Single-Collision Statistics Reveal a Global Mechanism Driven by Sample History for Contact Electrification in Granular Media

Paradoxical peeling patterns

Processes ranging from fracture of crystals to peeling of tape have been known for many decades to emit light through a mechanism believed to be associated with electrical charging of separating surfaces. This topic, broadly termed fractoluminescence, has been proposed to be involved in several remarkable phenomena, including medical diagnostics and the generation of X-rays in the lab and earthquake lightning in nature. Here we add the paradoxical finding that two separating surfaces produce entirely different charge patterns, despite originating from the same interface. Further, we report the discovery of a rich variety of new and unexplained patterns, and we examine the hypothesis that the patterns are produced by migration of either polar or non-polar discharge ions onto contact-charged surfaces. This hypothesis may first explain prior findings that charge patterns can extend far beyond points of contact, and second suggests that the ultimate charge imparted on surfaces depends both on well-characterized mechanisms of surface potential and on highly variable discharge ions in the surrounding environment.

Torque about electrostatically charged spheres makes them more attractive

The strength of interparticle interactions in a granular system controls how a collection of insulating particles flow, cohere and fragment. Forces due to electrostatic charging, particularly in free-fall or low gravity environments, can dominate the static and dynamic interactions with important implications for understanding natural and industrial processes. Here we show that shaking of homogeneous, spherical particles can result in a non-uniform surface charge distribution. The measured dipole moment and torque for each particle are found to be strongly correlated. However, our model shows that to predict the torque and force requires one to consider the full surface charge distribution. This overlooked torque is not only significant, but would amplify attractive interactions through particle reorientation.

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.

Multiple charge carrier species as a possible cause for triboelectric cycles

The tendency of materials to order in triboelectric series has prompted suggestions that contact electrification might have a single, unified underlying description. However, the possibility of "triboelectric cycles," i.e., series that loop back onto themselves, is seemingly at odds with such a coherent description. In this work, we propose that if multiple charge carrying species are at play, both triboelectric series and cycles are possible. We show how series arise naturally if only a single charge carrier species is involved and if the driving mechanism is approach toward thermodynamic equilibrium, and simultaneously, that cycles are forbidden under such conditions. Suspecting multiple carriers might relax the situation, we affirm this is the case by explicit construction of a cycle involving two carriers, and then extend this to show how more complex cycles emerge. Our work highlights the importance of series and cycles towards determining the underlying mechanism(s) and carrier(s) in contact electrification.

Electrostatic interactions between rough dielectric particles

From colloid suspension to particle aggregation in protoplanetary formation, electrostatic attraction and repulsion between particles is a key mechanism behind the aggregation and clustering of particles. Although most studies have focused on canonical spherical particles, it remains unclear how nonspherical and rough dielectric particles interact and whether the complicated interplay between roughness and charge distribution affects their force couplings. Here a boundary-element method model was leveraged to study electrostatic interactions between charged dielectric particles with modular, axisymmetric surface features. Charge accumulation at convex surface asperities decreases the strength of electrostatic interactions between particles, and the sensitivity of the electrostatic force to the particle surface roughness and orientation is especially apparent at small particle separations. Surface interactions between the particle near-contact regions were isolated to determine the degree that near-contact interactions dictate the relationship between the net electrostatic force and the particle roughness and orientation. A correction factor ΔF is introduced to recover higher order dielectric effects from a low order analytical model. Finally, implications of surface charge asymmetries produced for different particle orientations and surface roughnesses on the long-standing problem of triboelectrification are discussed.

Suppression and Control of Bipolar Powder Charging by Turbulence

Current models predict particles of the same material but different sizes to charge bipolar upon contacts; the resulting charge peaks endanger process safety. However, we found wall-bounded turbulence to suppress the powder's electrostatic charging. Aerodynamic forces skew the collision frequency and narrow the charge distribution's bandwidth. Bipolar charging reduces, especially in moderately polydisperse systems of a low Stokes number. Not the smallest but midsized particles charge most negatively. Moreover, turbulence separates charge, producing pockets of high electric potential in low-vorticity regions.

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.

Tribocharging of granular materials and influence on their flow

Once granular materials flow, particles charge because of the triboelectric effect. When particles touch each other, charges are exchanged during contact whether they are made of the same material or not. Surprisingly, when different sizes of particles are mixed together, large particles tend to charge positively while small particles charge negatively. If the particles are relatively small (typically smaller than a millimeter), the electrostatic interaction between the particles becomes significant and leads to aggregation or sticking on the surface of the container holding them. Studying those effects is challenging as the mechanisms that govern the triboelectric effect are not fully understood yet. We show that the patch model (or mosaic model) is suitable to reproduce numerically the flow of triboelectrically charged granular materials as the specific charging of bi-disperse granular materials can be retrieved. We investigate the influence of charging on the cohesion of granular materials and highlight the relevant parameters related to the patch model that influence cohesion. Our results shed new light on the mechanisms of the triboelectric effect as well as on how the charging of granular materials influences cohesion using numerical simulations.

Influence of Wettability and Geometry on Contact Electrification between Nonionic Insulators

Contact electrification is an interfacial process in which two surfaces exchange electrical charges when they are in contact with one another. Consequently, the surfaces may gain opposite polarity, inducing an electrostatic attraction. Therefore, this principle can be exploited to generate electricity, which has been precisely done in triboelectric nanogenerators (TENGs) over the last decades. The details of the underlying mechanisms are still ill-understood, especially the influence of relative humidity (RH). Using the colloidal probe technique, we convincingly show that water plays an important role in the charge exchange process when two distinct insulators with different wettability are contacted and separated in <1 s at ambient conditions. The charging process is faster, and more charge is acquired with increasing relative humidity, also beyond RH = 40% (at which TENGs have their maximum power generation), due to the geometrical asymmetry (curved colloid surface vs planar substrate) introduced in the system. In addition, the charging time constant is determined, which is found to decrease with increasing relative humidity. Altogether, the current study adds to our understanding of how humidity levels affect the charging process between two solid surfaces, which is even enhanced up to RH = 90% as long as the curved surface is hydrophilic, paving the way for designing novel and more efficient TENGs, eco-energy harvesting devices which utilize water and solid charge interaction mechanism, self-powered sensors, and tribotronics.

Accurate determination of the shapes of granular charge distributions

Experiments have shown that charge distributions of granular materials are non-Gaussian, with broad tails that indicate many particles with high charge. This observation has consequences for the behavior of granular materials in many settings, and may bear relevance to the underlying charge transfer mechanism. However, there is the unaddressed possibility that broad tails arise due to experimental uncertainties, as determining the shapes of tails is nontrivial. Here we show that measurement uncertainties can indeed account for most of the tail broadening previously observed. The clue that reveals this is that distributions are sensitive to the electric field at which they are measured; ones measured at low (high) fields have larger (smaller) tails. Accounting for sources of uncertainty, we reproduce this broadening in silico. Finally, we use our results to back out the true charge distribution without broadening, which we find is still non-Guassian, though with substantially different behavior at the tails and indicating significantly fewer highly charged particles. These results have implications in many natural settings where electrostatic interactions, especially among highly charged particles, strongly affect granular behavior.