@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}, } @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{14514, abstract = {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.}, author = {Binysh, Jack and Chakraborty, Indrajit and Chubynsky, Mykyta V. and Diaz Melian, Vicente L and Waitukaitis, Scott R and Sprittles, James E. and Souslov, Anton}, issn = {1079-7114}, journal = {Physical Review Letters}, number = {16}, publisher = {American Physical Society}, title = {{Modeling Leidenfrost levitation of soft elastic solids}}, doi = {10.1103/PhysRevLett.131.168201}, volume = {131}, year = {2023}, } @misc{14523, abstract = {see Readme file}, author = {Binysh, Jack and Chakraborty, Indrajit and Chubynsky, Mykyta and Diaz Melian, Vicente L and Waitukaitis, Scott R and Sprittles, James and Souslov, Anton}, publisher = {Zenodo}, title = {{SouslovLab/PRL2023-ModellingLeidenfrostLevitationofSoftElasticSolids: v1.0.1}}, doi = {10.5281/ZENODO.8329143}, 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}, } @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}, } @article{6976, abstract = {Origami is rapidly transforming the design of robots1,2, deployable structures3,4,5,6 and metamaterials7,8,9,10,11,12,13,14. However, as foldability requires a large number of complex compatibility conditions that are difficult to satisfy, the design of crease patterns is limited to heuristics and computer optimization. Here we introduce a systematic strategy that enables intuitive and effective design of complex crease patterns that are guaranteed to fold. First, we exploit symmetries to construct 140 distinct foldable motifs, and represent these as jigsaw puzzle pieces. We then show that when these pieces are fitted together they encode foldable crease patterns. This maps origami design to solving combinatorial problems, which allows us to systematically create, count and classify a vast number of crease patterns. We show that all of these crease patterns are pluripotent—capable of folding into multiple shapes—and solve exactly for the number of possible shapes for each pattern. Finally, we employ our framework to rationally design a crease pattern that folds into two independently defined target shapes, and fabricate such pluripotent origami. Our results provide physicists, mathematicians and engineers with a powerful new design strategy.}, author = {Dieleman, Peter and Vasmel, Niek and Waitukaitis, Scott R and van Hecke, Martin}, issn = {1745-2481}, journal = {Nature Physics}, number = {1}, pages = {63–68}, publisher = {Springer Nature}, title = {{Jigsaw puzzle design of pluripotent origami}}, doi = {10.1038/s41567-019-0677-3}, volume = {16}, year = {2020}, } @article{8101, abstract = {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.}, author = {Grosjean, Galien M and Wald, Sebastian and Sobarzo Ponce, Juan Carlos A and Waitukaitis, Scott R}, issn = {2475-9953}, journal = {Physical Review Materials}, keywords = {electric charge, tribocharging, soft matter, granular materials, polymers}, number = {8}, publisher = {American Physical Society}, title = {{Quantitatively consistent scale-spanning model for same-material tribocharging}}, doi = {10.1103/PhysRevMaterials.4.082602}, volume = {4}, year = {2020}, } @article{6763, abstract = {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.}, author = {Khattak, Hamza K. and Waitukaitis, Scott R and Slepkov, Aaron D.}, issn = {17446848}, journal = {Soft Matter}, number = {29}, pages = {5804--5809}, publisher = {Royal Society of Chemistry}, title = {{Microwave induced mechanical activation of hydrogel dimers}}, doi = {10.1039/c9sm00756c}, volume = {15}, year = {2019}, } @article{124, abstract = {By investigating the in situ chemical and O-isotope compositions of olivine in lightly sintered dust agglomerates from the early Solar System, we constrain their origins and the retention of dust in the protoplanetary disk. The grain sizes of silicates in these agglomeratic olivine (AO) chondrules indicate that the grain sizes of chondrule precursors in the Renazzo-like carbonaceous (CR) chondrites ranged from <1 to 80 µm. We infer this grain size range to be equivalent to the size range for dust in the early Solar System. AO chondrules may contain, but are not solely composed of, recycled fragments of earlier formed chondrules. They also contain 16O-rich olivine related to amoeboid olivine aggregates and represent the best record of chondrule-precursor materials. AO chondrules contain one or more large grains, sometimes similar to FeO-poor (type I) and/or FeO-rich (type II) chondrules, while others contain a type II chondrule core. These morphologies are consistent with particle agglomeration by electrostatic charging of grains during collision, a process that may explain solid agglomeration in the protoplanetary disk in the micrometer size regime. The petrographic, isotopic, and chemical compositions of AO chondrules are consistent with chondrule formation by large-scale shocks, bow shocks, and current sheets. The petrographic, isotopic, and chemical similarities between AO chondrules in CR chondrites and chondrule-like objects from comet 81P/Wild 2 indicate that comets contain AO chondrules. We infer that these AO chondrules likely formed in the inner Solar System and migrated to the comet forming region at least 3 Ma after the formation of the first Solar System solids. Observations made in this study imply that the protoplanetary disk retained a dusty disk at least ∼3.7 Ma after the formation of the first Solar System solids, longer than half of the dusty accretion disks observed around other stars.}, author = {Waitukaitis, Scott R and Schrader, Devin and Nagashima, Kazuhide and Davidson, Jemma and Mccoy, Timothy and Conolly Jr, Harold and Lauretta, Dante}, journal = {Geochimica et Cosmochimica Acta}, pages = {405 -- 421}, publisher = {Elsevier}, title = {{The retention of dust in protoplanetary disks: evidence from agglomeration olivine chondrules from the outer solar system}}, doi = {10.1016/j.gca.2017.12.014}, volume = {223}, year = {2018}, } @article{125, abstract = {Many fields of study, including medical imaging, granular physics, colloidal physics, and active matter, require the precise identification and tracking of particle-like objects in images. While many algorithms exist to track particles in diffuse conditions, these often perform poorly when particles are densely packed together—as in, for example, solid-like systems of granular materials. Incorrect particle identification can have significant effects on the calculation of physical quantities, which makes the development of more precise and faster tracking algorithms a worthwhile endeavor. In this work, we present a new tracking algorithm to identify particles in dense systems that is both highly accurate and fast. We demonstrate the efficacy of our approach by analyzing images of dense, solid-state granular media, where we achieve an identification error of 5% in the worst evaluated cases. Going further, we propose a parallelization strategy for our algorithm using a GPU, which results in a speedup of up to 10× when compared to a sequential CPU implementation in C and up to 40× when compared to the reference MATLAB library widely used for particle tracking. Our results extend the capabilities of state-of-the-art particle tracking methods by allowing fast, high-fidelity detection in dense media at high resolutions.}, author = {Cerda, Mauricio and Waitukaitis, Scott R and Navarro, Cristóbal and Silva, Juan and Mujica, Nicolás and Hitschfeld, Nancy}, journal = {Computer Physics Communications}, pages = {8 -- 16}, publisher = {Elsevier}, title = {{A high-speed tracking algorithm for dense granular media}}, doi = {10.1016/j.cpc.2018.02.010}, volume = {227}, year = {2018}, } @article{126, abstract = {The Leidenfrost effect occurs when a liquid or stiff sublimable solid near a hot surface creates enough vapor beneath it to lift itself up and float. In contrast, vaporizable soft solids, e.g., hydrogels, have been shown to exhibit persistent bouncing - the elastic Leidenfrost effect. By carefully lowering hydrogel spheres towards a hot surface, we discover that they are also capable of floating. The bounce-to-float transition is controlled by the approach velocity and temperature, analogously to the "dynamic Leidenfrost effect." For the floating regime, we measure power-law scalings for the gap geometry, which we explain with a model that couples the vaporization rate to the spherical shape. Our results reveal that hydrogels are a promising pathway for controlling floating Leidenfrost objects through shape.}, author = {Waitukaitis, Scott R and Harth, Kirsten and Van Hecke, Martin}, journal = {Physical Review Letters}, number = {4}, publisher = {American Physical Society}, title = {{From bouncing to floating: the Leidenfrost effect with hydrogel spheres}}, doi = {10.1103/PhysRevLett.121.048001}, volume = {121}, year = {2018}, } @article{127, abstract = {The ideas of topology are breaking ground in origami-based metamaterials. Experiments now show that certain shapes — doughnuts included — exhibit topological bistability, and can be made to click between different topologically stable states.}, author = {Waitukaitis, Scott R}, journal = {Nature Physics}, number = {8}, pages = {777 -- 778}, publisher = {Nature Publishing Group}, title = {{Clicks for doughnuts}}, doi = {10.1038/s41567-018-0160-6}, volume = {14}, year = {2018}, } @article{95, abstract = {Electrostatic charging of insulating fine particles can be responsible for numerous phenomena ranging from lightning in volcanic plumes to dust explosions. However, even basic aspects of how fine particles become charged are still unclear. Studying particle charging is challenging because it usually involves the complexities associated with many-particle collisions. To address these issues, we introduce a method based on acoustic levitation, which makes it possible to initiate sequences of repeated collisions of a single submillimeter particle with a flat plate, and to precisely measure the particle charge in situ after each collision. We show that collisional charge transfer between insulators is dependent on the hydrophobicity of the contacting surfaces. We use glass, which we modify by attaching nonpolar molecules to the particle, the plate, or both. We find that hydrophilic surfaces develop significant positive charges after contacting hydrophobic surfaces. Moreover, we demonstrate that charging between a hydrophilic and a hydrophobic surface is suppressed in an acidic environment and enhanced in a basic one. Application of an electric field during each collision is found to modify the charge transfer, again depending on surface hydrophobicity. We discuss these results within the context of contact charging due to ion transfer, and we show that they lend strong support to OH− ions as the charge carriers.}, author = {Lee, Victor and James, Nicole and Waitukaitis, Scott R and Jaeger, Heinrich}, journal = {Physical Review Materials}, number = {3}, publisher = {American Physical Society}, title = {{Collisional charging of individual submillimeter particles: Using ultrasonic levitation to initiate and track charge transfer}}, doi = {10.1103/PhysRevMaterials.2.035602}, volume = {2}, year = {2018}, } @article{123, abstract = {The Leidenfrost effect occurs when an object near a hot surface vaporizes rapidly enough to lift itself up and hover. Although well understood for liquids and stiff sublimable solids, nothing is known about the effect with materials whose stiffness lies between these extremes. Here we introduce a new phenomenon that occurs with vaporizable soft solids - the elastic Leidenfrost effect. By dropping hydrogel spheres onto hot surfaces we find that, rather than hovering, they energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, we uncover high-frequency microscopic gap dynamics at the sphere/substrate interface. We show how these otherwise-hidden agitations constitute work cycles that harvest mechanical energy from the vapour and sustain the bouncing. Our findings suggest a new strategy for injecting mechanical energy into a widely used class of soft materials, with potential relevance to fields such as active matter, soft robotics and microfluidics.}, author = {Waitukaitis, Scott R and Zuiderwijk, Antal and Souslov, Anton and Coulais, Corentin and Van Hecke, Martin}, journal = {Nature Physics}, number = {11}, pages = {1095 -- 1099}, publisher = {Nature Publishing Group}, title = {{Coupling the Leidenfrost effect and elastic deformations to power sustained bouncing}}, doi = {10.1038/nphys4194}, volume = {13}, year = {2017}, } @article{122, abstract = {Four rigid panels connected by hinges that meet at a point form a four-vertex, the fundamental building block of origami metamaterials. Most materials designed so far are based on the same four-vertex geometry, and little is known regarding how different geometries affect folding behavior. Here we systematically categorize and analyze the geometries and resulting folding motions of Euclidean four-vertices. Comparing the relative sizes of sector angles, we identify three types of generic vertices and two accompanying subtypes. We determine which folds can fully close and the possible mountain-valley assignments. Next, we consider what occurs when sector angles or sums thereof are set equal, which results in 16 special vertex types. One of these, flat-foldable vertices, has been studied extensively, but we show that a wide variety of qualitatively different folding motions exist for the other 15 special and 3 generic types. Our work establishes a straightforward set of rules for understanding the folding motion of both generic and special four-vertices and serves as a roadmap for designing origami metamaterials.}, author = {Waitukaitis, Scott R and Van Hecke, Martin}, journal = {Physical Review E - Statistical, Nonlinear, and Soft Matter Physics}, number = {2}, publisher = {American Physiological Society}, title = {{Origami building blocks: Generic and special four-vertices}}, doi = {10.1103/PhysRevE.93.023003}, volume = {93}, year = {2016}, } @article{120, abstract = {Clustering of fine particles is of crucial importance in settings ranging from the early stages of planet formation to the coagulation of industrial powders and airborne pollutants. Models of such clustering typically focus on inelastic deformation and cohesion. However, even in charge-neutral particle systems comprising grains of the same dielectric material, tribocharging can generate large amounts of net positive or negative charge on individual particles, resulting in long-range electrostatic forces. The effects of such forces on cluster formation are not well understood and have so far not been studied in situ. Here we report the first observations of individual collide-and-capture events between charged submillimetre particles, including Kepler-like orbits. Charged particles can become trapped in their mutual electrostatic energy well and aggregate via multiple bounces. This enables the initiation of clustering at relative velocities much larger than the upper limit for sticking after a head-on collision, a long-standing issue known from pre-planetary dust aggregation. Moreover, Coulomb interactions together with dielectric polarization are found to stabilize characteristic molecule-like configurations, providing new insights for the modelling of clustering dynamics in a wide range of microscopic dielectric systems, such as charged polarizable ions, biomolecules and colloids.}, author = {Lee, Victor and Waitukaitis, Scott R and Miskin, Marc and Jaeger, Heinrich}, journal = {Nature Physics}, number = {9}, pages = {733 -- 737}, publisher = {Nature Publishing Group}, title = {{Direct observation of particle interactions and clustering in charged granular streams}}, doi = {10.1038/nphys3396}, volume = {11}, year = {2015}, } @article{121, abstract = {We show that the simplest building blocks of origami-based materials - rigid, degree-four vertices - are generically multistable. The existence of two distinct branches of folding motion emerging from the flat state suggests at least bistability, but we show how nonlinearities in the folding motions allow generic vertex geometries to have as many as five stable states. In special geometries with collinear folds and symmetry, more branches emerge leading to as many as six stable states. Tuning the fold energy parameters, we show how monostability is also possible. Finally, we show how to program the stability features of a single vertex into a periodic fold tessellation. The resulting metasheets provide a previously unanticipated functionality - tunable and switchable shape and size via multistability.}, author = {Waitukaitis, Scott R and Menaut, Rémi and Chen, Bryan and Van Hecke, Martin}, journal = {APS Physics, Physical Review Letters}, number = {5}, publisher = {American Physical Society}, title = {{Origami multistability: From single vertices to metasheets}}, doi = {10.1103/PhysRevLett.114.055503}, volume = {114}, year = {2015}, } @article{118, abstract = {While the penetration of objects into granular media is well-studied, there is little understanding of how objects settle in gravities, geff, different from that of Earth - a scenario potentially relevant to the geomorphology of planets and asteroids and also to their exploration using man-made devices. By conducting experiments in an accelerating frame, we explore geff ranging from 0.4 g to 1.2 g. Surprisingly, we find that the rest depth is independent of geff and also that the time required for the object to come to rest scales like geff-1/2. With discrete element modeling simulations, we reproduce the experimental results and extend the range of geff to objects as small as asteroids and as large as Jupiter. Our results shed light on the initial stage of sedimentation into dry granular media across a range of celestial bodies and also have implications for the design of man-made, extraterrestrial vehicles and structures. Key Points The settling depth in granular media is independent of gravity The settling time scales like g-1/2 Layering driven by granular sedimentation should be similar.}, author = {Altshuler, Ernesto and Torres, H and González_Pita, A and Sánchez, Colina G and Pérez Penichet, Carlos and Waitukaitis, Scott R and Hidalgo, Rauól}, journal = {Geophysical Research Letters}, number = {9}, pages = {3032 -- 3037}, publisher = {Wiley-Blackwell}, title = {{Settling into dry granular media in different gravities}}, doi = {10.1002/2014GL059229}, volume = {41}, year = {2014}, } @article{119, abstract = {Observations of flowing granular matter have suggested that same-material tribocharging depends on particle size, typically rendering large grains positive and small ones negative. Models assuming the transfer of trapped electrons can account for this trend, but have not been validated. Tracking individual grains in an electric field, we show quantitatively that charge is transferred based on size between materially identical grains. However, the surface density of trapped electrons, measured independently by thermoluminescence techniques, is orders of magnitude too small to account for the scale of charge transferred. This reveals that trapped electrons are not a necessary ingredient for same-material tribocharging.}, author = {Waitukaitis, Scott R and Lee, Victor and Pierson, James and Forman, Steven and Jaeger, Heinrich}, journal = {APS Physics, Physical Review Letters}, number = {21}, publisher = {American Physical Society}, title = {{Size-dependent same-material tribocharging in insulating grains}}, doi = {10.1103/PhysRevLett.112.218001}, volume = {112}, year = {2014}, } @article{115, abstract = {We present the design and performance characterization of a new experimental technique for measuring individual particle charges in large ensembles of macroscopic grains. The measurement principle is qualitatively similar to that used in determining the elementary charge by Millikan in that it follows individual particle trajectories. However, by taking advantage of new technology we are able to work with macroscopic grains and achieve several orders of magnitude better resolution in charge to mass ratios. By observing freely falling grains accelerated in a horizontal electric field with a co-falling, high-speed video camera, we dramatically increase particle tracking time and measurement precision. Keeping the granular medium under vacuum, we eliminate air drag, leaving the electrostatic force as the primary source of particle accelerations in the co-moving frame. Because the technique is based on direct imaging, we can distinguish between different particle types during the experiment, opening up the possibility of studying charge transfer processes between different particle species. For the ∼300 μm diameter grains reported here, we achieve an average acceleration resolution of ∼0.008 m/s2, a force resolution of ∼500 pN, and a median charge resolution ∼6× 104 elementary charges per grain (corresponding to surface charge densities ∼1 elementary charges per μm2). The primary source of error is indeterminacy in the grain mass, but with higher resolution cameras and better optics this can be further improved. The high degree of resolution and the ability to visually identify particles of different species or sizes with direct imaging make this a powerful new tool to characterize charging processes in granular media.}, author = {Waitukaitis, Scott R and Jaeger, Heinrich}, journal = {Review of Scientific Instruments}, number = {2}, publisher = {AIP}, title = {{In situ granular charge measurement by free-fall videography}}, doi = {10.1063/1.4789496}, volume = {84}, year = {2013}, } @article{116, abstract = {We describe a model experiment for dynamic jamming: a two-dimensional collection of initially unjammed disks that are forced into the jammed state by uniaxial compression via a rake. This leads to a stable densification front that travels ahead of the rake, leaving regions behind it jammed. Using disk conservation in conjunction with an upper limit to the packing fraction at jamming onset, we predict the front speed as a function of packing fraction and rake speed. However, we find that the jamming front has a finite width, a feature that cannot be explained by disk conservation alone. This width appears to diverge on approach to jamming, which suggests that it may be related to growing lengthscales encountered in other jamming studies.}, author = {Waitukaitis, Scott R and Roth, Leah and Vitelli, Vincenzo and Jaeger, Heinrich}, journal = {EPL}, number = {4}, publisher = {Elsevier}, title = {{Dynamic jamming fronts}}, doi = {10.1209/0295-5075/102/44001}, volume = {102}, year = {2013}, } @inproceedings{117, abstract = {The packing arrangement of individual particles inside a granular material and the resulting response to applied stresses depend critically on particle-particle interactions. One aspect that recently received attention are nanoscale surface features of particles, which play an important role in determining the strength of cohesive van der Waals and capillary interactions and also affect tribo-charging of grains. We describe experiments on freely falling granular streams that can detect the contributions from all three of these forces. We show that it is possible to measure the charge of individual grains and build up distributions that are detailed enough to provide stringent tests of tribo-charging models currently available. A second aspect concerns particle shape. In this case steric interactions become important and new types of aggregate behavior can be expected when non-convex particle shapes are considered that can interlock or entangle. However, a general connection between the mechanical response of a granular material and the constituents\' shape remains unknown. This has made it infeasible to tackle the "inverse packing problem", namely to start from a given, desired behavior for the aggregate as a whole and then find the particle shape the produces it. We discuss a new approach, using concepts rooted in artificial evolution that provides a way to solve this inverse problem. This approach facilitates exploring the role of arbitrary particle geometry in jammed systems and invites the discovery and design of granular matter with optimized properties.}, author = {Jaeger, Heinrich and Miskin, Marc and Waitukaitis, Scott R}, booktitle = { AIP Conference Proceedings}, location = {Sydney, Australia}, pages = {3 -- 6}, publisher = {AIP}, title = {{From nanoscale cohesion to macroscale entanglement: opportunities for designing granular aggregate behaviour by tailoring grain shape and interactions}}, doi = {10.1063/1.4811858}, volume = {1542}, year = {2013}, } @article{113, abstract = {Although liquids typically flow around intruding objects, a counterintuitive phenomenon occurs in dense suspensions of micrometre-sized particles: they become liquid-like when perturbed lightly, but harden when driven strongly. Rheological experiments have investigated how such thickening arises under shear, and linked it to hydrodynamic interactions or granular dilation. However, neither of these mechanisms alone can explain the ability of suspensions to generate very large, positive normal stresses under impact. To illustrate the phenomenon, such stresses can be large enough to allow a person to run across a suspension without sinking, and far exceed the upper limit observed under shear or extension. Here we show that these stresses originate from an impact-generated solidification front that transforms an initially compressible particle matrix into a rapidly growing jammed region, ultimately leading to extraordinary amounts of momentum absorption. Using high-speed videography, embedded force sensing and X-ray imaging, we capture the detailed dynamics of this process as it decelerates a metal rod hitting a suspension of cornflour (cornstarch) in water. We develop a model for the dynamic solidification and its effect on the surrounding suspension that reproduces the observed behaviour quantitatively. Our findings suggest that prior interpretations of the impact resistance as dominated by shear thickening need to be revisited.}, author = {Waitukaitis, Scott R and Jaeger, Heinrich}, journal = {Nature}, number = {7406}, pages = {205 -- 209}, publisher = {Nature Publishing Group}, title = {{Impact-activated solidification of dense suspensions via dynamic jamming fronts}}, doi = {10.1038/nature11187}, volume = {487}, year = {2012}, } @article{114, abstract = {We report on an investigation of the solidification of a cornstarch and water suspension during normal impact on its surface. We find that a finite time after impact, the suspension displays characteristics reminiscent of a solid, including localized stress transmission, the development of a yield stress, and some elastic energy storage. The time dependence of these characteristics depends on the thickness of the cornstarch layer, showing that the solidification is a dynamic process driven by the impacting object. These findings confirm previous speculations that rapidly applied normal stress transforms the normally fluid-like suspension into a temporarily jammed solid and draw a clear distinction between the effects of normal stress and shear stress in dense suspensions.}, author = {Waitukaitis, Scott R and Jaeger, Heinrich}, journal = {Revista Cubana de Fisica}, number = {1E}, pages = {1E31 -- 1E33}, publisher = {Universidad de La Habana}, title = {{Solidification of a cornstarch and water suspension}}, volume = {29}, year = {2012}, } @article{112, abstract = {Particle beams are important tools for probing atomic and molecular interactions. Here we demonstrate that particle beams also offer a unique opportunity to investigate interactions in macroscopic systems, such as granular media. Motivated by recent experiments on streams of grains that exhibit liquid-like breakup into droplets, we use molecular dynamics simulations to investigate the evolution of a dense stream of macroscopic spheres accelerating out of an opening at the bottom of a reservoir. We show how nanoscale details associated with energy dissipation during collisions modify the stream\'s macroscopic behavior. We find that inelastic collisions collimate the stream, while the presence of short-range attractive interactions drives structure formation. Parameterizing the collision dynamics by the coefficient of restitution (i.e., the ratio of relative velocities before and after impact) and the strength of the cohesive interaction, we map out a spectrum of behaviors that ranges from gaslike jets in which all grains drift apart to liquid-like streams that break into large droplets containing hundreds of grains. We also find a new, intermediate regime in which small aggregates form by capture from the gas phase, similar to what can be observed in molecular beams. Our results show that nearly all aspects of stream behavior are closely related to the velocity gradient associated with vertical free fall. Led by this observation, we propose a simple energy balance model to explain the droplet formation process. The qualitative as well as many quantitative features of the simulations and the model compare well with available experimental data and provide a first quantitative measure of the role of attractions in freely cooling granular streams.}, author = {Waitukaitis, Scott R and Grütjen, Helge and Royer, John and Jaeger, Heinrich}, journal = {Physical Review E}, number = {5}, publisher = {American Physical Society}, title = {{Droplet and cluster formation in freely falling granular streams}}, doi = {10.1103/PhysRevE.83.051302}, volume = {83}, year = {2011}, } @article{110, abstract = {In order to better understand magnetic reconnection and particle acceleration in solar flares, we compare the RHESSI hard X-ray (HXR) footpoint motions of three flares with a detailed study of the corresponding topology given by a Magnetic Charge Topology model. We analyze the relationship between the footpoint motions and topological spine lines and find that the examined footpoint sources move along spine lines. We present a three-dimensional topological model in which this movement can be understood. As reconnection proceeds, flux is transferred between the reconnecting domains, causing the separator to move. The movement of the separator\'s chromospheric ends, identified with the HXR footpoints, is along those spine lines on which the separator ends.}, author = {Des Jardins, Angela and Canfield, Richard and Longcope, Dana and Fordyce, Crystal and Waitukaitis, Scott R}, journal = {The Astrophysical Journal}, number = {2}, pages = {1628 -- 1636}, publisher = {IOP Publishing Ltd.}, title = {{Reconnection in three dimensions: The role of spines in three eruptive flares}}, doi = {10.1088/0004-637X/693/2/1628}, volume = {693}, year = {2009}, } @article{111, abstract = {Thin streams of liquid commonly break up into characteristic droplet patterns owing to the surface-tension-driven PlateauRayleigh instability 1-3. Very similar patterns are observed when initially uniform streams of dry granular material break up into clusters of grains4-6, even though flows of macroscopic particles are considered to lack surface tension7,8. Recent studies on freely falling granular streams tracked fluctuations in the stream profile9, but the clustering mechanism remained unresolved because the full evolution of the instability could not be observed. Here we demonstrate that the cluster formation is driven by minute, nanoNewton cohesive forces that arise from a combination of van der Waals interactions and capillary bridges between nanometre-scale surface asperities. Our experiments involve high-speed video imaging of the granular stream in the co-moving frame, control over the properties of the grain surfaces and the use of atomic force microscopy to measure grain-grain interactions. The cohesive forces that we measure correspond to an equivalent surface tension five orders of magnitude below that, of ordinary liquids. We find that, the shapes of these weakly cohesive, non-thermal clusters of macroscopic particles closely resemble droplets resulting from thermally induced rupture of liquid nanojets 10-12.}, author = {Royer, John and Evans, Daniel and Oyarte, Loreto and Guo, Qiti and Kapit, Eliot and Möbius, Matthias and Waitukaitis, Scott R and Jaeger, Heinrich}, journal = {Nature}, number = {7250}, pages = {1110 -- 1113}, publisher = {Nature Publishing Group}, title = {{High-speed tracking of rupture and clustering in freely falling granular streams}}, doi = {10.1038/nature08115}, volume = {459}, year = {2009}, } @article{128, abstract = {A 671 nm diode laser with a mode-hop-free tuning range of 40 GHz is described. This long tuning range is achieved by simultaneously ramping the external cavity length with the laser injection current. The laser output pointing remains fixed, independent of its frequency because of the cover slip cavity design. This system is simple, economical, robust, and easy to use for spectroscopy, as we demonstrate with lithium vapor and lithium atom beam experiments. }, author = {Carr, Adra and Serchest, Yancey and Waitukaitis, Scott R and Perreault, John and Lonij, Vincent and Cronin, Alexander}, journal = {Review of Scientific Instruments}, number = {10}, publisher = {American Institute of Physics}, title = {{Cover slip external cavity diode laser}}, doi = {10.1063/1.2801006}, volume = {78}, year = {2007}, }