@article{21485, abstract = {Insulating oxides are among the most abundant solid materials in the universe1,2,3. Of the many ways in which they influence natural phenomena, perhaps the most consequential is their capacity to transfer electrical charge during contact4,5,6,7,8,9,10—which occurs even between samples of the same oxide—yet the symmetry-breaking parameter that causes this remains unidentified11,12. Here we show that adventitious carbonaceous molecules adsorbed from the environment are the symmetry-breaking factor in same-material oxide contact electrification (CE). We use acoustic levitation to measure charge exchange between a sphere and a plate composed of identical amorphous silicon dioxide (SiO2). Although charging polarity is random for co-prepared samples, we control it with baking or plasma treatment. Observing the charge-exchange relaxation afterwards, we see dynamics over a timescale of hours and connect this directly to the presence of adventitious carbon with time-of-flight mass spectrometry, low-energy ion scattering and infrared spectroscopy. Going further, we confirm that adventitious carbon can even determine charge exchange among different oxides. Our results identify the symmetry-breaking parameter that causes insulating oxides to exchange charge in settings ranging from desert sands4 to volcanic plumes5,6, while simultaneously highlighting an overlooked factor in CE more broadly.}, author = {Grosjean, Galien M and Ostermann, Markus and Sauer, Markus and Hahn, Michael and Pichler, Christian M. and Fahrnberger, Florian and Pertl, Felix and Balazs, Daniel and Link, Mason M. and Kim, Seong H. and Schrader, Devin L. and Blanco, Adriana and Gracia, Francisco and Mujica, Nicolás and Waitukaitis, Scott R}, issn = {1476-4687}, journal = {Nature}, number = {8106}, pages = {626--631}, publisher = {Springer Nature}, title = {{Adventitious carbon breaks symmetry in oxide contact electrification}}, doi = {10.1038/s41586-025-10088-w}, volume = {651}, year = {2026}, } @article{20295, abstract = {Scanning Kelvin probe microscopy (SKPM) is a powerful technique for macroscopic imaging of the electrostatic potential above a surface. Though most often used to image work-function variations of conductive surfaces, it can also be used to probe the surface charge on insulating surfaces. In both cases, relating the measured potential to the underlying signal is non-trivial. Here, general relationships are derived between the measured SKPM voltage and the underlying source, revealing either can be cast as a convolution with an appropriately scaled point spread function (PSF). For charge that exists on a thin insulating layer above a conductor, the PSF has the same shape as what would occur from a work-function variation alone, differing by a simple scaling factor. This relationship is confirmed by: (1) backing it out from finite-element simulations of work-function and charge signals, and (2) experimentally comparing the measured PSF from a small work-function target to that from a small charge spot. This scaling factor is further validated by comparing SKPM charge measurements with Faraday cup measurements for highly charged samples from contact-charging experiments. These results highlight a heretofore unappreciated connection between SKPM voltage and charge signals, offering a rigorous recipe to extract either from experimental data.}, author = {Lenton, Isaac C and Pertl, Felix and Shafeek, Lubuna B and Waitukaitis, Scott R}, issn = {2196-7350}, journal = {Advanced Materials Interfaces}, number = {19}, publisher = {Wiley}, title = {{A duality between surface charge and work function in scanning Kelvin probe microscopy}}, doi = {10.1002/admi.202500521}, volume = {12}, year = {2025}, } @article{20481, abstract = {Kelvin probe force microscopy (KPFM) is widely used in stationary and dynamic studies of contact electrification. An obvious question that connects these two has been overlooked: when are charge dynamics too fast for stationary studies to be meaningful? Using a rapid transfer system to quickly perform KPFM after contact, we find the dynamics are too fast in all but the best insulators. Our data further suggest that dynamics are caused by bulk as opposed to surface conductivity, and that charge-transfer heterogeneity is less prevalent than previously suggested.}, author = {Pertl, Felix and Lenton, Isaac C and Cramer, Tobias and Waitukaitis, Scott R}, issn = {1079-7114}, journal = {Physical Review Letters}, number = {14}, publisher = {American Physical Society}, title = {{No time for surface charge: How bulk conductivity hides charge patterns from Kelvin probe force microscopy in contact-electrified surfaces}}, doi = {10.1103/lcsm-xxty}, volume = {135}, year = {2025}, } @misc{20523, abstract = {Includes all data and Python code needed to reproduce figures for the publication: No Time for Surface Charge: How Bulk Conductivity Hides Charge Patterns from Kelvin Probe Force Microscopy in Contact-Electrified Surfaces.}, author = {Pertl, Felix}, publisher = {Zenodo}, title = {{No Time for Surface Charge: How Bulk Conductivity Hides Charge Patterns from Kelvin Probe Force Microscopy in Contact-Electrified Surfaces}}, doi = {10.5281/ZENODO.14888054}, year = {2025}, } @article{20847, abstract = {We report on an experimental active matter system with motion restricted to four cardinal directions. Our particles are magnetite-doped colloidal spheres driven by the Quincke electrorotational instability. The absence of a magnetic field (|𝑩|=0) leads to circular trajectories interspersed with short spontaneous runs. Intermediate fields (|𝑩|≲20mT) linearize the motion along the axis perpendicular to 𝑩. At high magnetic fields, we observe the surprising emergence of a second, distinct linearization along the axis parallel to 𝑩. With numerical simulations, we show that this behavior can be explained by anisotropic magnetic susceptibility.}, author = {Fitzgerald, Eavan and Clavaud, Cécile and Das, Debasish and Lenton, Isaac C and Waitukaitis, Scott R}, issn = {2470-0053}, journal = {Physical Review E}, number = {6}, publisher = {American Physical Society}, title = {{Rolling at right angles: Magnetic anisotropy enables dual-anisotropic active matter}}, doi = {10.1103/1ss8-31rb}, volume = {112}, year = {2025}, } @article{20705, abstract = {Optical tweezers are widely used as a highly sensitive tool to measure forces on micron-scale particles. One such application is the measurement of the electric charge of a particle, which can be done with high precision in liquids, air, or vacuum. We experimentally investigate how the trapping laser itself can electrically charge such a particle, in our case a ∼1  μ⁢m SiO2 sphere in air. We model the charging mechanism as a two-photon process which reproduces the experimental data with high fidelity.}, author = {Stöllner, Andrea and Lenton, Isaac C and Volosniev, Artem and Millen, James and Shibuya, Renjiro and Ishii, Hisao and Rak, Dmytro and Alpichshev, Zhanybek and David, Grégory and Signorell, Ruth and Muller, Caroline J and Waitukaitis, Scott R}, issn = {1079-7114}, journal = {Physical Review Letters}, number = {21}, publisher = {American Physical Society}, title = {{Using optical tweezers to simultaneously trap, charge, and measure the charge of a microparticle in air}}, doi = {10.1103/5xd9-4tjj}, volume = {135}, year = {2025}, } @article{19278, abstract = {When two insulating, neutral materials are contacted and separated, they exchange electrical charge1. Experiments have long suggested that this ‘contact electrification’ is transitive, with different materials ordering into ‘triboelectric series’ based on the sign of charge acquired2. At the same time, the effect is plagued by unpredictability, preventing consensus on the mechanism and casting doubt on the rhyme and reason that series imply3. Here we expose an unanticipated connection between the unpredictability and order in contact electrification: nominally identical materials initially exchange charge randomly and intransitively, but—over repeated experiments—order into triboelectric series. We find that this evolution is driven by the act of contact itself—samples with more contacts in their history charge negatively to ones with fewer contacts. Capturing this ‘contact bias’ in a minimal model, we recreate both the initial randomness and ultimate order in numerical simulations and use it experimentally to force the appearance of a triboelectric series of our choosing. With a set of surface-sensitive techniques to search for the underlying alterations contact creates, we only find evidence of nanoscale morphological changes, pointing to a mechanism strongly coupled with mechanics. Our results highlight the centrality of contact history in contact electrification and suggest that focusing on the unpredictability that has long plagued the effect may hold the key to understanding it.}, author = {Sobarzo Ponce, Juan Carlos A and Pertl, Felix and Balazs, Daniel and Costanzo, Tommaso and Sauer, Markus and Foelske, Annette and Ostermann, Markus and Pichler, Christian M. and Wang, Yongkang and Nagata, Yuki and Bonn, Mischa and Waitukaitis, Scott R}, issn = {1476-4687}, journal = {Nature}, number = {8051}, publisher = {Springer Nature}, title = {{Spontaneous ordering of identical materials into a triboelectric series}}, doi = {10.1038/s41586-024-08530-6}, volume = {638}, year = {2025}, } @phdthesis{20203, abstract = {Tribocharging, or contact electrification, is the phenomenon in which two initially neutral materials exchange electric charge through contact and subsequent separation. While it is widely observed in everyday life and crucial to numerous natural processes, even the most basic aspects of tribocharging are still a mystery—what are the charge carriers involved and what drives their exchange? This work spans three separate projects that address different aspects of tribocharging. First, we introduce a novel strategy combining Finite Element Method (FEM) simulations with Kelvin Probe Force Microscopy (KPFM) to quantitatively extract surface charge density from surface voltage maps. Second, we present a simple theoretical model that allows for the existence of triboelectric cycles, under the assumption that multiple charge carrying species are involved. Third, we present experimental evidence that identical materials can spontaneously evolve into a triboelectric series, driven by contact history. Modeling this behavior enables the replication of experimental results with simulations, and even experimentally forcing the appearance of a pre-designed series by manipulating contact history. Together, the findings from these projects challenge traditional views on tribocharging, provide new tools for probing it, and open up new avenues of research—all with the hopes of bringing us closer to understanding this puzzling phenomenon.}, author = {Sobarzo Ponce, Juan Carlos A}, isbn = {978-3-99078-062-6}, issn = {2663-337X}, pages = {96}, publisher = {Institute of Science and Technology Austria}, title = {{Tribocharging of identical insulators : Triboelectric series, triboelectric cycles and surface charges}}, doi = {10.15479/AT-ISTA-20203}, year = {2025}, } @article{17373, abstract = {Scanning Kelvin probe microscopy (SKPM) is a powerful technique for investigating the electrostatic properties of material surfaces, enabling the imaging of variations in work function, topology, surface charge density, or combinations thereof. Regardless of the underlying signal source, SKPM results in a voltage image, which is spatially distorted due to the finite size of the probe, long-range electrostatic interactions, mechanical and electrical noise, and the finite response time of the electronics. In order to recover the underlying signal, it is necessary to deconvolve the measurement with an appropriate point spread function (PSF) that accounts the aforementioned distortions, but determining this PSF is difficult. Here, we describe how such PSFs can be determined experimentally and show how they can be used to recover the underlying information of interest. We first consider the physical principles that enable SKPM and discuss how these affect the system PSF. We then show how one can experimentally measure PSFs by looking at well-defined features, and that these compare well to simulated PSFs, provided scans are performed extremely slowly and carefully. Next, we work at realistic scan speeds and show that the idealized PSFs fail to capture temporal distortions in the scan direction. While simulating PSFs for these situations would be quite challenging, we show that measuring PSFs with similar scan conditions works well. Our approach clarifies the basic principles and inherent challenges to SKPM measurements and gives practical methods to improve results.}, author = {Lenton, Isaac C and Pertl, Felix and Shafeek, Lubuna B and Waitukaitis, Scott R}, issn = {1089-7550}, journal = {Journal of Applied Physics}, number = {4}, publisher = {AIP Publishing}, title = {{Beyond the blur: Using experimentally determined point spread functions to improve scanning Kelvin probe imaging}}, doi = {10.1063/5.0215151}, volume = {136}, year = {2024}, } @article{12697, abstract = {Models for same-material contact electrification in granular media often rely on a local charge-driving parameter whose spatial variations lead to a stochastic origin for charge exchange. Measuring the charge transfer from individual granular spheres after contacts with substrates of the same material, we find instead a “global” charging behavior, coherent over the sample’s whole surface. Cleaning and baking samples fully resets charging magnitude and direction, which indicates the underlying global parameter is not intrinsic to the material, but acquired from its history. Charging behavior is randomly and irreversibly affected by changes in relative humidity, hinting at a mechanism where adsorbates, in particular, water, are fundamental to the charge-transfer process.}, author = {Grosjean, Galien M and Waitukaitis, Scott R}, issn = {1079-7114}, journal = {Physical Review Letters}, keywords = {General Physics, Electrostatics, Triboelectricity, Soft Matter, Acoustic Levitation, Granular Materials}, number = {9}, publisher = {American Physical Society}, title = {{Single-collision statistics reveal a global mechanism driven by sample history for contact electrification in granular media}}, doi = {10.1103/physrevlett.130.098202}, volume = {130}, year = {2023}, } @article{12789, abstract = {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.}, author = {Mujica, Nicolás and Waitukaitis, Scott R}, issn = {2470-0053}, journal = {Physical Review E}, number = {3}, publisher = {American Physical Society}, title = {{Accurate determination of the shapes of granular charge distributions}}, doi = {10.1103/PhysRevE.107.034901}, volume = {107}, year = {2023}, } @article{13197, abstract = {Nominally identical materials exchange net electric charge during contact through a mechanism that is still debated. ‘Mosaic models’, in which surfaces are presumed to consist of a random patchwork of microscopic donor/acceptor sites, offer an appealing explanation for this phenomenon. However, recent experiments have shown that global differences persist even between same-material samples, which the standard mosaic framework does not account for. Here, we expand the mosaic framework by incorporating global differences in the densities of donor/acceptor sites. We develop an analytical model, backed by numerical simulations, that smoothly connects the global and deterministic charge transfer of different materials to the local and stochastic mosaic picture normally associated with identical materials. Going further, we extend our model to explain the effect of contact asymmetries during sliding, providing a plausible explanation for reversal of charging sign that has been observed experimentally.}, author = {Grosjean, Galien M and Waitukaitis, Scott R}, issn = {2475-9953}, journal = {Physical Review Materials}, keywords = {Physics and Astronomy (miscellaneous), General Materials Science}, number = {6}, publisher = {American Physical Society}, title = {{Asymmetries in triboelectric charging: Generalizing mosaic models to different-material samples and sliding contacts}}, doi = {10.1103/physrevmaterials.7.065601}, volume = {7}, year = {2023}, } @inproceedings{14864, author = {Stöllner, Andrea and Lenton, Isaac C and Muller, Caroline J and Waitukaitis, Scott R}, booktitle = {EGU General Assembly 2023}, location = {Vienna, Austria & Virtual}, publisher = {European Geosciences Union}, title = {{Measuring spontaneous charging of single aerosol particles}}, doi = {10.5194/egusphere-egu23-6166}, year = {2023}, } @article{12109, abstract = {Kelvin probe force microscopy (KPFM) is a powerful tool for studying contact electrification (CE) at the nanoscale, but converting KPFM voltage maps to charge density maps is nontrivial due to long-range forces and complex system geometry. Here we present a strategy using finite-element method (FEM) simulations to determine the Green's function of the KPFM probe/insulator/ground system, which allows us to quantitatively extract surface charge. Testing our approach with synthetic data, we find that accounting for the atomic force microscope (AFM) tip, cone, and cantilever is necessary to recover a known input and that existing methods lead to gross miscalculation or even the incorrect sign of the underlying charge. Applying it to experimental data, we demonstrate its capacity to extract realistic surface charge densities and fine details from contact-charged surfaces. Our method gives a straightforward recipe to convert qualitative KPFM voltage data into quantitative charge data over a range of experimental conditions, enabling quantitative CE at the nanoscale.}, author = {Pertl, Felix and Sobarzo Ponce, Juan Carlos A and Shafeek, Lubuna B and Cramer, Tobias and Waitukaitis, Scott R}, issn = {2475-9953}, journal = {Physical Review Materials}, number = {12}, publisher = {American Physical Society}, title = {{Quantifying nanoscale charge density features of contact-charged surfaces with an FEM/KPFM-hybrid approach}}, doi = {10.1103/PhysRevMaterials.6.125605}, volume = {6}, year = {2022}, }