A Tool for Studying Contact Electrification in Systems Comprising Metals and Insulating Polymers

Spontaneous Wetting Induced by Contact-Electrification at Liquid-Solid Interface

Wettability significantly influences various surface interactions and applications at the liquid-solid interface. However, the understanding is complicated by the intricate charge exchange occurring through contact electrification (CE) during this process. The understanding of the influence of triboelectric charge on wettability remains challenging, especially due to the complexities involved in concurrently measuring contact angles and interfacial electrical signals. Here, the relationship is investigated between surface charge density and change of contact angle of dielectric films after contact with water droplets. It is observed that the charge exchange when water spared lead to a spontaneous wetting phenomenon, which is termed as the contact electrification induced wetting (CEW). Notably, these results demonstrate a linear dependence between the change of contact angle (CA) of the materials and the density of surface charge on the solid surface. Continuous CEW tests show that not only the static CA but also the dynamics of wetting are influenced by the accumulation charges at the interface. The mechanism behind CEW involves the redistribution of surface charges on a solid surface and polar water molecules within liquid. This interaction results in a decrease in interface energy, leading to a reduction in the CA. Ab initio calculations suggest that the reduction in interface energy may stem from the enhanced surface charge on the substrate, which strengthens the hydrogen bond interaction between water and the substrate. These findings have the potential to advance the understanding of CE and wetting phenomena, with applications in energy harvesting, catalysis, and droplet manipulation at liquid-solid interfaces.

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.

Microporous polymer membrane assisted water induced electricity generation based on triboelectrification and electrostatic induction

Water in its various forms has been found to be one of the most abundant sources of energy on the planet after solar energy, and hydroelectric power plays a key role in renewable-energy supplies. Traditionally, harvesting tremendous amounts of hydrodynamic energy requires the deployment of complex, bulky, and expensive electromagnetic generators, which become inefficient at lower volumes of flowing or falling water, and then the energy is stored when there is an excess, but these techniques remain largely unperfected. Regardless of the diversity of development strategies, adopted methodologies, and working mechanisms, there are a wide range of energy scavengers, to effectively harness environmental friendly alternative energy sources. Robust, sustainable and technologically effective water energy harvesting devices, especially hydroelectric nanogenerators, are in the research spotlight globally, due to their numerous benefits to society, including cost effectiveness, clean and continuous electricity generation, and environmental applicability. Here the design and working mechanism involved in the development of a microporous polymer membrane assisted unique hydroelectric generator (MPA-HEG) based on triboelectrification and electrostatic induction phenomena is reported, which scavenges energy from continuously dripping water droplets sliding onto the surface of a hydrophobic microporous polymer membrane. MPA-HEG utilizes a very simple architecture that consists of a hydrophobic microporous polymer, poly(tetrafluoroethylene) (PTFE), membrane on a single-sided copper-clad laminate as a substrate and an aluminium electrode. Unlike other reported water energy harvesting devices with similar functionalities, the rational design of MPA-HEG does not necessitate any technologically complex structures to be embedded in the substrate. It has also been revealed that the interaction of water droplets on the smooth, water-resistant solid polymer surface in MPA-HEG switches ‘ON’ and connects the originally disconnected equivalent electrical components at the solid–liquid–solid interfaces, giving an uninterrupted electrical circuit, and transmuting the conservative interfacial effects into a bulk mechanism. Consequently, the instantaneous power output shows a vast increase over equivalent devices that are constrained either to triboelectric interfacial effects or moisture-induced electricity generation. This could serve the purpose of validating the inherent advantages of developing self-powered electronic devices, and this approach can also be effectively exploited for boosted power generation with realistic future applications.

Water in its various forms has been found to be one of the most abundant sources of energy on the planet after solar energy, and hydroelectric power plays a key role in renewable-energy supplies.

Dynamic Analysis to Enhance the Performance of a Rotating-Disk-Based Triboelectric Nanogenerator by Injected Gas

A rotating-disk-based triboelectric nanogenerator (TENG) generating electrical energy from wind usually includes a propeller. TENGs are widely used because their high frequency of rotation allows them to generate a relatively high output current. Deep analysis of the gas flow in a TENG is essential to improve its energy conversion efficiency. However, previous studies have isolated the propeller and the TENG as separate entities that harvest wind energy and generate electrical energy, respectively. Most studies focused on each entity because considering both the dynamics and the TENG operation together is an intricate process. This paper introduces a dynamic analysis of the gas flow by dividing it into four vertical and horizontal directions and carrying out a COMSOL simulation to verify the pressure on the propeller and the flow of the gas. The electrical outputs are measured while varying the height and angle of the inlet and the number of wings on the propeller. After optimization, the P-TENG generated a high output power density of 283.95 mW/m², which can light up 205 light-emitting diodes and drive a commercial small electronic appliance. In addition, optimizing the P-TENG through a variety of analyses allowed it to provide sustainable power to a self-powered wireless sensor system.

Power Generation from a Hybrid Generator (TENG-EMG) Run by a Thermomagnetic Engine Harnessing Low Temperature Waste Heat

This work explored the scavenging of low temperature waste heat and conversion of it into electrical energy through the operation of a gadolinium (Gd) based thermomagnetic engine. Gd is one of the unique materials whose magnetic property changes from ferromagnetic to paramagnetic depending on the temperature (“the Curie temperature”), which is around 20 °C. In the present work, two different types of generators were designed and applied to the rotating shaft of a Gd-based thermomagnetic engine developed for low temperature differential (LTD) applications. Of these, one is the so-called triboelectric nanogenerator (TENG), and the other is the electromagnetic generator (EMG). These have been designed to produce electricity from the rotating shaft of the thermomagnetic engine, exploiting both the electromagnetic and triboelectric effects. When operated at a rotational speed of 251 rpm with a temperature difference of 45 °C between the hot and cold water jets, the hybrid (TENG-EMG) generator produced a combined pulsating DC open circuit voltage of 5 V and a short circuit current of 0.7 mA. The hybrid generator effectively produced a maximum output power of 0.75 mW at a loading resistance of 10 kΩ.

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.

Simultaneous measurement of triboelectrification and triboluminescence of crystalline materials

Triboelectrification has been studied for over 2500 years, yet there is still a lack of fundamental understanding as to its origin. Given its utility in areas such as xerography, powder spray painting, and energy harvesting, many devices have been made to investigate triboelectrification at many length-scales, though few seek to additionally make use of triboluminescence: the emission of electromagnetic radiation immediately following a charge separation event. As devices for measuring triboelectrification became smaller and smaller, now measuring down to the atomic scale with atomic force microscope based designs, an appreciation for the collective and multi-scale nature of triboelectrification has perhaps abated. Consider that the energy required to move a unit charge is very large compared to a van der Waals interaction, yet peeling Scotch tape (whose adhesion is derived from van der Waals forces) can provide strong enough energy-focusing to generate X-ray emission. This paper presents a device to press approximately cm-sized materials together in a vacuum, with in situ alignment. Residual surface charge, force, and position and X-ray, visible light, and RF emission are measured for single crystal samples. Charge is therefore tracked throughout the charging and discharging processes, resulting in a more complete picture of triboelectrification, with controllable and measurable environmental influence. Macroscale charging is directly measured, whilst triboluminescence, originating in atomic-scale processes, probes the microscale. The apparatus was built with the goal of obtaining an ab initio-level explanation of triboelectrification for well-defined materials, at the micro- and macro-scale, which has eluded scientists for millennia.

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.

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.

Recent Progress on Flexible Triboelectric Nanogenerators for SelfPowered Electronics

Recently, smart systems have met with large success. At the origin of the internet of things, they are a key driving force for the development of wireless, sustainable, and independent autonomous smart systems. In this context, autonomy is critical, and despite all the progress that has been made in low-power electronics and batteries, energy harvesters are becoming increasingly important. Thus, harvesting mechanical energy is essential, as it is widespread and abundant in our daily life environment. Among harvesters, flexible triboelectric nanogenerators (TENGs) exhibit good performance, and they are easy to integrate, which makes them perfect candidates for many applications and, therefore, crucial to develop. In this review paper, we first introduce the fundamentals of TENGs, including their four basic operation modes. Then, we discuss the different improvement parameters. We review some progress made in terms of performance and integration that have been possible through the understanding of each operation mode and the development of innovative structures. Finally, we present the latest trends, structures, and materials in view of future improvements and applications.

Coplanar induction enabled by asymmetric permittivity of dielectric materials for mechanical energy conversion

Triboelectric nanogenerator (TENG) is a newly emerged technology for harvesting mechanical energy, which has the promise for various practical applications. Here, we introduce a new principle of TENG in which induced current is generated between two coplanar electrodes because of different dielectric fillers of distinct permittivities. The manipulation of permittivity of dielectric materials for TENG is first reported, demonstrating a novel route in designing high-performance TENGs. When repeatedly contacting with an object, a TENG having lateral dimensions of 21 mm × 10 mm can produce an open-circuit voltage of 58.5 V and a short-circuit current of 44.7 μA. The instantaneous output power density can reach up to 708 μW. Besides, the new design incorporates all electrodes into a single plane, greatly simplifying the structure, promoting robustness, and providing a viable solution for device miniaturization.

Highly transparent triboelectric nanogenerator for harvesting water-related energy reinforced by antireflection coating

Water-related energy is an inexhaustible and renewable energy resource in our environment, which has huge amount of energy and is not largely dictated by daytime and sunlight. The transparent characteristic plays a key role in practical applications for some devices designed for harvesting water-related energy. In this paper, a highly transparent triboelectric nanogenerator (T-TENG) was designed to harvest the electrostatic energy from flowing water. The instantaneous output power density of the T-TENG is 11.56 mW/m(2). Moreover, with the PTFE film acting as an antireflection coating, the maximum transmittance of the fabricated T-TENG is 87.4%, which is larger than that of individual glass substrate. The T-TENG can be integrated with silicon-based solar cell, building glass and car glass, which demonstrates its potential applications for harvesting waste water energy in our living environment and on smart home system and smart car system.

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).

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.

Noncontact free-rotating disk triboelectric nanogenerator as a sustainable energy harvester and self-powered mechanical sensor

In this work, we introduced an innovative noncontact, free-rotating disk triboelectric nanogenerator (FRD-TENG) for sustainably scavenging the mechanical energy from rotary motions. Its working principle was clarified through numerical calculations of the relative-rotation-induced potential difference, which serves as the driving force for the electricity generation. The unique characteristic of the FRD-TENG enables its high output performance compared to its working at the contact mode, with an effective output power density of 1.22 W/m(2) for continuously driving 100 light-emitting diodes. Ultrahigh stability of the output and exceptional durability of the device structure were achieved, and the reliable output was utilized for fast/effective charging of a lithium ion battery. Based on the relationship between its output performance and the parameters of the mechanical stimuli, the FRD-TENG could be employed as a self-powered mechanical sensor, for simultaneously detecting the vertical displacement and rotation speed. The FRD-TENG has superior advantages over the existing disk triboelectric nanogenerator, and exhibits significant progress toward practical applications of nanogenerators for both energy harvesting and self-powered sensor networks.

Simultaneously harvesting electrostatic and mechanical energies from flowing water by a hybridized triboelectric nanogenerator

Flowing water contains not only mechanical kinetic energy, but also the electrostatic energy owing to the triboelectric charges caused by its contact with surrounding media such as air. In this paper, a water wheel hybridized triboelectric nanogenerator (TENG), composed of a water-TENG part and a disk-TENG part, has been developed for simultaneously harvesting the two types of energies from the tap water flowing from a household faucet. The wheel blades of the hybridized TENG are composed by superhydrophobic polytetrafluoroethylene (PTFE) thin films with nanostructures, which are used as water-TENG to harvest the electrostatic energy from the flowing water. In addition, the flowing water impacted on the wheel blades also causes the rotation motion of disk-TENG and can be used to harvest the mechanical kinetic energy. The short-circuit current of the water-TENG and the disk-TENG at a flowing water rate of 54 mL/s can reach 12.9 and 3.8 μA, respectively. The hybridized TENG is also demonstrated to harvest wind energy and acts as a self-powered sensor to detect the flowing water rate and wind speed. All these results show the potentials of the hybridized TENG for harvesting multiple types of energies from the environment.

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.

Pulsed nanogenerator with huge instantaneous output power density

A nanogenerator (NG) usually gives a high output voltage but low output current, so that the output power is low. In this paper, we developed a general approach that gives a hugely improved instantaneous output power of the NG, while the entire output energy stays the same. Our design is based on an off-on-off contact based switching during mechanical triggering that largely reduces the duration of the charging/discharge process, so that the instantaneous output current pulse is hugely improved without sacrificing the output voltage. For a vertical contact-separation mode triboelectric NG (TENG), the instantaneous output current and power peak can reach as high as 0.53 A and 142 W at a load of 500 Ω, respectively. The corresponding instantaneous output current and power density peak even approach 1325 A/m(2) and 3.6 × 10(5) W/m(2), which are more than 2500 and 1100 times higher than the previous records of TENG, respectively. For the rotation disk based TENG in the lateral sliding mode, the instantaneous output current and power density of 104 A/m(2) and 1.4 × 10(4) W/m(2) have been demonstrated at a frequency of 106.7 Hz. The approach presented here applies to both a piezoelectric NG and a triboelectric NG, and it is a major advance toward practical applications of a NG as a high pulsed power source.

Cylindrical rotating triboelectric nanogenerator

We demonstrate a cylindrical rotating triboelectric nanogenerator (TENG) based on sliding electrification for harvesting mechanical energy from rotational motion. The rotating TENG is based on a core-shell structure that is made of distinctly different triboelectric materials with alternative strip structures on the surface. The charge transfer is strengthened with the formation of polymer nanoparticles on surfaces. During coaxial rotation, a contact-induced electrification and the relative sliding between the contact surfaces of the core and the shell result in an "in-plane" lateral polarization, which drives the flow of electrons in the external load. A power density of 36.9 W/m(2) (short-circuit current of 90 μA and open-circuit voltage of 410 V) has been achieved by a rotating TENG with 8 strip units at a linear rotational velocity of 1.33 m/s (a rotation rate of 1000 r/min). The output can be further enhanced by integrating more strip units and/or applying larger linear rotational velocity. This rotating TENG can be used as a direct power source to drive small electronics, such as LED bulbs. This study proves the possibility to harvest mechanical energy by TENGs from rotational motion, demonstrating its potential for harvesting the flow energy of air or water for applications such as self-powered environmental sensors and wildlife tracking devices.

Electrokinetics of polar liquids in contact with nonpolar surfaces

Zeta potentials of several polar protic (water, ethylene glycol, and formamide) as well as polar aprotic (dimethyl sulfoxide) liquids were measured in contact with three nonpolar surfaces using closed-cell electroosmosis. The test surfaces were chemisorbed monolayers of alkyl siloxanes, fluoroalkyl siloxanes, and polydimethylsiloxanes (PDMS) grafted on glass slides. All these liquids exhibited substantial electrokinetics in contact with the nonpolar surfaces with these observations: the electrokinetic effect on the fluorocarbon-coated surface is the strongest and on a PDMS grafted surface, the effect is the weakest. Even though these hygroscopic liquids contain small amounts of water, the current models of charging based on the adsorption of hydroxide ions at the interface or the dissociation of pre-existing functionalities (e.g., silanol groups) appear to be insufficient to account for the various facets of the experimental observations. The results illustrate how ubiquitous the phenomenon of electrokinetics is with polar liquids contacting such apparently passive nonpolar surfaces. We hope that these results will inspire further experimental and theoretical studies in this important area of research that has potential practical implications.

In situ quantitative study of nanoscale triboelectrification and patterning

By combining contact-mode atomic force microscopy (AFM) and scanning Kevin probe microscopy (SKPM), we demonstrated an in situ method for quantitative characterization of the triboelectrification process at the nanoscale. We systematically characterized the triboelectric charge distribution, multifriction effect on charge transfer, as well as subsequent charge diffusion on the dielectric surface: (i) the SiO2 surface can be either positively or negatively charged through triboelectric process using Si-based AFM probes with and without Pt coating, respectively; (ii) the triboelectric charges accumulated from multifriction and eventually reached to saturated concentrations of (-150 ± 8) μC/m(2) and (105 ± 6) μC/m(2), respectively; (iii) the charge diffusion coefficients on SiO2 surface were measured to be (1.10 ± 0.03) × 10(-15) m(2)/s for the positive charge and (0.19 ± 0.01) × 10(-15) m(2)/s for the negative charges. These quantifications will facilitate a fundamental understanding about the triboelectric and de-electrification process, which is important for designing high performance triboelectric nanogenerators. In addition, we demonstrated a technique for nanopatterning of surface charges without assistance of external electric field, which has a promising potential application for directed self-assembly of charged nanostructures for nanoelectronic devices.

Segmentally structured disk triboelectric nanogenerator for harvesting rotational mechanical energy

We introduce an innovative design of a disk triboelectric nanogenerator (TENG) with segmental structures for harvesting rotational mechanical energy. Based on a cyclic in-plane charge separation between the segments that have distinct triboelectric polarities, the disk TENG generates electricity with unique characteristics, which have been studied by conjunction of experimental results with finite element calculations. The role played by the segmentation number is studied for maximizing output. A distinct relationship between the rotation speed and the electrical output has been thoroughly investigated, which not only shows power enhancement at high speed but also illuminates its potential application as a self-powered angular speed sensor. Owing to the nonintermittent and ultrafast rotation-induced charge transfer, the disk TENG has been demonstrated as an efficient power source for instantaneously or even continuously driving electronic devices and/or charging an energy storage unit. This work presents a novel working mode of TENGs and opens up many potential applications of nanogenerators for harvesting even large-scale energy.

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.

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.

Electrokinetic assembly of selenium and silver nanowires into macroscopic fibers

Solution-phase synthesized nanowires with high aspect ratios can be a challenge to assemble into desired structures. As synthesized, these nanostructures readily bend and entangle with each other to form larger aggregates. This manuscript reports a general procedure for directing the assembly of semiconducting and metallic nanowires into fibers that can easily span distances >1 cm. Dispersions of these nanostructures in a low dielectric solution are organized by electrokinetic techniques into fibers that can be isolated from solution. Theoretical studies suggest that the assembled fibers adopt an orientation along electric field lines in the solution. The number of assembled fibers is a function of the duration of the assembly process, the magnitude of the electric potential, and the initial concentration of nanowires dispersed in solution. These findings offer a general method for the assembly of nanowires into macroscopic fibers of tunable dimensions. Fibers of selenium nanowires isolated from solution can reversibly bend in response to a source of electrostatic charges positioned in close proximity to the free-standing fiber. These flexible selenium fibers also exhibit a photoconductive response when illuminated with white light.

Controlling the kinetics of contact electrification with patterned surfaces

This communication describes a new approach for controlling static charging (contact electrification), and resulting electrical discharging, that occurs when two contacting materials separate. The prevention of contact electrification is an important problem; unwanted adhesion between oppositely charged materials, spark-initiated explosions, and damage to microelectronic circuitry are some of the deleterious effects of static charging. Current strategies for controlling contact electrification rely upon dissipating an accumulated charge by making contacting surfaces conductive and, therefore, can be difficult to implement with electrically insulating materials. Specifically, using our understanding of the ion-transfer mechanism of contact electrification, we patterned glass slides with negatively charging areas (clean glass) and positively charging areas (glass silanized with a cationic siloxane terminated with a quaternary ammonium group). The rate of charge separation due to a steel sphere rolling on the patterned glass surface correlated linearly with the percentage of the glass surface that was silanized; the rate of charge transfer was minimal when 50% of the glass surface area was silanized. Patterned surfaces also prevented electrical discharges between electrically conducting (bare steel) or insulating (acrylate-coated steel) spheres rolling on the glass, because the rate of charging was sufficiently slow to prevent electric fields greater than the dielectric strength of air to develop. This strategy for preventing static charging therefore does not require one of the two contacting surfaces to be electrically conductive. More generally, these results show that our enhanced understanding of the ion-transfer mechanism of contact electrification enables the rational design of chemically tailored surfaces for functional electrets.

Electrostatic electrochemistry at insulators

The identity of charges generated by contact electrification on dielectrics has remained unknown for centuries and the precise determination of the charge density is also a long-standing challenge. Here, electrostatic charges on Teflon (polytetrafluoroethylene) produced by rubbing with Lucite (polymethylmethacrylate) were directly identified as electrons rather than ions by electrochemical (redox) experiments with charged Teflon used as a single electrode in solution causing various chemical reactions: pH increases; hydrogen formation; metal deposition; Fe(CN)(6)(3-) reduction; and chemiluminescence in the system of Teflon(-)/Ru(bpy)(3)(2+)/S(2)O(8)(2-) (analogous to electrogenerated chemiluminescence). Moreover, copper deposition could be amplified by depositing Pd first in a predetermined pattern, followed by electroless deposition to produce Cu lines. This process could be potentially important for microelectronic and other applications because Teflon has desirable properties including a low dielectric constant and good thermal stability. Charge density was determined using Faraday's law and the significance of electron transfer processes on charged polymers and potentially other insulators have been demonstrated.

Kinetics of Contact Electrification between Metals and Polymers

Kinetics of charge transfer between metals and polymers was studied using an analytical rolling-sphere tool. The rates of charge transfer were related to the area of contact between contacting surfaces and the tunneling current between them. The derived rate equations accounted for the experimentally observed sigmoidal charging curves. Furthermore, for a model system of steel spheres rolling on modified polystyrene supports, it was shown that the magnitudes of separated charges can be varied by adjusting the polymer's surface properties and/or ambient conditions.