@phdthesis{22684,
  abstract     = {Contact electrification (CE) is a simple yet elusive phenomenon that occurs when two materials come into contact and separate, leaving behind net electrical charge. Despite its ubiquity, the microscopic origin of CE remains unclear. In this thesis, we investigate CE from three complementary perspectives: developing a quantitative method to measure charge at the nanoscale, exploring the dynamic behavior of charge on insulating surfaces, and uncovering the role of mechanical history in forming a triboelectric series.

In the first part, we establish a rigorous framework for converting qualitative Kelvin probe force microscopy (KPFM) voltage maps into quantitative charge density distributions. Using finite element method (FEM) simulations, we determine the point-spread function of the KPFM tip–sample geometry and demonstrate that the true surface charge can be reconstructed by numerical deconvolution. This procedure enables the recovery of both the magnitude and sign of charge density with high fidelity, resolving nanoscale features that are otherwise obscured. Applying the method to contact-charged SiO$_2$ surfaces, we show that existing analytical approximations, such as parallel plate or spherical models, can miscalculate charge magnitude by orders of magnitude. Our hybrid FEM/KPFM approach therefore provides a fast and general method to convert qualitative KPFM signals into quantitative charge data, enabling nanoscale charge mapping under realistic experimental conditions.

In the second part, we study the temporal stability of CE-induced charges and identify the key material factors that determine whether KPFM can capture meaningful charge patterns. Through time-resolved experiments combining a custom-built transfer system with both microscopic and macroscopic measurements, we demonstrate that only the best insulators, such as SiO$_2$, preserve CE charge long enough for stationary imaging. For less conductive polymers, such as PDMS, charge decays within the duration of a single KPFM scan due to bulk conduction. Using a simple capacitor-based model, we reproduce the observed decay dynamics and confirm that the transferred charge decays characteristic to the sample's bulk conductivity. Further, we always observe homogeneous charge transfer.

In the third part, we address the question: can we form a triboelectric series with identical materials? Using controlled repetitive contact experiments, we show that nominally identical materials can progressively order themselves into a triboelectric series, where surfaces with more contact history charge negatively relative to fresher ones. By constructing a minimal model based on this ``contact bias'', we replicate the evolution from random to ordered charging observed in experiments. Supporting surface analyses, including atomic force microscopy, reveal that repeated contact induces nanoscale morphological changes, suggesting a mechanism tightly coupled to mechanical strain. These results highlight the crucial role of surface history and nanoscale mechanics in dictating charge transfer, motivating further exploration of mechanisms such as mechanochemical bond cleavage and flexoelectric polarization.},
  author       = {Pertl, Felix},
  isbn         = {978-3-99078-083-1},
  issn         = {2663-337X},
  pages        = {107},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Experimental probing of nanoscale charge features and surface morphology changes during tribocharging}},
  doi          = {10.15479/AT-ISTA-22684},
  year         = {2026},
}

@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},
}

@phdthesis{18588,
  abstract     = {This thesis is an experimental work about two distinct research projects that evolved from a single project: non-equilibrium dynamics of an acoustically vibrated particle and microfabrication of particles with nano-scale 3D printing. The first project explores non equilibrium dynamics of a particle driven by ultrasonic vibrations. We design an experimental system consisting of an electromechanical vibration scheme to drive the particle’s vibrations and an imaging scheme to track its trajectories. We study the trajectories to determine how the particle’s dynamics evolve under the driven conditions, considering out of equilibrium systems in the context of equilibrium statistical mechanics. Using a Langevin framework and the Boltzmann factor, we characterize the particle’s dynamics as complex; the particle motion
is not purely diffusive. We extract physical parameters like spring constant, effective temperature, damping coefficient and resonance frequency.

In the second project, we explore and develop techniques in the design and microfabrication of particles across scales. Microfabrication involves building structures at the micron or submicron scale. These designed miniaturized patterns, objects, or devices are useful in biophysics, pharmacology, medical biology, and nanotechnology. We specifically apply two-photon polymerization, a form of 3D nano printing. We print millimetric particles, characterizing different designs to evaluate and showcase the resolution, aspect ratio integrity and print quality of the printing process. We also design and fabricate a microsensor to deflect under applicable force of order 0.1 pN. We present fundamental concepts needed to design the microsensor, showcasing 3D printing at considerably smaller scales down to the µm or below.},
  author       = {Mweka, Cecelia N},
  issn         = {2791-4585},
  pages        = {61},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Non equilibrium dynamics of driven individual particles and 3D printing across scales}},
  doi          = {10.15479/at:ista:18588},
  year         = {2024},
}