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Search processes often involve multiple agents that collectively look for a randomly located target. While increasing the number of agents usually decreases the time at which the first agent finds the target, it also requires resources to create and sustain more agents. In this letter, we consider a collective search cost that not only accounts for the search time but also for the cost associated to the creation and the maintenance of an agent. We first present a general formalism for independent agents in terms of the survival probability of the target for a single-agent search s(t), where we allow agents to be introduced in the system one after the other. From this, we first derive analytically the optimal number of searchers to launch initially in the system. Then, we identify the optimal strategies for exponential and algebraic single-agent survival probabilities by pointing out the ideal times at which new searchers should be launched in the system. Our results show that all searchers should be launched simultaneously in the exponential case, while some should be launched at later times in the algebraic case. Finally, we compare these results with numerical simulations of a strongly interacting collective search, the true self-avoiding walk, and show how the optimal strategy differ from the non-interacting case.
We undertake a numerical study of the ordering kinetics in the two-dimensional (2d) active Ising model (AIM), a discrete flocking model with a non-conserved scalar order parameter. We find that for a quench into the liquid-gas coexistence region and in the ordered liquid region, the characteristic length scale of both the density and magnetization domains follows the Lifshitz-Cahn-Allen (LCA) growth law: R(t)∼t1/2, consistent with the growth law of passive systems with scalar order parameter and non-conserved dynamics. The system morphology is analyzed with the two-point correlation function and its Fourier transform, the structure factor, which conforms to the well-known Porod's law, a manifestation of the coarsening of compact domains with smooth boundaries. We also find the domain growth exponent unaffected by different noise strengths and self-propulsion velocities of the active particles. However, transverse diffusion is found to play the most significant role in the growth kinetics of the AIM. We extract the same growth exponent by solving the hydrodynamic equations of the AIM.
We numerically study a discretized Vicsek model (DVM) with particles orienting in q possible orientations in two dimensions. The study probes the significance of anisotropic orientation and microscopic interaction on the macroscopic behavior. The DVM is an off-lattice flocking model like the active clock model [ACM; EPL {\bf 138}, 41001 (2022)] but the dynamical rules of particle alignment and movement are inspired by the prototypical Vicsek model (VM). The DVM shows qualitatively similar properties as the ACM for intermediate noise strength where a transition from macrophase to microphase separation of the coexistence region is observed as q is increased. But for small q and noise strength, the liquid phase appearing in the ACM at low temperatures is replaced in the DVM by a configuration of multiple clusters with different polarization which does not exhibit any long-range order. We find that the dynamical rules have a profound influence on the overarching features of the flocking phase. We further identify the metastability of the ordered liquid phase subjected to a perturbation.
We numerically study a discretized Vicsek model (DVM) with particles orienting in q possible orientations in two dimensions. The study probes the significance of anisotropic orientation and microscopic interaction on the macroscopic behavior. The DVM is an off-lattice flocking model like the active clock model [ACM; EPL {\bf 138}, 41001 (2022)] but the dynamical rules of particle alignment and movement are inspired by the prototypical Vicsek model (VM). The DVM shows qualitatively similar properties as the ACM for intermediate noise strength where a transition from macrophase to microphase separation of the coexistence region is observed as q is increased. But for small q and noise strength, the liquid phase appearing in the ACM at low temperatures is replaced in the DVM by a configuration of multiple clusters with different polarization which does not exhibit any long-range order. We find that the dynamical rules have a profound influence on the overarching features of the flocking phase. We further identify the metastability of the ordered liquid phase subjected to a perturbation.
Ein am INM entwickeltes Verfahren zur lichtinduzierten Silberabscheidung sollte verwendet werden, um Mikro- und Submikrostrukturen mit optischer Funtionalität für photonische Bauelemente herzustellen. Unter Ausnutzung von Effekten der Oberflächenplasmonenresonanz sollten dabei insbesondere diffraktive Elemente und Wellenleiter gezeigt werden, um daraus eine Plattform für unterschiedliche Anwendungen, wie z.B. optische Sicherheitsmerkmale, aber auch integrierte Optik oder Biosensoren zu entwickeln. Das erwähnte Verfahren beruht auf der durch UV-Licht initiierten Zersetzung eines gelösten Silberkomplexes an einer mit photokatalytischen Anatas-Nanopartikeln belegten Oberfläche. Hierdurch wird unlösliches elementares Silber freigesetzt, das sich am Ort der Belichtung niederschlägt. Diese Silberabscheidung erfolgt zunächst in Form kolloidaler Silbernanopartikel, die mit zunehmender Belichtungsdosis zu leitfähigen Silberflächen zusammenwachsen können. Durch örtliche Modulation der Lichtverteilung kann die Verteilung des Silbers auf der photokatalytischen Grenzfläche mit beugungsbegrenzter Auflösung strukturiert werden. In diesem Vorhaben sollte primär der kolloidale Zustand genutzt werden. Edelmetallkolloide zeigen eine charakteristische Oberflächenplasmonenresonanz, die sich sowohl in einer starken Absorption und Lichtstreuung bei der Resonanzfrequenz als auch in einer Änderung des Brechungsindex des aus kolloidalen Partikeln und der umgebebenden Matrix bestehenden Kompositmaterials äußert. Die durch Strukturierung der Silberverteilung erreichte örtliche Modulation der optischen Materialeigenschaften sollte demnach die Erzeugung photonischer Bauelemente wie z.B. optischer Beugungsgitter erlauben. Da die Änderung des Brechungsindex auch Frequenzbereiche fernab der eigentlichen Resonanzfrequenz betrifft, in denen die durch die Resonanz bedingte Absorption gering ist, sollten auch indexgeführte planare Welleneleiter ("photonische Wellenleiter") auf diesem Weg möglich werden. Zudem sollte die Option, auch strukturierte leitfähige Flächen herzustellen, die Integration mit plasmonischen Wellenleitern erlauben. Das Ziel dieses Vorhabens bestand in der Realisierung der oben skiziierten Möglichkeiten, um die Grundlagen einer neuen Technologieplattform zur Herstellung photonischer Bauelemente zu schaffen. Neben kolloidbasierten Beugungsgittern sollten auch photonische und plasmonische Wellenleiter gezeigt werden und auf dieser Basis Prototypen einzelner anwendungsbezogener Devices hergestellt werden.
The 2D materials exhibit excellent tribological properties due to their weak inter-plane interactions, such as the ultra-low friction, which can be further tuned by number of layers, application of electric bias, stacking of different materials into a van der Waals heterostructure, and change of substrate. In this work, the tribological properties of 2D materials were investigated experimentally by means of atomic force microscopy techniques in ultra-high vacuum and theoretically with atomistic simulations. Friction measurements on epitaxial graphene on SiC(0001) show that the ultra-low friction is limited by a normal load threshold, above which friction increases by one order of magnitude. Simulations suggest that, at contact pressures above 10 GPa, the high-friction regime is a result of an intermittent sp3 rehybridization of graphene and the formation of covalent bonds. Friction on the MoS2/graphene heterostructure is dominated by adhesion due to the out-of-plane deformation of the MoS2 layers. Increasing the number of MoS2 layers decreases friction as the flexural compliance decreases. Higher friction was recorded on MoSe2/hBN compared to graphene/hBN heterostructure or pristine hBN. Work on exfoliated materials was facilitated by the application of navigational microstructures.
The cytoskeleton is a dynamic network of filaments comprising actin filaments, microtubules, and intermediate filaments in the cytoplasm of cells. Vimentin is an intermediate filament protein that plays a crucial role in adhesion, migration, and signalling. These functions of vimentin have broader implications on inflammation, wound healing, cell physiology, and immune response. Inside the cell, the vimentin network extends from the nucleus to the cell's periphery. However, vimentin gets out to the extracellular environment under the circumstances such as activation, stress, and senescence. However, the influence of such vimentin on general cellular functions and its characteristics is not well established. This thesis demonstrates that extracellular addition of vimentin enhances proliferation, adhesion and migration prominently in cancer cells (MCF-7), overexpressing insulin-like growth factor 1 (IGF1-R). Interestingly, in SARS-CoV-2 infection, the extracellular vimentin preincubation with the SARS-CoV-2 receptor binding domain protected the cancer (MCF-7) cell's monolayer integrity. Further investigation on the characteristics of extracellular vimentin found secretion of vimentin from the back of activated macrophages in the form of small fragments, enhancing phagocytosis and migration of activated macrophages. Collectively, this work demonstrates new insights into vimentin secretion and its implications on cellular functionality.
The remarkable properties of bio-inspired microstructures make them extensively accessible for various applications, including industrial, medical, and space applications. However, their implementation especially as grippers for pick-and-place robotics can be compromised by multiple factors. The most common ones are alignment imperfections with the target object, unbalanced stress distribution, contamination, defects, and roughness at the gripping interface. In the present work, three different approaches to assess the contact phenomena between patterned structures and the target object are presented. First, in-situ observation and machine learning are combined to realize accurate real-time predictions of adhesion performance. The trained supervised learning models successfully predict the adhesion performance from the contact signature. Second, two newly developed optical systems are compared to observe the correct grasping of various target objects (rough or transparent) by looking through the microstructures. And last, model experiments are provided for a direct comparison with simulation efforts aiming at a prediction of the contact signature and an analysis of the rate and preload-dependency of the adhesion strength of a soft polymer film in contact with roughness-like surface topography. The results of this thesis open new perspectives for improving the reliability of handling systems using bioinspired microstructures.
We numerically study a discretized Vicsek model (DVM) with particles orienting in q possible orientations in two dimensions. The study investigates the significance of anisotropic orientation and microscopic interaction on macroscopic behavior. The DVM is an off-lattice flocking model like the active clock model (ACM; Chatterjee et al 2022 Europhys. Lett.138 41001) but the dynamical rules of particle alignment and movement are inspired by the prototypical Vicsek model (VM). The DVM shows qualitatively similar properties as the ACM for intermediate noise strength where a transition from macrophase to microphase separation of the coexistence region is observed as q is increased. But for small q and noise strength, the liquid phase appearing in the ACM at low temperatures is replaced in the DVM by a configuration of multiple clusters with different polarizations, which does not exhibit any long-range order. We find that the dynamical rules have a profound influence on the overarching features of the flocking phase. We further identify the metastability of the ordered liquid phase subjected to a perturbation.
Many biological active agents respond to gradients of environmental cues by redirecting their motion. In addition to the well-studied prominent examples such as phototaxis and chemotaxis, there has been considerable recent interest in topotaxis, i.e., the ability to sense and follow topographic environmental cues. A trivial topotaxis is achievable through a spatial gradient of obstacle density, though over limited length scales. Here, we introduce a type of topotaxis based on sliding of particles along obstacles—as observed, e.g., in bacterial dynamics near surfaces. We numerically demonstrate how imposing a gradient in the angle of sliding along pillars breaks the spatial symmetry and biases the direction of motion, resulting in an efficient topotaxis in a uniform pillar park. By repeating blocks of pillars with a strong gradient of sliding angle, we propose an efficient method for guiding particles over arbitrary long distances. We provide an explanation for this spectacular phenomenon based on effective reflection at the borders of neighboring blocks. Our results are of technological and medical importance for design of efficient taxis devices for living agents.
