Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session B07: Tribocharging and the Role of Water |
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Sponsoring Units: DSOFT Chair: Ignaas Jimidar, Vrije universiteit Brussel; Scott Waitukaitis, IST Austria Room: Room 130 |
Monday, March 6, 2023 11:30AM - 11:42AM |
B07.00001: A quantitatively consistent, scale-spanning model for same-material tribocharging Scott R Waitukaitis, Galien Grosjean, Juan Carlos A Sobarzo Ponce, Sebastian Wald Although tribocharging occurs ubiquitously and is widely studied, we still don't know how it works. It is not difficult to imagine why charge exchange might occur with different materials—any number of parameters (e.g. electron affinity, hydrophobicity, acidity/basicity) could break symmetry to drive transfer. Yet there is a little known but perfectly reproducible fact that makes tribocharging even more perplexing: even identical materials, when contacted together, still exchange electrical charge. It has been proposed that such 'same-material' tribocharging arises due to surface heterogeneity of some hereto unknown charge-driving parameter. This model correctly predicts that the scale of charge transfer grows with the square root of the contacting area. However, to reconcile it with experimental data, the size of individual donors/acceptors must be 1/100th the size of a hydrogen atom—a physical impossibility. To resolve this conundrum, we propose that charge donors/acceptors come in mesoscale patches. By accounting for the spatial correlations of these patches, we develop a model for same-material tribocharging that is able to make sense of several inconsistencies in the literature, including the impossibly small donors/acceptors previously mentioned, thus shedding potential light on this perplexing phenomenon. |
Monday, March 6, 2023 11:42AM - 11:54AM |
B07.00002: Are granular charge distributions really non-Gaussian? Nicolas Mujica, Scott R Waitukaitis Measurements have shown that charge distributions of grains comprised of the same material are non-Gaussian, which seem to imply that the underlying charge transfer process defies the central limit theorem. However, such measurements are largely made with a single experimental technique, videographic particle tracking, and common sources of error that may affect the shape of distributions---especially at tails where data is sparse---have not been considered. In this article, we show that the shape of such gathered distributions is sensitive to the field at which it is measured; distributions measured at low (high) fields have larger (smaller) tails. We reveal that this is a signal to noise issue, generating synthetic data and subjecting it to the same sources of noise to recreate such broadening in silico. Testing whether or not we can recreate experimentally measured distributions by broadening an underlying Gaussian distribution, we find we cannot. Testing the same for a charge distribution from experiments recovered at high fields, we find correspondence. Our results confirm that granular charge distributions are non-Gaussian, but that the precise shape can only be determined at sufficiently large fields. |
Monday, March 6, 2023 11:54AM - 12:06PM |
B07.00003: Tribocharging of patchy particles Nicolas Preud'homme, Geoffroy Lumay, Eric Opsomer When granular materials flow, particles charge significantly resulting in large electric fields between them and a strong alteration of the material’s flowing properties. The charging of granular materials arises from a transfer of charges at contact between the particles as a consequence of the triboelectric effect. It has been shown that many parameters such as ambient humidity or particle size influence the charging of granular materials making this effect complex to understand. The existence of positively and negatively charged patches at the surface of the objects in contact has been postulated to interpret same-materials tribocharging and show good reproducibility. However, this model has never been applied to granular materials. We performed DEM numerical simulations of patchy particles and obtained charge transfers with our model which are consistent with experimental results. We also show that this model correctly reproduces the specific charging of bi-disperse granular materials where small particles charge negatively while large particles charge positively. Our results thus provide another evidence that the existence of patches at the surface of objects could explain same-materials tribocharging. They also strengthen the current hypothesis on the role of water as we compare our model to the transfer of ions from wet to dry patches on the particles and make the connection with ambient humidity. |
Monday, March 6, 2023 12:06PM - 12:18PM |
B07.00004: Combined effect of moisture and electrostatic charges on granular materials: an experimental approach Geoffroy Lumay It is well known empirically by industrialists that the relative air humidity and the presence of electrostatic charges influence drastically granular material properties. The relative air humidity induces the formation of capillary bridges and modify the grain surface conductivity. The presence of capillary bridges produces cohesive forces. On the other hand, the apparition of electrostatic charges due to the triboelectric effect at the contacts between the grains and at the contacts between the grains and the container produces electrostatic forces. Therefore, in many cases, the cohesiveness is the result of the interplay between capillary and electrostatic forces. Unfortunately, the triboelectric effect is still poorly understood, particularly inside a granular material. Moreover, reproducible electrostatic measurements are difficult to perform. In this presentation, we will discuss a set of experimental results showing the sensitivity of tribocharging to relative air humidity with an optimum around 37%RH. Moreover, we show the consequences of this optimum on the flow and packing fraction dynamics. The fundamental aspects but also the consequences on a short selection of industrial processes will be discussed. From a fundamental point of view, we will link our experimental results with the recent advances in the field. In particular with the model based on patchy grains (donor/acceptor sites for electrostatic charges) and the models based on water ions exchange. |
Monday, March 6, 2023 12:18PM - 12:30PM |
B07.00005: Influence of ambient humidity on contact electrification between nonionic insulators Ignaas Jimidar, Gijs Roozendaal, Stefan Kooij, Han Gardeniers, Gert Desmet, Kai Sotthewes Contact electrification is an interfacial process in which two surfaces exchange electrical charges when rubbed against each other. Consequently, the surfaces may gain opposite polarity, leading to the onset of an electrostatic attraction. This principle is applied in triboelectric nanogenerators (TENGs). However, the underlying mechanisms' details are still ill-understood, especially the influence of relative humidity (RH). Using the colloidal probe technique it is convincingly shown that when two insulators touch at ambient conditions, water plays an important role in the charge accumulation process due to the presence of H+ and OH- ions. The charging process is enhanced and faster with increasing relative humidity, also beyond RH = 40\%, due to the geometrical asymmetry introduced in the system. In addition, the time constant of charging is determined, which depends heavily on the relative humidity. |
Monday, March 6, 2023 12:30PM - 12:42PM |
B07.00006: Can Thermodynamics Explain Contact Charging between Insulating Polymer Surfaces? Michael A Webb, Hang Zhang, Sankaran Sundaresan Contact charging is a well-known phenomenon wherein material surfaces become asymmetrically charged following their contact and separation. Despite its ubiquity in natural processes and importance in engineering technology, the underlying mechanisms and driving forces remain unknown. For insulating materials, water is thought to play a central role, perhaps through the transport of water ions, but there is yet no molecular basis to understand observed behavior. In this talk, I will describe our efforts in using molecular dynamics simulations to characterize possible thermodynamic driving forces for water ions to transfer from one surface to another. By comparing results across a variety of polymer surfaces, we assess to what extent our calculations are consistent with experimental observations derived from contact charging series and what molecular phenomena dictate these thermodynamics. In the end, this work should provide a firmer molecular basis to understand the behavior of water ions on polymer surfaces and the extent to which they may contribute to observed contact charging phenomena. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B07.00007: Enhanced stability of triboelectric charge by hydrophilic air-stable radicals Sooik Im, Ethan Frey, Jan Genzer, Michael D Dickey The stability of triboelectric charge has received much attention due to academic interest and numerous applications. An example of the latter includes increasing the lifetime of air filtration that uses the charges to capture microdust. We studied charge retention on different self-assembled monolayers (SAMs) on Si wafers. The goal is to investigate the role of radicals on charge retention and eventually find a way to increase charge stability. Pt-coated atomic force microscopy probes were brought into contact with different SAMs to tribocharge the SAMs. The charge retention was recorded over time by the non-contact tapping mode of Kelvin Probe Force Microscopy (KPFM). The KPFM images showed that charges on hydrophilic surfaces generally tend to dissipate faster than those on hydrophobic surfaces, which indicates water vapor could enhance the charge dissipation. Interestingly, the charge retention in the presence of stable radicals (TEMPO) dissipated much slower (>12 h), showing out of the general trend. To verify the role of radicals in enhancing charge retention, radical density on the surface was tuned by two separate approaches: the treatments of TEMPO SAMs with tetrabutylammonium fluoride as a SAM etchant and ascorbic acid as a radical scavenger. After these treatments, the charge destabilized by lowering radical density on the surface. |
Monday, March 6, 2023 12:54PM - 1:06PM |
B07.00008: A triboelectric series of identical materials. Juan Carlos A Sobarzo Ponce, Scott Waitukaitis A triboelectric series is an ordering of materials according to their tendency to acquire positive or negative charge when in contact with each other. Despite the fact that the first triboelectric series was experimentally established over 250 years ago, the underlying principle that orders the materials is still unresolved. Considerable work has been done to try to solve this mystery, with hypotheses atributting it to diverse properties such as water content, acidity/basicity, among others. However, without a way to isolate and change a single potential driving parameter, work with different materials so far has not led to any definitive answer. |
Monday, March 6, 2023 1:06PM - 1:18PM Author not Attending |
B07.00009: Probing Surface Charge Densities of Evaporated Polymers Abdulmalik Alghonaim, Peng Zhang, Yinfeng Xu, Nischal Maharjan, Jamilya Nauruzbayeva, Himanshu Mishra Electrified interfaces of water manifest across natural and applied sciences, e.g., thundercloud charging, |
Monday, March 6, 2023 1:18PM - 1:30PM |
B07.00010: Towards long term observation of individual aerosol particle charging with optical tweezers Isaac Lenton, Andrea Stoellner, Scott R Waitukaitis Aerosol particles, including dust, pollen and viruses, surround us in our everyday lives. These particles are often electrostatically charged. Charge can accumulate on aerosol particles due to various mechanisms, including interactions with other particles and scavenging of ions. The charge of microscopic particles and aerosols can influence aggregation and how long particles remain airborne: understanding the microscopic nature of aerosol charging can give important insights into the behaviour of larger systems such as dust transport across oceans and air filtration. In order to study how the charge evolves on individual micro-meter sized aerosol particles, we need a non-contact technique capable of holding both charged and uncharged particles. Optical tweezers are one such tool. Our group has been using optical tweezers to study charging of aerosol particles: preliminary results suggest that the rate at which particles charge depends on numerous factors, most importantly, the humidity of the surrounding air. Our hypothesis is that this occurs due to preferential adsorption/desorption of OH- or H+ ions. When we artificially increase the number of ions surrounding our particle, we observe rapid discharging. Currently we have only studied charging behavior over relative short time frames (several hours) due to limitations in system stability. We are working towards extending the capabilities of our system to study charging over longer time frames. |
Monday, March 6, 2023 1:30PM - 1:42PM |
B07.00011: Quantifying nanoscale charge density features of contact-charged surfaces with an FEM/KPFM-hybrid approach Felix Pertl, Juan Carlos Sobarzo, Scott Waitukaitis, Lubuna Shafeek, Tobias Cramer A powerful tool for studying contact electrification is Kelvin Probe Force Microscopy (KPFM), where electrical signals from an AFM tip allow one to spatially map a voltage above a surface that is caused by the presence of charge. However, a fundamental challenge from KPFM is to convert the voltage map to a surface charge density map. Without a method to convert voltage to charge, the signal from KPFM remains qualitative. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B07.00012: Same-material tribocharging: single-collision statistics point to a global, surface-water driven mechanism Galien Grosjean, Scott Waitukaitis Models for same-material contact electrification often rely on a yet-to-be-identified local parameter whose stochastic fluctuations would drive charge exchange. Using acoustic levitation, we measure the charge gained by individual granular spheres after repeated contacts with substrates of the same material. By looking at the charge distributions, we find that the charge-driving parameter is global instead of local. Furthermore, we show that this parameter is not intrinsic to the samples, but acquired during sample preparation, as cleaning the samples fully resets their charging behavior. Changes in humidity affect charging irreversibly and in a random direction. Ultimately, our results point to a mechanism where adsorbates, likely water, recruited on the sample throughout its history play a driving role. |
Monday, March 6, 2023 1:54PM - 2:06PM |
B07.00013: Role of thermoelectricity as a driving force in tribocharging Euicheol Shin, Jae-Hyeon Ko, Ho-Ki Lyeo, Yong-Hyun Kim Triboelectricity has historically been challenging to pinpoint the origin of the thermodynamic driving force that underlies its omnipresence on any material interface. Here, we propose a ubiquitous triboelectric model based on thermoelectricity, which is generally applicable to solids, liquids, and even gas [1]. The driving force for tribocharing is quantified by the electrostatic potential drop at the interface caused by thermoelectric charge redistribution for frictional heat. This unprecedented quantification of triboelectricity leads to the first theoretical triboelectric series based on experimental thermodynamic parameters and theoretical Seebeck coefficients. Our ad hoc choice of Fermi energy from universal band alignment between semiconductors and water for theoretical Seebeck coefficients offers remarkable triboelectric series similar to experiments [2]. The water might be a feasible defect source in ambient friction determining the most likely Fermi energy. Our discoveries will lead not only to a deeper understanding of triboelectricity but also to a practical way of boosting the efficiency of triboelectric nanogenerators.n |
Monday, March 6, 2023 2:06PM - 2:18PM |
B07.00014: Mechanism of dynamic friction in a humid nanocontact Igor Stankovic, Olivier Noel, Pierre-Emmanuel Mazeran The physics of dynamic friction on water molecule-contaminated surfaces is still poorly understood. In line with the growing interest in hydrophobic contact for industrial applications, this paper focuses on friction mechanisms in such interfaces. As a commonly used material, contact with graphite is considered in a twin-fold approach based on experimental investigations using the circular mode atomic force microscopy technique combined with molecular dynamic simulations. We demonstrate that an intuitive paradigm should be refined, which asserts that water molecules are squeezed out of a hydrophobic contact. Consequently, we introduce a mechanism considering a droplet produced within the sliding nano-contact by the accumulation of water adsorbed on the substrate. Then we show that a full slip regime of the droplet sliding on the hydrophobic substrate explains the experimental tribological behavior. |
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