08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access The role of dimensionality and geometry in quench-induced nonequilibrium forces(2021) Nejad, Mehrana Raeisian; Khalilian, Hamidreza; Rohwer, Christian M.; Moghaddam, Ali GhorbanzadehWe present an analytical formalism, supported by numerical simulations, for studying forces that act on curved walls following temperature quenches of the surrounding ideal Brownian fluid. We show that, for curved surfaces, the post-quench forces initially evolve rapidly to an extremal value, whereafter they approach their steady state value algebraically in time. In contrast to the previously-studied case of flat boundaries (lines or planes), the algebraic decay for curved geometries depends on the dimension of the system. Specifically, steady-state values of the force are approached in time as t-d/2 in d-dimensional spherical (curved) geometries. For systems consisting of concentric circles or spheres, the exponent does not change for the force on the outer circle or sphere. However, the force exerted on the inner circles or sphere experiences an overshoot and, as a result, does not evolve to the steady state in a simple algebraic manner. The extremal value of the force also depends on the dimension of the system, and originates from curved boundaries and the fact that particles inside a sphere or circle are locally more confined, and diffuse less freely than particles outside the circle or sphere.Item Open Access Localization and diffusion of positive muons in metals(1979) Orth, Helmut; Döring, Klaus-Peter; Gladisch, Michael; Herlach, Dierk; Maysenhölder, Waldemar; Metz, Harald; Putlitz, Gisbert zu; Seeger, Alfred; Vetter, J.; Wahl, W.; Wigand, M.; Yagi, EiichiDie Grundeigenschaften positiver Müonen (positive Elementarladung, Spin 1/2, Massenverhältnis Müon/Proton ≈ 1/9, großes magnetisches Moment, paritätsverletzender Zerfall in ein Positron und zwei Neutrinos) und ihre Verfügbarkeit in Form intensiver spinpolarisierter Strahlen in "Mesonen-Fabriken" gestatten es, mit Hilfe der μ+SR ("Müon-Spin-Rotation")-Methode die Wechselwirkung der Müonen mit Magnetfeldern in Metallen zu studieren. In dieser Arbeit geben wir einen kurzen Überblick über die μ+SR-Methode und ihre Anwendung auf das Studium der Lokalisierung und Diffusion von Müonen in Metallen für den Fall der dipolaren Wechselwirkung mit den magnetischen Momenten der Kerne. Die Zusammenhänge zwischen den gemessenen Relaxationsraten und der Müonenbeweglichkeit und dem Einfang und Entweichen an Verunreinigungen werden dargestellt und anhand von Messungen an hochreinen Be-, Nb- und Ta-Einkristallen veranschaulicht.Item Open Access Ionic liquids in conducting nanoslits : how important is the range of the screened electrostatic interactions?(2022) Groda, Yaroslav; Dudka, Maxym; Oshanin, Gleb; Kornyshev, Alexei A.; Kondrat, SvyatoslavAnalytical models for capacitive energy storage in nanopores attract growing interest as they can provide in-depth analytical insights into charging mechanisms. So far, such approaches have been limited to models with nearest-neighbor interactions. This assumption is seemingly justified due to a strong screening of inter-ionic interactions in narrow conducting pores. However, how important is the extent of these interactions? Does it affect the energy storage and phase behavior of confined ionic liquids? Herein, we address these questions using a two-dimensional lattice model with next-nearest and further neighbor interactions developed to describe ionic liquids in conducting slit confinements. With simulations and analytical calculations, we find that next-nearest interactions enhance capacitance and stored energy densities and may considerably affect the phase behavior. In particular, in some range of voltages, we reveal the emergence of large-scale mesophases that have not been reported before but may play an important role in energy storage.Item Open Access Entropy by neighbor distance as a new measure for characterizing spatiotemporal orders in microscopic collective systems(2023) Fu, Yulei; Wu, Zongyuan; Zhan, Sirui; Yang, Jiacheng; Gardi, Gaurav; Kishore, Vimal; Malgaretti, Paolo; Wang, WendongCollective systems self-organize to form globally ordered spatiotemporal patterns. Finding appropriate measures to characterize the order in these patterns will contribute to our understanding of the principles of self-organization in all collective systems. Here we examine a new measure based on the entropy of the neighbor distance distributions in the characterization of collective patterns. We study three types of systems: a simulated self-propelled boid system, two active colloidal systems, and one centimeter-scale robotic swarm system. In all these systems, the new measure proves sensitive in revealing active phase transitions and in distinguishing steady states. We envision that the entropy by neighbor distance could be useful for characterizing biological swarms such as bird flocks and for designing robotic swarms.Item Open Access Magnetic domains and domain wall pinning in atomically thin CrBr3 revealed by nanoscale imaging(2021) Sun, Qi-Chao; Song, Tiancheng; Anderson, Eric; Brunner, Andreas; Förster, Johannes; Shalomayeva, Tetyana; Taniguchi, Takashi; Watanabe, Kenji; Gräfe, Joachim; Stöhr, Rainer; Xu, Xiaodong; Wrachtrup, JörgThe emergence of atomically thin van der Waals magnets provides a new platform for the studies of two-dimensional magnetism and its applications. However, the widely used measurement methods in recent studies cannot provide quantitative information of the magnetization nor achieve nanoscale spatial resolution. These capabilities are essential to explore the rich properties of magnetic domains and spin textures. Here, we employ cryogenic scanning magnetometry using a single-electron spin of a nitrogen-vacancy center in a diamond probe to unambiguously prove the existence of magnetic domains and study their dynamics in atomically thin CrBr3. By controlling the magnetic domain evolution as a function of magnetic field, we find that the pinning effect is a dominant coercivity mechanism and determine the magnetization of a CrBr3 bilayer to be about 26 Bohr magnetons per square nanometer. The high spatial resolution of this technique enables imaging of magnetic domains and allows to locate the sites of defects that pin the domain walls and nucleate the reverse domains. Our work highlights scanning nitrogen-vacancy center magnetometry as a quantitative probe to explore nanoscale features in two-dimensional magnets.Item Open Access Axisymmetric spheroidal squirmers and self-diffusiophoretic particles(2020) Pöhnl, Ruben; Popescu, Mihail Nicolae; Uspal, William E.We study, by means of an exact analytical solution, the motion of a spheroidal, axisymmetric squirmer in an unbounded fluid, as well as the low Reynolds number hydrodynamic flow associated to it. In contrast to the case of a spherical squirmer - for which, e.g. the velocity of the squirmer and the magnitude of the stresslet associated with the flow induced by the squirmer are respectively determined by the amplitudes of the first two slip (‘squirming’) modes - for the spheroidal squirmer each squirming mode either contributes to the velocity, or contributes to the stresslet. The results are straightforwardly extended to the self-phoresis of axisymmetric, spheroidal, chemically active particles in the case when the phoretic slip approximation holds.Item Open Access How to speed up ion transport in nanopores(2020) Breitsprecher, Konrad; Janssen, Mathijs; Srimuk, Pattarachai; Mehdi, B. Layla; Presser, Volker; Holm, Christian; Kondrat, SvyatoslavElectrolyte-filled subnanometre pores exhibit exciting physics and play an increasingly important role in science and technology. In supercapacitors, for instance, ultranarrow pores provide excellent capacitive characteristics. However, ions experience difficulties in entering and leaving such pores, which slows down charging and discharging processes. In an earlier work we showed for a simple model that a slow voltage sweep charges ultranarrow pores quicker than an abrupt voltage step. A slowly applied voltage avoids ionic clogging and co-ion trapping - a problem known to occur when the applied potential is varied too quickly - causing sluggish dynamics. Herein, we verify this finding experimentally. Guided by theoretical considerations, we also develop a non-linear voltage sweep and demonstrate, with molecular dynamics simulations, that it can charge a nanopore even faster than the corresponding optimized linear sweep. For discharging we find, with simulations and in experiments, that if we reverse the applied potential and then sweep it to zero, the pores lose their charge much quicker than they do for a short-circuited discharge over their internal resistance. Our findings open up opportunities to greatly accelerate charging and discharging of subnanometre pores without compromising the capacitive characteristics, improving their importance for energy storage, capacitive deionization, and electrochemical heat harvesting.Item Open Access Non-equilibrium work distribution for interacting colloidal particles under friction(2015) Gomez-Solano, Juan Ruben; July, Christoph; Mehl, Jakob; Bechinger, ClemensWe experimentally investigate the non-equilibrium steady-state distribution of the work done by an external force on a mesoscopic system with many coupled degrees of freedom: a colloidal crystal mechanically driven across a commensurate periodic light field. Since this system mimics the spatiotemporal dynamics of a crystalline surface moving on a corrugated substrate, our results show general properties of the work distribution for atomically flat surfaces undergoing friction. We address the role of several parameters which can influence the shape of the work distribution, e.g. the number of particles used to locally probe the properties of the system and the time interval to measure the work. We find that, when tuning the control parameters to induce particle depinning from the substrate, there is an abrupt change of the shape of the work distribution. While in the completely static and sliding friction regimes the work distribution is Gaussian, non-Gaussian tails show up due to the spatiotemporal heterogeneity of the particle dynamics during the transition between these two regimes.Item Open Access Microrobot collectives with reconfigurable morphologies, behaviors, and functions(2022) Gardi, Gaurav; Ceron, Steven; Wang, Wendong; Petersen, Kirstin; Sitti, MetinMobile microrobots, which can navigate, sense, and interact with their environment, could potentially revolutionize biomedicine and environmental remediation. Many self-organizing microrobotic collectives have been developed to overcome inherent limits in actuation, sensing, and manipulation of individual microrobots; however, reconfigurable collectives with robust transitions between behaviors are rare. Such systems that perform multiple functions are advantageous to operate in complex environments. Here, we present a versatile microrobotic collective system capable of on-demand reconfiguration to adapt to and utilize their environments to perform various functions at the air-water interface. Our system exhibits diverse modes ranging from isotropic to anisotrpic behaviors and transitions between a globally driven and a novel self-propelling behavior. We show the transition between different modes in experiments and simulations, and demonstrate various functions, using the reconfigurability of our system to navigate, explore, and interact with the environment. Such versatile microrobot collectives with globally driven and self-propelled behaviors have great potential in future medical and environmental applications.Item Open Access Micro- and nanofabrication of dynamic hydrogels with multichannel information(2023) Zhang, Mingchao; Lee, Yohan; Zheng, Zhiqiang; Khan, Muhammad Turab Ali; Lyu, Xianglong; Byun, Junghwan; Giessen, Harald; Sitti, MetinCreating micro/nanostructures containing multi-channel information within responsive hydrogels presents exciting opportunities for dynamically changing functionalities. However, fabricating these structures is immensely challenging due to the soft and dynamic nature of hydrogels, often resulting in unintended structural deformations or destruction. Here, we demonstrate that dehydrated hydrogels, treated by a programmable femtosecond laser, can allow for a robust fabrication of micro/nanostructures. The dehydration enhances the rigidity of the hydrogels and temporarily locks the dynamic behaviours, significantly promoting their structural integrity during the fabrication process. By utilizing versatile dosage domains of the femtosecond laser, we create micro-grooves on the hydrogel surface through the use of a high-dosage mode, while also altering the fluorescent intensity within the rest of the non-ablated areas via a low-dosage laser. In this way, we rationally design a pixel unit containing three-channel information: structural color, polarization state, and fluorescent intensity, and encode three complex image information sets into these channels. Distinct images at the same location were simultaneously printed onto the hydrogel, which can be observed individually under different imaging modes without cross-talk. Notably, the recovered dynamic responsiveness of the hydrogel enables a multi-information-encoded surface that can sequentially display different information as the temperature changes.