Ionic electrets: electrostatic charging of surfaces by transferring mobile ions upon contact

Thermodynamic driving forces in contact electrification between polymeric materials

Contact electrification, or contact charging, refers to the process of static charge accumulation after rubbing, or even simple touching, of two materials. Despite its relevance in static electricity, various natural phenomena, and numerous technologies, contact charging remains poorly understood. For insulating materials, even the species of charge carrier may be unknown, and the direction of charge-transfer lacks firm molecular-level explanation. Here, we use all-atom molecular dynamics simulations to investigate whether thermodynamics can explain contact charging between insulating polymers. Based on prior work suggesting that water-ions, such as hydronium and hydroxide ions, are potential charge carriers, we predict preferred directions of charge-transfer between polymer surfaces according to the free energy of water-ions within water droplets on such surfaces. Broad agreement between our predictions and experimental triboelectric series indicate that thermodynamically driven ion-transfer likely influences contact charging of polymers. Furthermore, simulation analyses reveal how specific interactions of water and water-ions proximate to the polymer-water interface explain observed trends. This study establishes relevance of thermodynamic driving forces in contact charging of insulators with new evidence informed by molecular-level interactions. These insights have direct implications for future mechanistic studies and applications of contact charging involving polymeric materials.

Molecular Adsorption and Physicochemical Properties at Liquid/Liquid Nanoemulsion Soft Interfaces: Effect of Charge and Hydrophobicity

Contrary to the popular adage, "Oil and water do not mix", evidence of mixtures comprising the two "immiscible" liquids is universal. In the presence of an emulsifier, oil and water mix to form a colloidal suspension known as emulsion. Their utility in many areas such as food chemistry, biomedical health sectors, catalysis, and the petroleum industry is well recognized. While their application in our society is pervasive, tantalizing fundamental questions regarding the chemistry that takes place at the oil/water soft interface still linger. For instance, do organic compounds show proclivity for this molecularly thin boundary and, if so, what forces, hydrophobic or pure electrostatic among others, drive the molecular interactions? The focus of this Article is on molecular adsorption at the interface of oil-in-water (O/W) nanoemulsion (NE) droplets. The effect of the interfacial surfactant charge (positive, negative, zwitterionic, and neutral) on the affinity of aromatic organic compounds on the O/W NEs has been studied. Using a second harmonic generation (SHG), a nonlinear light scattering technique, we have explored the adsorption equilibrium of charged and neutral organic dyes. By variation of the surfactant functional group and thereby the interfacial charge properties, the source of the adsorption interaction, if any, has been deduced. The population of surfactants containing a charged functional group at the O/W interface is found to be sparse, yet adsorption at some of these interfaces has been observed. A purely electrostatic Coulomb interaction plays a key role, but the presence of a charged interface does not necessitate molecular adsorption. Hydrophobic interactions are not a major driving force of adsorption for the SHG dyes studied. However, a possible pi-interaction is likely in explaining the accumulation of neutral aromatic compounds at the O/W NE interface. These intricate adsorption features are discussed in the context of NE interfacial charge properties and their stability upon molecular adsorption.

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.

Uncertainty and Irreproducibility of Triboelectricity Based on Interface Mechanochemistry

Triboelectrification mechanism is still not understood, despite centuries of investigations. Here, we propose a model showing that mechanochemistry is key to elucidate triboelectrification fundamental properties. Studying contact between gold and silicate glasses, we observe that the experimental triboelectric output is subject to large variations and polarity inversions. First principles analysis shows that electronic transfer is activated by mechanochemistry and the tribopolarity is determined by the termination exposed to contact, depending on the material composition, which can result in different charging at the macroscale. The electron transfer mechanism is driven by the interface barrier dynamics, regulated by mechanical forces. The model provides a unified framework to explain several experimental observations, including the systematic variations in the triboelectric output and the mixed positive-negative "mosaic" charging patterns, and paves the way to the theoretical prediction of the triboelectric properties.

Recent Advances in Triboelectric Nanogenerators: From Technological Progress to Commercial Applications

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.

Effects of initial charge on triboelectrification of plastics

Plastic recycling is one of the most economical and environment-friendly solutions for effective plastic waste management. Triboelectric separation is one of beneficial methods to accomplish this. A method and device capable of analyzing the triboelectrification of materials with specific initial charges are proposed in this study. The process of triboelectrification is experimentally analyzed for various initial-charge conditions using the proposed method and device. The triboelectrification process can be divided into two groups depending on the initial-charge conditions. For the specific initial conditions, termed Group 2 in this work, it is observed that the initial charge of one material is first released into the control volume, following which the two materials exchange charges, unlike in the conventional triboelectrification process. This study is expected to provide valuable insights into triboelectrification analysis, thereby advancing the multistage plastic-separation processes.

Understanding Contact Electrification at Water/Polymer Interface

Contact electrification (CE) involves a complex interplay of physical interactions in realistic material systems. For this reason, scientific consensus on the qualitative and quantitative importance of different physical mechanisms on CE remains a formidable task. The CE mechanism at a water/polymer interface is a crucial challenge owing to the poor understanding of charge transfer at the atomic level. First-principle density functional theory (DFT), used in the present work, proposes a new paradigm to address CE. Our results indicate that CE follows the same trend as the gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of polymers. Electron transfer occurs at the outmost atomic layer of the water/polymer interface and is closely linked to the functional groups and atom locations. When the polymer chains are parallel to the water layer, most electrons are transferred; conversely, if they are perpendicular to each other, the transfer of charges can be ignored. We demonstrate that a decrease in the interface distance between water and the polymer chains leads to CE in quantitative agreement with the electron cloud overlap model. We finally use DFT calculations to predict the properties of CE materials and their potential for triboelectric nanogenerator energy harvesting devices.

Harvesting of flow current through implanted hydrophobic PTFE surface within silicone-pipe as liquid nanogenerator

Harvesting of flow current through implanted hydrophobic surface within silicone pipe as liquid nanogenerators where Tap water (TW), and DI water (DIw) as liquid reservoirs to successfully convert induced mechanical energy into electrical energy. Here, we used a commercial PTFE film for the generation of a hydrophobic surface as a source of mechanical energy. The surface roughness of the hydrophobic surface is confirmed using atomic force microscopy, and contact angle analyses. The generation of power through the interaction of TW and DI with inbuilt PTFE in silicone tube is described. The higher output voltage (Voc), and short circuit currents (Isc) were attained through an interaction of TW and DIw with N-PTFE. The lower Voc, and Isc's were produced when DI water interacts with N-PTFE electrode, whereas TW produced higher Voc and Isc's, respectively, due to a lack of free mobile ions in DIw than TW. The TW-Sh-TENG and DIw-Sh-TENG are produced the maximum peak-to-peak Voc, and Isc of 29.5 V and 17.4 V and 3.7 μA, and 2.9 μA, respectively. Significant power output enhancement of ~ 300% from TW-Sh-TENG from DIw-N-TENG due to the formation of higher surface roughness and lead to the slipping of water droplets by super-hydrophobicity.

Violation of triboelectric charge conservation on colliding particles

In microgravity experiments, we quantified the net charge on systems of two identical, 434-μm-diameter glass spheres before and after a collision. We find that charge conservation is significantly violated. Independent of the sign of the total charge, the systems regularly lose some of their net charges, that is, they slightly discharge. This implies that positive as well as negative charge carriers become entrained into the surrounding atmosphere during a collision.

Direct Measurement of Water-Assisted Ion Desorption and Solvation on Isolated Carbon Nanotubes

We have investigated the change in mean residence time of gaseous ions adsorbed on the surface of suspended carbon nanotube field-effect transistors (CNT-FETs) with and without native surface water layers that exists in atmospheric conditions. Devices were characterized electrically before and after dehydration by thermal, dry gas, and vacuum desiccation and in each scenario were found to have substantially higher mean ion residence times. It is proposed that water molecules native to the CNT surface in ambient conditions provide a reduction pathway for incoming gaseous ions, yielding hydronium ions (H₃O⁺). This is supported by the appearance of frequent clustered readsorption events in the presence of surface water, caused by the rapid hopping of H⁺ between the device surface and the lowest water layer, which are not present in data collected from desiccated devices. After desiccation of the device, a thermal trial was conducted to determine the adsorption energy of N₂⁺ ions on the CNT surface. This work has profound implications for our understanding of wetting in one-dimensional systems and the chemistry of ion chemisorption and solvation on the surfaces of materials in general.

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.

Recharging and rejuvenation of decontaminated N95 masks

N95 respirators comprise a critical part of the personal protective equipment used by frontline health-care workers and are typically meant for one-time usage. However, the recent COVID-19 pandemic has resulted in a serious shortage of these masks leading to a worldwide effort to develop decontamination and re-use procedures. A major factor contributing to the filtration efficiency of N95 masks is the presence of an intermediate layer of charged polypropylene electret fibers that trap particles through electrostatic or electrophoretic effects. This charge can degrade when the mask is used. Moreover, simple decontamination procedures (e.g., use of alcohol) can degrade any remaining charge from the polypropylene, thus severely impacting the filtration efficiency post-decontamination. In this report, we summarize our results on the development of a simple laboratory setup allowing measurement of charge and filtration efficiency in N95 masks. In particular, we propose and show that it is possible to recharge the masks post-decontamination and recover filtration efficiency.

Effects of Surface Functional Groups on Electron Transfer at Liquid-Solid Interfacial Contact Electrification

Contact electrification (CE) at interfaces is sensitive to the functional groups on the solid surface, but its mechanism is poorly understood, especially for the liquid-solid cases. A core controversy is the identity of the charge carriers (electrons or/and ions) in the CE between liquids and solids. Here, the CE between SiO₂ surfaces with different functional groups and different liquids, including DI water and organic solutions, is systematically studied, and the contribution of electron transfer is distinguished from that of ion transfer according to the charge decay behavior at surfaces at specific temperature, because electron release follows the thermionic emission theory. It is revealed that electron transfer plays an important role in the CE between liquids and functional group modified SiO₂. Moreover, the electron transfer between the DI water and the SiO₂ is found highly related to the electron affinity of the functional groups on the SiO₂ surfaces, while the electron transfer between organic solutions and the SiO₂ is independent of the functional groups, due to the limited ability of organic solutions to donate or gain electrons. An energy band model for the electron transfer between liquids and solids is further proposed, in which the effects of functional groups are considered. The discoveries in this work support the "two-step" model about the formation of an electric double-layer (Wang model), in which the electron transfer occurs first when the liquids contact the solids for the very first time.

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.

Sublimation Electrification of Organic Compounds

The electrification of crystalline deposits of organic compounds under high vacuum was measured and quantified. A group of compounds that produce multiply charged ions by matrix-assisted ionization were deposited on a metal plate, and the current was amplified and recorded; the total charge was obtained by integration of the current. Signals of several hundred picoamperes were obtained within 10 s of the application of high vacuum and persisted for several minutes as the compounds sublimed. The magnitude and sign of the charge were matrix dependent and were affected by the presence of organic or mineral acid in the crystalline deposit, as well as by the application of an electric field. The observations are interpreted as surface electrification caused by the emission of small charged matrix particles during sublimation, with ionic charge carriers comprising protons and hydroxide ions.

Advances in the study of piezoelectric polymers

The literature review based on the works published over the last decade concerns the progress in research on innovative piezoelectric materials with current or potential practical applications. At the beginning, the nature of piezoelectric phenomenon is clarified. The main emphasis is put on presentation of polymers, biopolymers and polymer composites as well as hybrid materials with piezoelectric properties. Moreover, carbon nanomaterials are also included. These materials have recently become an intensively developing field, as evidenced by numerous scientific publications. Furthermore, the recently reported main methods of characterizations and selected examples of modern applications of piezoelectric materials in various fields (electronics, industry, medicine) have been discussed.

The bibliography includes 217 references.

Minimizing friction, wear, and energy losses by eliminating contact charging

One-fourth of the global energy losses result from friction and wear. Although friction and tribocharging were presented to be mutually related, reduction of friction and wear by eliminating tribocharges on common polymers, and decrease of power losses in devices with polymer parts were not shown to date. Here, we demonstrate that for common polymers, friction-which is strongly related to surface charge density-can be notably reduced by various methods of tribocharge mitigation, namely, corona discharging, solvent treatment, or placing a grounded conductor on the backside of one of the shearing materials. In our simple demonstrations, we found that by preventing tribocharge accumulation, a remarkable two-thirds of power loss during operation of simple mechanical devices with common polymers and plastic parts can be saved and wear can be reduced by a factor of 10. These demonstrations indicate important practical ramifications in mechanical systems with insulating parts.

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.

Double origin of stochastic granular tribocharging

The mechanisms underlying triboelectric charging have a stochastic nature. We investigate how this randomness affects the distributions of charges generated on granular particles during either a single or many collisions. The charge distributions we find in our experiments are more heavy-tailed than normal distributions with an exponential decay of the probability, they are asymmetric, and exhibit charges of both signs. Moreover, we find a linear correlation between the width and mean of these distributions. We rationalize these findings with a model for triboelectric charging which combines stochastic charge separation during contact and stochastic charge recombination after separation of the surfaces. Our results further imply that subsequent charging events are not statistically independent.

On the Electron-Transfer Mechanism in the Contact-Electrification Effect

A long debate on the charge identity and the associated mechanisms occurring in contact-electrification (CE) (or triboelectrification) has persisted for many decades, while a conclusive model has not yet been reached for explaining this phenomenon known for more than 2600 years! Here, a new method is reported to quantitatively investigate real-time charge transfer in CE via triboelectric nanogenerator as a function of temperature, which reveals that electron transfer is the dominant process for CE between two inorganic solids. A study on the surface charge density evolution with time at various high temperatures is consistent with the electron thermionic emission theory for triboelectric pairs composed of Ti-SiO₂ and Ti-Al₂ O₃ . Moreover, it is found that a potential barrier exists at the surface that prevents the charges generated by CE from flowing back to the solid where they are escaping from the surface after the contacting. This pinpoints the main reason why the charges generated in CE are readily retained by the material as electrostatic charges for hours at room temperature. Furthermore, an electron-cloud-potential-well model is proposed based on the electron-emission-dominatedcharge-transfer mechanism, which can be generally applied to explain all types of CE in conventional materials.

Reversible and Continuously Tunable Control of Charge of Close Surfaces

Surfaces of almost all types of materials are often charged easily by contact electrification or deposition of ions; hence, surface charge is ubiquitous and has a vast range of influences in our lives and in industry. Since the 19th century, scientists have been measuring the charge of multiple materials collectively. The common expectation is that the total charge of multiple materials is equal to the sum of the charges of the individual materials. This study describes a previously unreported phenomenon in which the total charge of two insulating surfaces decreases when the surfaces are brought close to each other. The charge varies continuously and reversibly depending on the distance of separation between the surfaces. Experimental results derived from analyzing the movement of charge suggest that the changes are due to a rapid exchange of charge between the surfaces and their surrounding air. This change can be used to control the surface charge of the materials flexibly and reversibly.

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.

Control of Triboelectrification by Engineering Surface Dipole and Surface Electronic State

Although triboelectrification is a well-known phenomenon, fundamental understanding of its principle on a material surface has not been studied systematically. Here, we demonstrated that the surface potential, especially the surface dipoles and surface electronic states, governed the triboelectrification by controlling the surface with various electron-donating and -withdrawing functional groups. The functional groups critically affected the surface dipoles and surface electronic states followed by controlling the amount of and even the polarity of triboelectric charges. As a result, only one monolayer with a thickness of less than 1 nm significantly changed the conventional triboelectric series. First-principles simulations confirmed the atomistic origins of triboelectric charges and helped elucidate the triboelectrification mechanism. The simulation also revealed for the first time where charges are retained after triboelectrification. This study provides new insights to understand triboelectrification.

Effects of Ambient Air and Temperature on Ionic Gel Gated Single-Walled Carbon Nanotube Thin-Film Transistor and Circuits

Single-walled carbon nanotube thin-film transistor (SWCNT TFT) and circuits were fabricated by fully inkjet printing gold nanoparticles as source/drain electrodes, semiconducting SWCNT thin films as channel materials, PS-PMMA-PS/EMIM TFSI composite gel as gate dielectrics, and PEDOT/PSS as gate electrodes. The ionic gel gated SWCNT TFT shows reversible conversion from p-type transistor behavior in air to ambipolar features under vacuum due to reversible oxygen doping in semiconducting SWCNT thin films. The threshold voltages of ionic gel gated SWCNT TFT and inverters are largely shifted to the low value (0.5 V for p-region and 1.0 V for n-region) by vacuum annealing at 140 °C to exhausively remove water that is incorporated in the ionic gel as floating gates. The vacuum annealed ionic gel gated SWCNT TFT shows linear temperature dependent transconductances and threshold voltages for both p- and n-regions. The strong temperature dependent transconductances (0.08 μS/K for p-region, 0.4 μS/K for n-region) indicate their potential application in thermal sensors. In the other hand, the weak temperature dependent threshold voltages (-1.5 mV/K for p-region, -1.1 mV/K for n-region) reflect their excellent thermal stability.

Charging of multiple interacting particles by contact electrification

Many processes involve the movement of a disordered collection of small particles (e.g., powders, grain, dust, and granular foods). These particles move chaotically, interact randomly among themselves, and gain electrical charge by contact electrification. Understanding the mechanisms of contact electrification of multiple interacting particles has been challenging, in part due to the complex movement and interactions of the particles. To examine the processes contributing to contact electrification at the level of single particles, a system was constructed in which an array of millimeter-sized polymeric beads of different materials were agitated on a dish. The dish was filled almost completely with beads, such that beads did not exchange positions. At the same time, during agitation, there was sufficient space for collisions with neighboring beads. The charge of the beads was measured individually after agitation. Results of systematic variations in the organization and composition of the interacting beads showed that three mechanisms determined the steady-state charge of the beads: (i) contact electrification (charging of beads of different materials), (ii) contact de-electrification (discharging of beads of the same charge polarity to the atmosphere), and (iii) a long-range influence across beads not in contact with one another (occurring, plausibly, by diffusion of charge from a bead with a higher charge to a bead with a lower charge of the same polarity).

Bipolar tribocharging signal during friction force fluctuations at metal-insulator interfaces

Friction and triboelectrification of materials show a strong correlation during sliding contacts. Friction force fluctuations are always accompanied by two tribocharging events at metal-insulator [e.g., polytetrafluoroethylene (PTFE)] interfaces: injection of charged species from the metal into PTFE followed by the flow of charges from PTFE to the metal surface. Adhesion maps that were obtained by atomic force microscopy (AFM) show that the region of contact increases the pull-off force from 10 to 150 nN, reflecting on a resilient electrostatic adhesion between PTFE and the metallic surface. The reported results suggest that friction and triboelectrification have a common origin that must be associated with the occurrence of strong electrostatic interactions at the interface.

Reversible photochemical tuning of net charge separation from contact electrification

This paper describes the photochemical tuning of net charge that develops from contact electrification. The net charge separation and/or rate of charging of photochromic spiropyran (SP)-containing polymer films can be controlled by irradiation with UV light. The isomerization of the SP form to the more hydrophilic merocyanine (MC) by irradiation with UV light always leads to films that charge more positively than before irradiation. The incorporation of a halogenated comonomer into ring-opening metathesis polymerization (ROMP) yields films that charged negatively before irradiation so that a change in sign of the net charge separation of the film could be observed upon irradiation. An important advantage to our photochromic system is that light can be used to tune the charging behavior of the material, with spatial or temporal control of irradiation, leading to films that resist charging. Furthermore, the observed trends can be reversed by the exposure of the SP-containing polymer films to heat or visible light.

Friction, tribochemistry and triboelectricity: recent progress and perspectives

Mechanochemical reactions during polymer friction or contact produce ionic fragments distributed on positive and negative domains at both surfaces.

Rectification of ion current in nanopipettes by external substrates

We describe ion distribution and the current-voltage (i-V) response of nanopipettes at different probe-to-substrate distances (Dps) as simulated by finite-element methods. Results suggest electrostatic interactions between a charged substrate and the nanopipette dominate electrophoretic ion transport through the nanopipette when Dps is within 1 order of magnitude of the Debye length (∼10 nm for a 1 mM solution as employed in the simulation). Ion current rectification (ICR) and permselectivity associated with a neutral or charged nanopipette can be reversibly enhanced or reduced dependent on Dps, charge polarity, and charge density (σ) of the substrate. Regulation of nanopipette current is a consequence of the enrichment or depletion of ions within the nanopipette interior, which influences conductivity of the nanopipette. When the external substrate is less negatively charged than the nanopipette, the substrate first reduces, and then enhances the ICR as Dps decreases. Surprisingly, both experimental and simulated data show that a neutral substrate was also able to reduce and reverse the ICR of a slightly negatively charged nanopipette. Simulated results ascribe such effects to the elimination of ion depletion within the nanopipette at positive potentials.

Unexpected and unusual triboelectrification behavior of polymer films containing carboxyl group

The triboelectrification behavior of polymer films when in contact with iron carrier beads was investigated using a homemade special instrument which can control relative humidity under a nitrogen atmosphere. The films showed a large humidity-dependent charging behavior; the charge amount increases with the ambient humidity in the range of a few percent RH to ca. 60% RH. This dependence is explained in terms of the model based on the water charging during the friction and the penetration of the positively charged water into the film. However, it was found that the humidity dependence is significantly depressed by the doping of carboxylic acids into the film or by the introduction of a carboxyl group in the polymer backbone. This depression is again explained by the above charged-water penetration model in which carboxylate ions trap the positively charged water on the film surface.

Linear-grating triboelectric generator based on sliding electrification

The triboelectric effect is known for many centuries and it is the cause of many charging phenomena. However, it has not been utilized for energy harvesting until very recently. (1-5) Here we developed a new principle of triboelectric generator (TEG) based on a fully contacted, sliding electrification process, which lays a fundamentally new mechanism for designing universal, high-performance TEGs to harvest diverse forms of mechanical energy in our daily life. Relative displacement between two sliding surfaces of opposite triboelectric polarities generates uncompensated surface triboelectric charges; the corresponding polarization created a voltage drop that results in a flow of induced electrons between electrodes. Grating of linear rows on the sliding surfaces enables substantial enhancements of total charges, output current, and current frequency. The TEG was demonstrated to be an efficient power source for simultaneously driving a number of small electronics. The principle established in this work can be applied to TEGs of different configurations that accommodate the needs of harvesting energy and/or sensing from diverse mechanical motions, such as contacted sliding, lateral translation, and rotation/rolling.

In situ granular charge measurement by free-fall videography

We present the design and performance characterization of a new experimental technique for measuring individual particle charges in large ensembles of macroscopic grains. The measurement principle is qualitatively similar to that used in determining the elementary charge by Millikan in that it follows individual particle trajectories. However, by taking advantage of new technology we are able to work with macroscopic grains and achieve several orders of magnitude better resolution in charge to mass ratios. By observing freely falling grains accelerated in a horizontal electric field with a co-falling, high-speed video camera, we dramatically increase particle tracking time and measurement precision. Keeping the granular medium under vacuum, we eliminate air drag, leaving the electrostatic force as the primary source of particle accelerations in the co-moving frame. Because the technique is based on direct imaging, we can distinguish between different particle types during the experiment, opening up the possibility of studying charge transfer processes between different particle species. For the ∼300 μm diameter grains reported here, we achieve an average acceleration resolution of ∼0.008 m/s(2), a force resolution of ∼500 pN, and a median charge resolution ∼6× 10(4) elementary charges per grain (corresponding to surface charge densities ∼1 elementary charges per μm(2)). The primary source of error is indeterminacy in the grain mass, but with higher resolution cameras and better optics this can be further improved. The high degree of resolution and the ability to visually identify particles of different species or sizes with direct imaging make this a powerful new tool to characterize charging processes in granular media.

Corona-treated polyethylene films are macroscopic charge bilayers

Top and bottom surfaces of polyethylene (PE) films exposed to corona discharge display large and opposite electrostatic potentials, forming an electric bilayer in agreement with recent and unexpected findings from Zhiqiang et al. Water wetting, chemical composition and roughness of the two surfaces are different. Surprisingly, the bottom surface, opposite to the corona electrode is charged but it is not oxidized, neither is it wetted with water. Moreover, its morphology is unaltered by charging, while the hydrophilic top surface is much rougher with protruding islands that are the result of oxidation followed by phase separation and polymer-polymer dewetting. Common liquids extract the oxidized, hydrophilic material formed at the upper surface, a result that explains the well-known sensitivity of adhesive joints made using corona-treated thermoplastics to liquids, especially water. These results show that poling the surface closer to the corona electrode triggers another but different charge build-up process at the opposite surface. The outcome is another poled PE surface showing high potential but with unchanged chemical composition, morphology and wetting behavior as the pristine surface, thus opening new possibilities for surface engineering.

Contact de-electrification of electrostatically charged polymers

The contact electrification of insulating organic polymers is still incompletely understood, in part because multiple fundamental mechanisms may contribute to the movement of charge. This study describes a mechanism previously unreported in the context of contact electrification: that is, "contact de-electrification", a process in which polymers charged to the same polarity discharge on contact. Both positively charged polymeric beads, e.g., polyamide 6/6 (Nylon) and polyoxymethylene (Delrin), and negatively charged polymeric beads, e.g., polytetrafluoroethylene (Teflon) and polyamide-imide (Torlon), discharge when the like-charged beads are brought into contact. The beads (both with charges of ∼±20 μC/m(2), or ∼100 charges/μm(2)) discharge on contact regardless of whether they are made of the same material, or of different materials. Discharge is rapid: discharge of flat slabs of like-charged Nylon and Teflon pieces is completed on a single contact (∼3 s). The charge lost from the polymers during contact de-electrification transfers onto molecules of gas in the atmosphere. When like-charged polymers are brought into contact, the increase in electric field at the point of contact exceeds the dielectric breakdown strength of the atmosphere and ionizes molecules of the gas; this ionization thus leads to discharge of the polymers. The detection (using a Faraday cup) of charges transferred to the cup by the ionized gas is compatible with the mechanism. Contact de-electrification occurs for different polymers and in atmospheres with different values of dielectric breakdown strength (helium, argon, oxygen, carbon dioxide, nitrogen, and sulfur hexafluoride): the mechanism thus appears to be general.

Interfacial ion-transfer mechanism for the intense luminescence observed when opening self-seal envelopes

The unusually intense luminescence (commonly called triboluminescence) observed when opening self-seal envelopes has been studied using spectroscopy. Emissions from gas-phase species due to electrical discharge were observed, which in the case of air consists of vibronic transitions of N(2)(C(3)Π(u)-B(3)Π(g)) in the ultraviolet (UV) region (280-400 nm). However, the major cause of the intense blue luminescence (around 435 nm) is attributed to optical brightening agents added to the white paper. The results suggest that the emission from the brightening agents may be caused by two mechanisms: (i) fluorescence due to excitation by the UV light from the gas discharge and (ii) nonoptically, by electron transfer. The electrical discharge results from contact electrification; we propose a mechanism for the charge transfer involving a net migration of hydroxide anions out of the paper into the wet latex-adhesive during drying.