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The theoretical framework of conventional contact mechanics is based on idealized as- sumptions that have shaped the field for more than 140 years. Unfortunately, these assumptions do not lend themselves to the modelling of thin films, viscoelastic materials and frictional interfaces. Therefore, the present thesis is concerned with the system- atic generalization of these assumptions and their GFMD implementation to simulate a variety of previously inaccessible, realistic contact problems. First, finite material thickness is considered in the design of film-terminated fibril struc- tures for skin adhesion. An elastic film resting on a hard foundation is effectively more stiff than its bulk counterpart, which reduces its ability to conform to counter-faces and therefore reduces the adhesion to roughness. Second, the velocity-dependence of soft, adhesive multi-asperity contacts is studied, revealing the importance of topographical saddle points and the initial configuration, from which detachment is initiated. Further- more, we identify a scaling relation describing how short-ranged microscopic interactions slow down the macroscopic relaxation of a contact. Finally, we explore the influence of interfacial friction, showing that it increases local stress concentrations and impedes the fluid flow through the interface. The reported results provide new insight into commonly neglected phenomena, whose practical significance is reinforced by direct comparisons to experiments.
Spectroscopic characterization of laser-induced luminescence for remote environmental thermometry
(2025)
Lanthanide-doped upconversion microparticles (UCMP) enable composites for luminescence thermometry with long luminescence lifetime and narrowband absorption and emission spectra. Being non-toxic, easily synthesizable, and having a bright, stable emission makes them an attractive candidate for in-vivo monitoring of key environmental parameters such as temperature. We use them to create soft, biodegradable, miniaturized seed-like robots endowed with fluorescence tags for the sustainable environmental monitoring of topsoil and air above soil environments. Our aim is an airborne platform with a sufficient signal-to-noise ratio to identify the concentration of targeted soil parameters. Here, we study the photoluminescence of Er, Yb: NaYF4 UCMPs embedded in polylactic acid (PLA) polymeric matrix to assess their suitability for remote read-out. We assessed the signal-to-noise ratio in terms of excitation intensity, UCMP concentration, working distance, and sample orientation. We evaluated the signal stability over long exposure time as well as for amplitude-modulated excitation. Finally, we carried out ratiometric and lifetime measurements of luminescence emission in order to demonstrate the feasibility of such sensors in measuring the variation of temperature. Overall, the rare-earth doped UCMPs embedded in biodegradable polymer can be used for remote thermometry, displaying a significant signal-to-noise ratio for luminescence emission detection and subsequent derivation of temperature.
Fingertip friction plays a key role in tactile interactions, but the perception of friction in active touch has rarely been studied. In this psychophysical study, we examine the impact of feature sizes on micro-structured surfaces on friction perception. First results indicate that the perception of friction differences is deteriorated when a surface with structures smaller than 100 µm is compared with larger structures, possibly in agreement with the two pathways proposed in Katz’ duplex theory of tactile perception.
Das Verständnis und die Kontrolle der Dynamik von Polymer-Oberflächen-Wechsel- wirkungen sind die Voraussetzung für das Design von Nanoobjekten und für das Verständnis biologischer Prozesse. Wir untersuchen dynamische Reibung und Adhäsion an einer Fest-Flüssig-Grenzfläche mit Hilfe des Rasterkraftmikroskops (AFM). Als Modellsystem wird ein einzelnes M13mp18-DNA-Molekül, mit einer Länge von 2.5 µm, mittels Biotin-Streptavidin-Wechselwirkung über einen Bead an einen Cantilever gebunden. Bei den Adhäsionsmessungen wird der Cantilever mehrfach gen Oberfläche gefahren, wobei er teilweise mehrere hundert Nanometer darüber verweilt, um eine Interaktion zwischen dem Bead und der Oberfläche zu vermeiden. Die Reibungsmessungen werden durchgeführt, indem der Cantilever seitlich parallel zur Oberfläche in einer Höhe von mehreren hundert Nanometern mit unterschiedlichen Geschwindigkeiten bewegt wird. Dies führt zur adhäsiven Wechselwirkung zwischen dem DNA-Molekül an verschiedenen Oberflächen und somit zu einer Verbiegung des Cantilevers. Die verwendeten Oberflächen sind eine mit Cellulosenitrat und Anti-Digoxigenin beschichtete Glasoberfläche, eine positiv geladene poröse Membran und eine mit Poly-L-Lysin beschichteter Glasoberfläche. Das Signal wird unter Berücksichtigung der Abrisskraft, Abrissposition und Frequenz sowie auf mögliche Hotspots bei den Reibungsmessungen analysiert, um typische Wechselwirkungen der DNA mit verschiedenen Oberflächen aufzudecken.
Carbon black (CB)-elastomers can serve as low-cost, highly deformable sensor materials, but hardly any work exists on their structure-property relationships. We report on flow-induced anisotropy, considering CB-silicone films generated via doctor blade coating. Cured films showed slight electrical anisotropy, with conductivity parallel to the coating direction being lower than perpendicular to it. Furthermore, piezoresistive sensitivity was much larger for stretch perpendicular to the coating direction than for parallel stretch. Structural analysis for length scales up to the CB agglomerate level yielded only weak evidence of anisotropy. Based on this evidence and insight from CB network simulations, we hypothesize that shear flow during coating fragments the CB network and then induces a preferential aggregate alignment, as well as increased inter-particle distances, parallel to the coating direction. As a practical conclusion, already weak anisotropic structuration suffices to cause significant electric anisotropy.
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.
