the opposite directions, the direction and amount of charge transfer could be modulated by the competition between flexoelectric and triboelectric effects, which leads to a distinctive phenomenon, that is, the charge transfer is reversed with varying forces. The subsequent results on the electrical power output signals from the triboelectrification support the proposed mechanism. Therefore, the present study emphasizes the key role of the flexoelectric effect through experimental approaches, and suggests that both the amount and direction of charge transfer can be modulated by manipulating the mixed triboelectric and flexoelectric effects. This finding may provide important information on the triboelectric effect and can be further extended to serve as a guideline for material selection during a nanopatterned device design.

Recent Progress in Development of Wearable Pressure Sensors Derived from Biological Materials

This review summarizes recent progress in the use of biological materials (biomaterials) in wearable pressure sensors. Biomaterials are abundant, sustainable, biocompatible, and biodegradable. Especially, many have sophisticated hierarchical structure and biological characteristics, which are attractive candidates for facile and ecologically-benign fabrication of wearable pressure sensors that are biocompatible, biodegradable, and highly sensitivity. The biomaterials and structures that use them in wearable pressure sensors that exploit sensing mechanisms such as piezoelectric, triboelectric, piezoresistive and capacitive effects are present. Finally, remaining impediments are discussed to use of biomaterials in wearable pressure sensors.

Fast dynamics and emergent topological defects in long-range interacting particle systems

Long-range interacting systems exhibit unusual physical properties not shared by systems with short-range interactions. Understanding the dynamical and statistical effects of long-range interactions yields insights into a host of physical systems in nature and industry. In this work, we investigate the classical microscopic dynamics of screened Coulomb interacting particles confined in the disk, and reveal the featured dynamics and emergent statistical regularities created by the long-range interaction. We highlight the long-range interaction driven fast single-particle and collective dynamics, and the emergent topological defect structure. This work suggests the rich physics arising from the interplay of long-range interaction, topology and dynamics.

Layer-Scale and Chip-Scale Transfer Techniques for Functional Devices and Systems: A Review

Hetero-integration of functional semiconductor layers and devices has received strong research interest from both academia and industry. While conventional techniques such as pick-and-place and wafer bonding can partially address this challenge, a variety of new layer transfer and chip-scale transfer technologies have been developed. In this review, we summarize such transfer techniques for heterogeneous integration of ultrathin semiconductor layers or chips to a receiving substrate for many applications, such as microdisplays and flexible electronics. We showed that a wide range of materials, devices, and systems with expanded functionalities and improved performance can be demonstrated by using these technologies. Finally, we give a detailed analysis of the advantages and disadvantages of these techniques, and discuss the future research directions of layer transfer and chip transfer techniques.

Quick self-assembly of bio-inspired multi-dimensional well-ordered structures induced by ultrasonic wave energy

Bottom-up self-assembly of components, inspired by hierarchically self-regulating aggregation of small subunits observed in nature, provides a strategy for constructing two- or three-dimensional intriguing biomimetic materials via the spontaneous combination of discrete building blocks. Herein, we report the methods of ultrasonic wave energy-assisted, fast, two- and three-dimensional mesoscale well-ordered self-assembly of microfabricated building blocks (100 μm in size). Mechanical vibration energy-driven self-assembly of microplatelets at the water-air interface of inverted water droplets is demonstrated, and the real-time formation process of the patterned structure is dynamically explored. 40 kHz ultrasonic wave is transferred into microplatelets suspended in a water environment to drive the self-assembly of predesigned well-ordered structures. Two-dimensional self-assembly of microplatelets inside the water phase with a large patterned area is achieved. Stable three-dimensional multi-layered self-assembled structures are quickly formed at the air-water interface. These demonstrations aim to open distinctive and effective ways for new two-dimensional surface coating technology with autonomous organization strategy, and three-dimensional complex hierarchical architectures built by the bottom-up method and commonly found in nature (such as nacre, bone or enamel, etc.).

Capturing the Moment of Emergence of Crystal Nucleus from Disorder

Crystallization is the process of atoms or molecules forming an organized solid via nucleation and growth. Being intrinsically stochastic, the research at an atomistic level has been a huge experimental challenge. We report herein in situ detection of a crystal nucleus forming during nucleation/growth of a NaCl nanocrystal, as video recorded in the interior of a vibrating conical carbon nanotube at 20-40 ms frame^-1 with localization precision of <0.1 nm. We saw NaCl units assembled to form a cluster fluctuating between featureless and semiordered states, which suddenly formed a crystal. Subsequent crystal growth at 298 K and shrinkage at 473 K took place also in a stochastic manner. Productive contributions of the graphitic surface and its mechanical vibration have been experimentally indicated.

Charging Organic Liquids by Static Charge

Aqueous liquids can be charged effectively by a number of methods for many important applications. Organic liquids, however, cannot be charged effectively by existing methods due to their low conductivities, especially the insulating nonpolar organic liquids; hence, there has not been any significant application developed based on charged organic liquids. This study describes an effective fundamental strategy for charging organic liquids, including nonpolar organic liquids: static charge is simply mixed into the liquid. Analyses suggested that the charged species are molecular ions that reside in the bulk of the liquid after charging. This method is simple and general, and the amount and polarity of charge can be flexibly tunable. The effectiveness of this method gives rise to opportunities for the development of novel applications. Charged organic droplets are manipulated for the first time by an electric field for controlling organic reactions. Particles with charge embedded in their bulk matrices are fabricated for the first time (i.e., via polymerizing the liquid monomers mixed with static charge). The charge in this novel class of bulk-charged particles is stable and permanent, especially when compared to the typical surface-charged particles. Simultaneous bulk-charged and bulk-magnetic particles are fabricated for the first time via simply mixing both the static charge and magnetic nanoparticles into the liquid monomers. These highly versatile particles are responsive to both electric and magnetic fields for practical applications.

Liquid-polymer triboelectricity: chemical mechanisms in the contact electrification process

Liquid-polymer contact electrification between sliding water drops and the surface of polytetrafluoroethylene (PTFE) was studied as a function of the pH and ionic strength of the drop as well as ambient relative humidity (RH). The PTFE surface was characterized by using SEM, water-contact-angle measurements, FTIR spectroscopy, XPS, and Raman spectroscopy. The charge acquired by the drops was calculated by detecting the transient voltage induced on a specifically designed capacitive sensor. It is shown that water drops become positively charged at pH > pH_zch (pH_zch being the zero charge point of the polymer) while they become negatively charged for pH < pH_zch. The addition of non-hydrolysable salts (NaCl or CaCl₂) to water decreases the electrical charge induced in the drop. The charge also decreases with increasing RH. These results suggest proton or hydroxyl transfer from the liquid to the hydrophobic polymer surface. A proposed thermodynamic model for the ion transfer process allows explaining the observed effects of RH, pH and ionic strength.

Spatial Segregation of Microspheres by Rubbing-Induced Triboelectrification on Patterned Surfaces

Particle (monolayer) assembly is essential to various scientific and industrial applications, such as the fabrication of photonic crystals, optical sensors, and surface coatings. Several methods, including rubbing, have been developed for this purpose. Here, we report on the serendipitous observation that microparticles preferentially partition onto the fluorocarbon-coated parts of patterned silicon and borosilicate glass wafers when rubbed with poly(dimethylsiloxane) slabs. To explore the extent of this effect, we varied the geometry of the pattern, the substrate material, the ambient humidity, and the material and size of the particles. Partitioning coefficients amounted up to a factor of 12 on silicon wafers and even ran in the 100s on borosilicate glass wafers at zero humidity. Using Kelvin probe force microscopy, the observations can be explained by triboelectrification, inducing a strong electrostatic attraction between the particles and the fluorocarbon zones, while the interaction with the noncoated zones is insignificant or even weakly repulsive.

The Relationship between Static Charge and Shape

The amount of charge of a material has always been regarded as a property (or state) of materials and can be measured precisely and specifically. This study describes for the first time a fundamental physical-chemical phenomenon in which the amount of charge of a material is actually a variable-it depends on the shape of the material. Materials are shown to have continuously variable and reversible ranges of charge states by changing their shapes. The phenomenon was general for different shapes, transformations, materials, atmospheric conditions, and methods of charging. The change in charge was probably due to a dynamic exchange of charge from the material to the surrounding atmosphere as the shape changed via the reversible ionization and deposition of air molecules. Similar changes in charge were observed for self-actuating materials that changed their shapes autonomously. This fundamental relationship between geometry and electrostatics via chemistry is important for the broad range of applications related to the charge of flexible materials.

Ionic solids from common colloids

From rock salt to nanoparticle superlattices, complex structure can emerge from simple building blocks that attract each other through Coulombic forces^1-4. On the micrometre scale, however, colloids in water defy the intuitively simple idea of forming crystals from oppositely charged partners, instead forming non-equilibrium structures such as clusters and gels^5-7. Although various systems have been engineered to grow binary crystals^8-11, native surface charge in aqueous conditions has not been used to assemble crystalline materials. Here we form ionic colloidal crystals in water through an approach that we refer to as polymer-attenuated Coulombic self-assembly. The key to crystallization is the use of a neutral polymer to keep particles separated by well defined distances, allowing us to tune the attractive overlap of electrical double layers, directing particles to disperse, crystallize or become permanently fixed on demand. The nucleation and growth of macroscopic single crystals is demonstrated by using the Debye screening length to fine-tune assembly. Using a variety of colloidal particles and commercial polymers, ionic colloidal crystals isostructural to caesium chloride, sodium chloride, aluminium diboride and K₄C₆₀ are selected according to particle size ratios. Once fixed by simply diluting out solution salts, crystals are pulled out of the water for further manipulation, demonstrating an accurate translation from solution-phase assembly to dried solid structures. In contrast to other assembly approaches, in which particles must be carefully engineered to encode binding information^12-18, polymer-attenuated Coulombic self-assembly enables conventional colloids to be used as model colloidal ions, primed for crystallization.

Polyethylene and Polyvinyl Chloride-Blended Polystyrene Nanofibrous Sorbents and Their Application in the Removal of Various Oil Spills

Polymers provide a wide range of properties, and these properties can be greatly enhanced and modified through polymer blending. Polymer blending combines the properties and advantages of their original polymers. This paper showcases hydrophobic polymers prepared through polymer blending; these blends are characterized and evaluated for their efficiency in the removal of crude oil spills from aqueous media. The application of these blends holds a great deal of importance in preserving the environment and the recovery of lost oil in spills. The blends are produced using polystyrene (PS) as the matrix polymer and individually blending poly(vinyl chloride) (PVC) and polyethylene (PE) with the PS consisting of proportions of 5–20 wt.% each. The blends are then electrospun into bead-free microfibers with interconnected porosities as shown by their respective scanned electron micrographs. All fibrous sorbents showed a high affinity towards the removal of crude oil, motor oil, and diesel spills. The highly viscous motor spill showed a different pattern of sorption onto fibers than that of crude oil and diesel spills. Upon comparing all the studied electrospun fibers to commercially available polypropylene fibrous sorbents, results show that the sorption efficiency of the electrospun fibers is superior. Most notably, both PS-PE5 and PS-PVC5 fibers showed to be highly more effective than commercially available polypropylene (PP) sorbents towards all types of oil spills.

Nonconductive Noncharging Composites: Tunable and Stretchable Materials for Adaptive Prevention of Charging by Contact Electrification

Static charge generated by contact electrification can cause a wide range of undesirable consequences in our lives and in industry (e.g., adhesion of particles on surfaces, damage to electronics, and explosions). It has, however, been challenging to develop methods to prevent charging due to the vast types of materials that charge easily by contact electrification and the frequent changes in process and environmental conditions. The most common method is to use conductive materials for dissipating charge away; however, it is ineffective for many circumstances. Here, we propose a general and effective materials framework that involves a two-level consideration for preparing noncharging materials: (1) the variation of the proportion of a two-material composite and (2) the extent of stretching the composite material. This materials strategy is achieved by infusing particles within a stretchable bulk material. Importantly, the preparation of the noncharging surface for (1) is based on a novel fundamental mechanism that involves combining an appropriate amount of a material (e.g., the particles) that tends to charge positively with another material (e.g., the bulk material) that tends to charge negatively. This mechanism does not rely on conductivity; both the contacting materials naturally prevent the generation of static charge even when only nonconductive materials are involved. When the composite material is stretchable, the change in proportion of the surface coverage of the particles allows the charging response to be changed. Therefore, the variation in composition and stretching provide a wide two-dimensional parameter space for achieving noncharging response for the vast range of contacting materials that are used in industry and our lives. In addition, stretchability allows the composite material to flexibly adapt to changes in process and environmental conditions. This stretchable composite material was also demonstrated to be capable of preventing the adhesion of particles and separating particles of different materials.

Global Alignment of Solution-Based Single-Wall Carbon Nanotube Films via Machine-Vision Controlled Filtration

Over the past decade, substantial progress has been made in the chemical control (chiral enrichment, length sorting, handedness selectivity, and filling substance) of single-wall carbon nanotubes (SWCNTs). Recently, it was shown that large, horizontally aligned films can be created out of postprocessed SWCNT solutions. Here, we use machine-vision automation and parallelization to simultaneously produce globally aligned SWCNT films using pressure-driven filtration. Feedback control enables filtration to occur with a constant flow rate that not only improves the nematic ordering of the SWCNT films but also provides the ability to align a wide range of SWCNT types and on a variety of nanoporous membranes using the same filtration parameters. Using polarized optical spectroscopic techniques, we show that under standard implementation, meniscus combing produces a two-dimensional radial SWCNT alignment on one side of the film. After we flatten the meniscus through silanization, spatially resolved nematicity maps on both sides of the SWCNT film reveal global alignment across the entire structure. From experiments changing ionic strength and membrane charging, we provide evidence that the SWCNT alignment mechanism stems from an interplay of intertube interactions and ordered membrane charging. This work opens up the possibility of creating globally aligned SWCNT film structures for a new generation of nanotube electronics and optical control elements.

Granular ionic crystals in a small nutshell

Ordered two-dimensional arrangements of triboelectrically oppositely charged granular particles have been reported several times, but observations of bulk ordered binary granular particle packings are singular. We attribute this suppression of triboelectrically induced order to the concurrent behaviour of granular particles to pack densest due to gravity. We show that triboelectrically induced order robustly emerges in a container that does not allow for crystallization into a dense packing under gravity. It turns out that the triboelectrically ordered structure follows Pauling's predictions for atomic ionic crystals in many aspects, but also exhibits systematic deviations. We discuss how the emergence of order in an incommensurate container, the deviations from Pauling's predictions and the gravitational potential energy of the particles are connected.

Quantifying the triboelectric series

Triboelectrification is a well-known phenomenon that commonly occurs in nature and in our lives at any time and any place. Although each and every material exhibits triboelectrification, its quantification has not been standardized. A triboelectric series has been qualitatively ranked with regards to triboelectric polarization. Here, we introduce a universal standard method to quantify the triboelectric series for a wide range of polymers, establishing quantitative triboelectrification as a fundamental materials property. By measuring the tested materials with a liquid metal in an environment under well-defined conditions, the proposed method standardizes the experimental set up for uniformly quantifying the surface triboelectrification of general materials. The normalized triboelectric charge density is derived to reveal the intrinsic character of polymers for gaining or losing electrons. This quantitative triboelectric series may serve as a textbook standard for implementing the application of triboelectrification for energy harvesting and self-powered sensing.

Triboelectric charging is a well-known phenomenon, but triboelectric polarization has only been ranked qualitatively. Here the authors develop a quantified triboelectric series for a wide range of polymers by measuring triboelectric charge density with respect to a liquid metal at well-defined conditions.

Control of triboelectric charges on common polymers by photoexcitation of organic dyes

Triboelectric charging of insulators, also known as contact charging in which electrical charges develop on surfaces upon contact, is a significant problem that is especially critical for various industries such as polymers, pharmaceuticals, electronics, and space. Several methods of tribocharge mitigation exist in practice; however, none can reach the practicality of using light in the process. Here we show a light-controlled manipulation of triboelectric charges on common polymers, in which the tribocharges are mitigated upon illumination with appropriate wavelengths of light in presence of a mediator organic dye. Our method provides spatial and temporal control of mitigation of static charges on common polymer surfaces by a mechanism that involves photoexcitation of organic dyes, which also allows additional control using wavelength. This control over charge mitigation provides a way to manipulate macroscopic objects by tribocharging followed by light-controlled discharging.

Contact charging of insulators is a significant problem for various industries, such as plastics, electronics, and space. Here the authors gain spatial and temporal control of discharge of triboelectrically charged polymers upon illumination of a set of common organic dyes.

Linear array of cesium atoms assisted by uracil molecules on Au(111)

A series of metal–organic U + Cs structures are achieved in which the Cs cations tend to form linear arrays.

Controlling Surface Charge Generated by Contact Electrification: Strategies and Applications

Contact electrification is the phenomenon in which charge is generated on the surfaces of materials after they come into contact. The surface charge generated has traditionally been known to cause a vast range of undesirable consequences in our lives and in industry; on the other hand, it can also give rise to many types of useful applications. In addition, there has been a lot of interest in recent years for fabricating devices and materials based on regulating a desired amount of surface charge. It is thus important to understand the general strategies for increasing, decreasing, or controlling the surface charge generated by contact electrification. Herein, the fundamental mechanisms for influencing the amount of charge generated, the methods used for implementing these mechanisms, and some of the recent interesting applications that require regulating the amount of surface charge generated by contact electrification, are briefly summarized.

A Self-Powered Six-Axis Tactile Sensor by Using Triboelectric Mechanism

Triboelectric nanogenerators (TENGs) are highly promising because they can scavenge energy from their working environment to sustainably power wearable/mobile electronics. In this work, we propose a novel and straightforward strategy for six-axis force detection and object controlling by using a self-powered sensor based on TENG. The self-powered sensor can be used in diversified tactile sensing and energy saving applications, which has been demonstrated to be able to detect normal force in the range of 0⁻18 N. Using the vector properties of external force, six-axis directions in three-dimensional (3D) space is detected. Additionally, it is fabricated with environmental friendly materials, i.e., galinstan and polydimethylsiloxane (PDMS), promoting its applications in more diversified situations. Because of the available and high output voltage as well as the low internal impedance, the self-powered sensor is readily compatible with commercial signal processing and management circuits. The device presented in this work shows robust structure and stable output performance, enabling itself as an ideal human machine interface in self-powered, batteryless, and electric energy saving applications.

Dynamic simulations of many-body electrostatic self-assembly

Two experimental studies relating to electrostatic self-assembly have been the subject of dynamic computer simulations, where the consequences of changing the charge and the dielectric constant of the materials concerned have been explored. One series of calculations relates to experiments on the assembly of polymer particles that have been subjected to tribocharging and the simulations successfully reproduce many of the observed patterns of behaviour. A second study explores events observed following collisions between single particles and small clusters composed of charged particles derived from a metal oxide composite. As before, observations recorded during the course of the experiments are reproduced by the calculations. One study in particular reveals how particle polarizability can influence the assembly process.This article is part of the theme issue 'Modern theoretical chemistry'.

From dynamic self-assembly to networked chemical systems

Although dynamic self-assembly, DySA, is a relatively new area of research, the past decade has brought numerous demonstrations of how various types of components - on scales from (macro)molecular to macroscopic - can be arranged into ordered structures thriving in non-equilibrium, steady states. At the same time, none of these dynamic assemblies has so far proven practically relevant, prompting questions about the field's prospects and ultimate objectives. The main thesis of this Review is that formation of dynamic assemblies cannot be an end in itself - instead, we should think more ambitiously of using such assemblies as control elements (reconfigurable catalysts, nanomachines, etc.) of larger, networked systems directing sequences of chemical reactions or assembly tasks. Such networked systems would be inspired by biology but intended to operate in environments and conditions incompatible with living matter (e.g., in organic solvents, elevated temperatures, etc.). To realize this vision, we need to start considering not only the interactions mediating dynamic self-assembly of individual components, but also how components of different types could coexist and communicate within larger, multicomponent ensembles. Along these lines, the review starts with the discussion of the conceptual foundations of self-assembly in equilibrium and non-equilibrium regimes. It discusses key examples of interactions and phenomena that can provide the basis for various DySA modalities (e.g., those driven by light, magnetic fields, flows, etc.). It then focuses on the recent examples where organization of components in steady states is coupled to other processes taking place in the system (catalysis, formation of dynamic supramolecular materials, control of chirality, etc.). With these examples of functional DySA, we then look forward and consider conditions that must be fulfilled to allow components of multiple types to coexist, function, and communicate with one another within the networked DySA systems of the future. As the closing examples show, such systems are already appearing heralding new opportunities - and, to be sure, new challenges - for DySA research.

Triboelectrification-Induced Self-Assembly of Macro-Sized Polymer Beads on a Nanostructured Surface for Self-Powered Patterning

Here we report an electrostatic-templated self-assembly (ETSA) method for arbitrarily patterning millimeter-sized polymer beads on a nanostructured surface without using an extra voltage source. A patterned electrode underneath an electrification layer generates "potential wells" of the corresponding pattern at predefined window sites, which capture and anchor the beads within the window sites by electrostatic force. Analytical calculation is combined with numerical modeling to derive the electrostatic force acting on the beads, which is in great agreement with experimentally measured values. The generated pattern is solely determined by the predefined underlying electrode, making it arbitrarily switchable by using different electrode patterns. By transferring the assembled beads into an elastomer matrix, possible applications of the ETSA in fabricating optical and flexible displays are demonstrated.

A Sliding-Mode Triboelectric Nanogenerator with Chemical Group Grated Structure by Shadow Mask Reactive Ion Etching

The sliding-mode triboelectric nanogenerator (S-TENG) with grated structure has important applications in energy harvest and active sensors; however its concavo-convex structure leads to large frictional resistance and abrasion. Here, we developed a S-TENG with a chemical group grated structure (S-TENG-CGG), in which the triboelectric layer's triboelectric potential has a positive-negative alternating charged structure. The triboelectric layer of the S-TENG-CGG was fabricated through a reactive ion etching process with a metal shadow mask with grated structure. In the etched region, the nylon film, originally positively charged as in friction with stainless steel, gained opposite triboelectric potential and became negatively charged because of the change of surface functional groups. The output signals of the S-TENG-CGG are alternating and the frequency is determined by both the segment numbers and the moving speed. The applications of the S-TENG-CGG in the charging capacitor and driving calculator are demonstrated. In the S-TENG-CGG, since there is no concavo-convex structure, the frictional resistance and abrasion are largely reduced, which enhances its performances in better stability and longer working time.

Transparent and Flexible Triboelectric Sensing Array for Touch Security Applications

Tactile sensors with large-scale array and high sensitivity is essential for human-machine interaction, smart wearable devices, and mobile networks. Here, a transparent and flexible triboelectric sensing array (TSA) with fingertip-sized pixels is demonstrated by integrating ITO electrodes, FEP film, and signal transmission circuits on an undivided palm-sized polyethylene terephthalate substrate. The sensing pixels can be triggered by the corresponding external contact to induce the electrostatic potential in the transparent electrodes without power consumption, which is individually recognized by the sensor. By testing the response of the pixels, the electrical characterization is systematically investigated. The proposed TSA exhibits excellent durability, independence, and synchronicity, which is able to realize real-time touch sensing, spatial mapping, and motion monitoring. The integrated TSA has great potential for an active tactile system, human-machine interface, wearable electronics, private communication, and advanced security identification.

Influence of humidity on tribo-electric charging and segregation in shaken granular media

We study the effect of humidity on the charge accumulation of polymer granulates shaken vertically in a stainless steel container. This setup allows us to control the humidity level from 5% to 100%RH while performing automated charge measurements in a Faraday cup directly connected to the shaking container. We find that samples of approximately 2000 polymer spheres become highly charged at low humidity levels (<30%RH), but acquire almost no charge for humidity levels above 80%RH. The transition between these two regimes does depend on the material, as does the sign of the charge. For the latter we find a correlation with the contact angle of the polymer with only very hydrophilic particles attaining positive charges. We show that this humidity dependence of tribo-charging can be used to control segregation in shaken binary mixtures.

Bipolar charge transfer induced by water: experimental and first-principles studies

Water plays an important role in the bipolar charge transfer generated by triboelectrification.

Contact Electrification of Individual Dielectric Microparticles Measured by Optical Tweezers in Air

We measure charging of single dielectric microparticles after interaction with a glass substrate using optical tweezers to control the particle, measure its charge with a sensitivity of a few electrons, and precisely contact the particle with the substrate. Polystyrene (PS) microparticles adhered to the substrate can be selected based on size, shape, or optical properties and repeatedly loaded into the optical trap using a piezoelectric (PZT) transducer. Separation from the substrate leads to charge transfer through contact electrification. The charge on the trapped microparticles is measured from the response of the particle motion to a step excitation of a uniform electric field. The particle is then placed onto a target location of the substrate in a controlled manner. Thus, the triboelectric charging profile of the selected PS microparticle can be measured and controlled through repeated cycles of trap loading followed by charge measurement. Reversible optical trap loading and manipulation of the selected particle leads to new capabilities to study and control successive and small changes in surface interactions.