TY - JOUR AB - 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. AU - Grosjean, Galien M AU - Waitukaitis, Scott R ID - 13197 IS - 6 JF - Physical Review Materials KW - Physics and Astronomy (miscellaneous) KW - General Materials Science SN - 2475-9953 TI - Asymmetries in triboelectric charging: Generalizing mosaic models to different-material samples and sliding contacts VL - 7 ER - TY - JOUR AB - 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. AU - Grosjean, Galien M AU - Waitukaitis, Scott R ID - 12697 IS - 9 JF - Physical Review Letters KW - General Physics KW - Electrostatics KW - Triboelectricity KW - Soft Matter KW - Acoustic Levitation KW - Granular Materials SN - 0031-9007 TI - Single-collision statistics reveal a global mechanism driven by sample history for contact electrification in granular media VL - 130 ER - TY - JOUR AB - The elastic Leidenfrost effect occurs when a vaporizable soft solid is lowered onto a hot surface. Evaporative flow couples to elastic deformation, giving spontaneous bouncing or steady-state floating. The effect embodies an unexplored interplay between thermodynamics, elasticity, and lubrication: despite being observed, its basic theoretical description remains a challenge. Here, we provide a theory of elastic Leidenfrost floating. As weight increases, a rigid solid sits closer to the hot surface. By contrast, we discover an elasticity-dominated regime where the heavier the solid, the higher it floats. This geometry-governed behavior is reminiscent of the dynamics of large liquid Leidenfrost drops. We show that this elastic regime is characterized by Hertzian behavior of the solid’s underbelly and derive how the float height scales with materials parameters. Introducing a dimensionless elastic Leidenfrost number, we capture the crossover between rigid and Hertzian behavior. Our results provide theoretical underpinning for recent experiments, and point to the design of novel soft machines. AU - Binysh, Jack AU - Chakraborty, Indrajit AU - Chubynsky, Mykyta V. AU - Diaz Melian, Vicente L AU - Waitukaitis, Scott R AU - Sprittles, James E. AU - Souslov, Anton ID - 14514 IS - 16 JF - Physical Review Letters SN - 0031-9007 TI - Modeling Leidenfrost levitation of soft elastic solids VL - 131 ER - TY - GEN AB - see Readme file AU - Binysh, Jack AU - Chakraborty, Indrajit AU - Chubynsky, Mykyta AU - Diaz Melian, Vicente L AU - Waitukaitis, Scott R AU - Sprittles, James AU - Souslov, Anton ID - 14523 TI - SouslovLab/PRL2023-ModellingLeidenfrostLevitationofSoftElasticSolids: v1.0.1 ER - TY - JOUR AB - 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. AU - Mujica, Nicolás AU - Waitukaitis, Scott R ID - 12789 IS - 3 JF - Physical Review E SN - 2470-0045 TI - Accurate determination of the shapes of granular charge distributions VL - 107 ER - TY - GEN AU - Stöllner, Andrea AU - Lenton, Isaac C AU - Muller, Caroline J AU - Waitukaitis, Scott R ID - 14864 T2 - EGU General Assembly 2023 TI - Measuring spontaneous charging of single aerosol particles ER - TY - JOUR AB - When a cylindrical object penetrates granular matter near a vertical boundary, it experiences two effects: its center of mass moves horizontally away from the wall, and it rotates around its symmetry axis. Here we show experimentally that, if two identical intruders instead of one are released side-by-side near the wall, both effects are also detected. However, unexpected phenomena appear due to a cooperative dynamics between the intruders. The net horizontal distance traveled by the common center of mass of the twin intruders is much larger than that traveled by one intruder released at the same initial distance from the wall, and the rotation is also larger. The experimental results are well described by the Discrete Element Method (DEM), which reveals that, as the number of intruders horizontally released side-by-side increases, the total energy dissipation per intruder decreases. Finally, DEM simulations demonstrate that the horizontal repulsion is substantially enhanced if groups of intruders are released forming a column near the wall. AU - Espinosa, M. AU - Diaz Melian, Vicente L AU - Serrano-Muñoz, A. AU - Altshuler, E. ID - 10733 IS - 1 JF - Granular Matter KW - granular matter KW - boundary effects KW - intruder penetration KW - sedimentation SN - 1434-5021 TI - Intruders cooperatively interact with a wall into granular matter VL - 24 ER - TY - JOUR AB - 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. AU - Pertl, Felix AU - Sobarzo Ponce, Juan Carlos A AU - Shafeek, Lubuna B AU - Cramer, Tobias AU - Waitukaitis, Scott R ID - 12109 IS - 12 JF - Physical Review Materials TI - Quantifying nanoscale charge density features of contact-charged surfaces with an FEM/KPFM-hybrid approach VL - 6 ER - TY - JOUR AB - The dynamics of a triangular magnetocapillary swimmer is studied using the lattice Boltzmann method. We extend on our previous work, which deals with the self-assembly and a specific type of the swimmer motion characterized by the swimmer’s maximum velocity centred around the particle’s inverse viscous time. Here, we identify additional regimes of motion. First, modifying the ratio of surface tension and magnetic forces allows to study the swimmer propagation in the regime of significantly lower frequencies mainly defined by the strength of the magnetocapillary potential. Second, introducing a constant magnetic contribution in each of the particles in addition to their magnetic moment induced by external fields leads to another regime characterized by strong in-plane swimmer reorientations that resemble experimental observations. AU - Sukhov, Alexander AU - Hubert, Maxime AU - Grosjean, Galien M AU - Trosman, Oleg AU - Ziegler, Sebastian AU - Collard, Ylona AU - Vandewalle, Nicolas AU - Smith, Ana Sunčana AU - Harting, Jens ID - 9411 IS - 4 JF - European Physical Journal E SN - 12928941 TI - Regimes of motion of magnetocapillary swimmers VL - 44 ER - TY - JOUR AB - When tiny soft ferromagnetic particles are placed along a liquid interface and exposed to a vertical magnetic field, the balance between capillary attraction and magnetic repulsion leads to self-organization into well-defined patterns. Here, we demonstrate experimentally that precessing magnetic fields induce metachronal waves on the periphery of these assemblies, similar to the ones observed in ciliates and some arthropods. The outermost layer of particles behaves like an array of cilia or legs whose sequential movement causes a net and controllable locomotion. This bioinspired many-particle swimming strategy is effective even at low Reynolds number, using only spatially uniform fields to generate the waves. AU - Collard, Ylona AU - Grosjean, Galien M AU - Vandewalle, Nicolas ID - 8036 JF - Communications Physics TI - Magnetically powered metachronal waves induce locomotion in self-assemblies VL - 3 ER - TY - JOUR AB - By rigorously accounting for mesoscale spatial correlations in donor/acceptor surface properties, we develop a scale-spanning model for same-material tribocharging. We find that mesoscale correlations affect not only the magnitude of charge transfer but also the fluctuations—suppressing otherwise overwhelming charge-transfer variability that is not observed experimentally. We furthermore propose a generic theoretical mechanism by which the mesoscale features might emerge, which is qualitatively consistent with other proposals in the literature. AU - Grosjean, Galien M AU - Wald, Sebastian AU - Sobarzo Ponce, Juan Carlos A AU - Waitukaitis, Scott R ID - 8101 IS - 8 JF - Physical Review Materials KW - electric charge KW - tribocharging KW - soft matter KW - granular materials KW - polymers SN - 2475-9953 TI - Quantitatively consistent scale-spanning model for same-material tribocharging VL - 4 ER - TY - JOUR AB - When grape-sized aqueous dimers are irradiated in a microwave oven, an intense electromagnetic hotspot forms at their point of contact, often igniting a plasma. Here we show that this irradiation can result in the injection of mechanical energy. By examining irradiated hydrogel dimers through high-speed imaging, we find that they repeatedly bounce off of each other while irradiated. We determine that an average of 1 lJ of mechanical energy is injected into the pair during each collision. Furthermore, a characteristic high-pitched audio signal is found to accompany each collision. We show that both the audio signal and the energy injection arise via an interplay between vaporization and elastic deformations in the region of contact, the so-called ‘elastic Liedenfrost effect’. Our results establish a novel, non-contact method of injecting mechanical energy into soft matter systems, suggesting application in fields such as soft robotics. AU - Khattak, Hamza K. AU - Waitukaitis, Scott R AU - Slepkov, Aaron D. ID - 6763 IS - 29 JF - Soft Matter SN - 1744683X TI - Microwave induced mechanical activation of hydrogel dimers VL - 15 ER -