12,628 research outputs found

    Chiral active fluids: Odd viscosity, active turbulence, and directed flows of hydrodynamic microrotors

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    While the number of publications on rotating active matter has rapidly increased in recent years, studies on purely hydrodynamically interacting rotors on the microscale are still rare, especially from the perspective of particle based hydrodynamic simulations. The work presented here targets to fill this gap. By means of high-performance computer simulations, performed in a highly parallelised fashion on graphics processing units, the dynamics of ensembles of up to 70,000 rotating colloids immersed in an explicit mesoscopic solvent consisting out of up to 30 million fluid particles, are investigated. Some of the results presented in this thesis have been worked out in collaboration with experimentalists, such that the theoretical considerations developed in this thesis are supported by experiments, and vice versa. The studied system, modelled in order to resemble the essential physics of the experimentally realisable system, consists out of rotating magnetic colloidal particles, i.e., (micro-)rotors, rotating in sync to an externally applied magnetic field, where the rotors solely interact via hydrodynamic and steric interactions. Overall, the agreement between simulations and experiments is very good, proving that hydrodynamic interactions play a key role in this and related systems. While already an isolated rotating colloid is driven out of equilibrium, only collections of two or more rotors have experimentally shown to be able to convert the rotational energy input into translational dynamics in an orbital rotating fashion. The rotating colloids inject circular flows into the fluid, such that detailed balance is broken, and it is not a priori known whether equilibrium properties of colloids can be extended to isolated rotating colloids. A joint theoretical and experimental analysis of isolated, pairs, and small groups of hydrodynamically interacting rotors is given in chapter 2. While the translational dynamics of isolated rotors effectively resemble the dynamics of non-rotating colloids, the orbital rotation of pairs of rotors can be described with leading order hydrodynamics and a two-dimensional analogy of Faxén’s law is derived. In chapter 3, a homogeneously distributed ensemble of rotors (bulk) as a realisation of a chiral active fluid is studied and it is explicitly shown computationally and experimentally that it carries odd viscosity. The mutual orbital translation of rotors and an increase of the effective solvent viscosity with rotor density lead to a non-monotonous behaviour of the average translational velocity. Meanwhile, the rotor suspension bears a finite osmotic compressibility resulting from the long-ranged nature of hydrody- namic interactions such that rotational and odd stresses are transmitted through the solvent also at small and intermediate rotor densities. Consequently, density inhomogeneities predicted for chiral active fluids with odd viscosity can be found and allow for an explicit measurement of odd viscosity in simulations and experiments. At intermediate densities, the collective dynamics shows the emergence of multi-scale vortices and chaotic motion which is identified as active turbulence with a self-similar power-law decay in the energy spectrum, showing that the injected energy on the rotor scale is transported to larger scales, similar to the inverse energy cascade of clas- sical two-dimensional turbulence. While either odd viscosity or active turbulence have been reported in chiral active matter previously, the system studied here shows that the emergence of both simultaneously is possible resulting from the osmotic compressibility and hydrodynamic mediation of odd and active stresses. The collective dynamics of colloids rotating out of phase, i.e., where a constant torque instead of a constant angular velocity is applied, is shown to be qualitatively very similar. However, at smaller densities, local density inhomogeneities imply position dependent angular velocities of the rotors resulting from inter-rotor friction. While the friction of a quasi-2D layer of active colloids with the substrate is often not easily modifiable in experiments, the incorporation of substrate friction into the simulation models typically implies a considerable increase in computational effort. In chapter 4, a very efficient way of incorporating the friction with a substrate into a two-dimensional multiparticle collision dynamics solvent is introduced, allowing for an explicit investigation of the influences of substrate on active dynamics. For the rotor fluid, it is explicitly shown that the influence of the substrate friction results in a cutoff of the hydrodynamic interaction length, such that the maximum size of the formed vortices is controlled by the substrate friction, also resulting in a cutoff in the energy spectrum, because energy is taken out of the system at the respective length. These findings are in agreement with the experiments. Since active particles in confinement are known to organise in states of collective dynamics, ensembles of rotationally actuated colloids are studied in circular confinement and in the presence of periodic obstacle lattices in chapters 5 and 6, respectively. The results show that the chaotic active turbulent transport of rotors in suspension can be enhanced and guided resulting from edge flows generated at the boundaries, as has recently been reported for a related chiral active system. The consequent collective rotor dynamics can be regarded as a superposition of active turbulent and imposed flows, leading to on average stationary flows. In contrast to the bulk dynamics, the imposed flows inject additional energy into the system on the long length scales, and the same scaling behaviour of the energy spectrum as in bulk is only obtained if the energy injection scales, due to the mutual generation of rotor translational dynamics throughout the system and the edge flows, are well separated. The combination of edge flow and entropic layering at the boundaries leads to oscillating hydrodynamic stresses and consequently to an oscillating vorticity profile. In the presence of odd viscosity, this consequently leads to non-trivial steady-state density modulations at the boundary, resulting from a balance of osmotic pressure and odd stresses. Relevant for the efficient dispersion and mixing of inert particles on the mesoscale by means of active turbulent mixing powered by rotors, a study of the dynamics of a binary mixture consisting out of rotors and passive particles is presented in chapter 7. Because the rotors are not self-propelled, but the translational dynamics is induced by the surrounding rotors, the passive particles, which do not inject further energy into the system, are transported according to the same mechanism as the rotors. The collective dynamics thus resembles the pure rotor bulk dynamics at the respective density of only rotors. However, since no odd stresses act between the passive particles, only mutual rotor interactions lead to odd stresses leading to the accumulation of rotors in the regions of positive vorticity. This density increase is associated with a pressure increase, which balances the odd stresses acting on the rotors. However, the passive particles are only subject to the accumulation induced pressure increase such that these particles are transported into the areas of low rotor concentration, i.e., the regions of negative vorticity. Under conditions of sustained vortex flow, this results in segregation of both particle types. Since local symmetry breaking can convert injected rotational into translational energy, microswimmers can be constructed out of rotor materials when a suitable breaking of symmetry is kept in the vicinity of a rotor. One hypothetical realisation, i.e., a coupled rotor pair consisting out of two rotors of opposite angular velocity and of fixed distance, termed a birotor, are studied in chapter 8. The birotor pumps the fluid into one direction and consequently translates into the opposite direction, and creates a flow field reminiscent of a source doublet, or sliplet flow field. Fixed in space the birotor might be an interesting realisation of a microfluidic pump. The trans- lational dynamics of a birotor can be mapped onto the active Brownian particle model for single swimmers. However, due to the hydrodynamic interactions among the rotors, the birotor ensemble dynamics do not show the emergence of stable motility induced clustering. The reason for this is the flow created by birotor in small aggregates which effectively pushes further arriving birotors away from small aggregates, which eventually are all dispersed by thermal fluctuations

    Multiscale Modelling of Self-assembly in Soft Matter

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    This thesis presents all-atom molecular dynamics simulations and the development of coarse-grained models for various classes of liquid crystals. The overall aim was to parametrise chemically specific models, propagating information between different resolutions through multiscale modelling approaches, to investigate hierarchical self-assembly in soft matter systems. Common coarse-graining methods were assessed in terms of their representability and transferability for applications involving thermotropic calamitic and discotic mesogens, and lyotropic chromonic liquid crystals. Extensive all-atom simulations were performed on: bent liquid crystal dimers, such as CB7CB; ionic cyanine dyes in aqueous solution (PIC, PCYN, TTBC and BIC); a chromonic perylene bisimide dye (PER); and its thermotropic discotic analogue (PEROEG). These serve as references to parametrise/validate lower resolution models and to provide insights into these systems at the molecular level. For CB7CB, the twist-bend nematic (NTB) phase is observed and characterised. The self-assembly of cyanine dyes and chromonic mesogens was studied by calculating ΔGassoc\Delta G_{\rm{assoc}}, ΔHassoc\Delta H_{\rm{assoc}} and ΔSassoc\Delta S_{\rm{assoc}} for the association of nn-mers (where nn = 2, 3 or 4). Structures of H-aggregate stacks, with shift and Y junction defects, and J-aggregates with a brickwork arrangement were detected. Coarse-graining approaches including iterative Boltzmann inversion (IBI), multiscale coarse-graining (MS-CG) in the form of hybrid force matching (FM) and the Martini 3 force field were utilised for the aforementioned systems. A FM model of CB7CB demonstrates high representability and transferability; the NTB phase is captured and the full phase diagram can be explored via heating or cooling. An optimised Martini model correctly exhibits the chromonic nematic and hexagonal phases for PER at the expected concentrations. For PEROEG, an IBI model was found to be superior in modelling the columnar-hexagonal phase. This thesis discusses, in detail, the successes and failures of the various coarse-graining strategies. While successful coarse-graining of liquid crystals remains a challenge, this thesis demonstrates that, with the right choice of method, high-quality coarse-grained models can be developed for both thermotropic and lyotropic systems

    Multi-scale computer models of lymphatic pumping

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    The lymphatic system maintains fluid homeostasis by returning interstitial fluid to the veins. Lymphatics pump fluid locally with contracting segments of the vessel (lymphangions) bounded by valves. Contractions are generated by specialized muscle exhibiting phasic and tonic contractions. Deficient pumping can result in accumulation of interstitial fluid, called lymphoedema. Lymphoedema treatments have limited effectiveness, partially attributable to a lack of understanding of contractions. A lumped parameter computational model of lymphangion pumping has previously been developed in the group. In this thesis I detail development of two multiscale models of lymphatic pumping to facilitate improved treatments for lymphoedema. The first model captures subcellular mechanisms of lymphatic muscle contraction. This model is based on the sliding filament model and its smooth muscle adaptation. Contractile elements are combined with passive viscoelastic elements to model a cell. Many arrangements were trialled but only one behaved physiologically. The muscle model was then combined with the lymphangion model for comparison with experiments. This model captures mechanical and energetic aspects of both contraction types. I show that the model provides results similar to published experiments from rat mesenteric lymphatics. The model predicted a peak efficiency of 35%, in the upper range from other muscle types. In the range of frequencies and amplitudes simulated, the direct effect of calcium oscillations can increase lymphangion outflow by up to 40% of the flow in their absence. The second model aims to improve our understanding of lymphangion interaction in large networks through computational homogenisation. In this model we do not directly simulate all lymphangions but sample lymphangions at evenly spaced intervals to reduce the computational intensity. We show through this model that increased external pressure at the network inlet collapses lymphangions and that this disruption of pumping for a few lymphangions reduces the outflow from the entire network.Open Acces

    Special Topics in Information Technology

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    This open access book presents thirteen outstanding doctoral dissertations in Information Technology from the Department of Electronics, Information and Bioengineering, Politecnico di Milano, Italy. Information Technology has always been highly interdisciplinary, as many aspects have to be considered in IT systems. The doctoral studies program in IT at Politecnico di Milano emphasizes this interdisciplinary nature, which is becoming more and more important in recent technological advances, in collaborative projects, and in the education of young researchers. Accordingly, the focus of advanced research is on pursuing a rigorous approach to specific research topics starting from a broad background in various areas of Information Technology, especially Computer Science and Engineering, Electronics, Systems and Control, and Telecommunications. Each year, more than 50 PhDs graduate from the program. This book gathers the outcomes of the thirteen best theses defended in 2020-21 and selected for the IT PhD Award. Each of the authors provides a chapter summarizing his/her findings, including an introduction, description of methods, main achievements and future work on the topic. Hence, the book provides a cutting-edge overview of the latest research trends in Information Technology at Politecnico di Milano, presented in an easy-to-read format that will also appeal to non-specialists

    Recent Advances in Single-Particle Tracking: Experiment and Analysis

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    This Special Issue of Entropy, titled “Recent Advances in Single-Particle Tracking: Experiment and Analysis”, contains a collection of 13 papers concerning different aspects of single-particle tracking, a popular experimental technique that has deeply penetrated molecular biology and statistical and chemical physics. Presenting original research, yet written in an accessible style, this collection will be useful for both newcomers to the field and more experienced researchers looking for some reference. Several papers are written by authorities in the field, and the topics cover aspects of experimental setups, analytical methods of tracking data analysis, a machine learning approach to data and, finally, some more general issues related to diffusion

    Electrical Properties of Model Lipid Membranes

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    Biological membranes are essential components of the living systems and processes occurring with their participation are related mainly to electric phenomena, such as signal transduction, the existence of membrane potentials, and transport through the membrane. It is well known that the universal model of the cell membrane structure is the lipid bilayer, which constitutes the environment for integral and surface membrane proteins. Thus, much attention has been given to the study of the organization and properties of these structures concerning both experimental and theoretical aspects. As systematic examinations are impeded by the complexity of the natural membranes, the best approach to conducting detailed physical and chemical studies of biological membranes is to use simplified well-defined model lipid membranes. Among the most commonly used are liposomes, planar lipid membranes, membranes on solid substrates, and lipid monolayers on the free surface.Studies of the electrical properties of model lipid membranes have been carried out for many years. However, there are still many issues that have not been verified experimentally and for which the existing results are incomplete or inconsistent. Therefore, the main objective of this book was to collect recent scientific and review articles on the electrical properties of model lipid membranes. This objective has been successfully achieved, for which I express heartfelt appreciation to all authors and reviewers for their excellent contributions

    Nanostructured piezoelectric materials for the design and development of self-sensing composite materials and energy harvesting devices

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    The work activities reported in this PhD thesis regard the functionalization of composite materials and the realization of energy harvesting devices by using nanostructured piezoelectric materials, which can be integrated in the composite without affecting its mechanical properties. The self-sensing composite materials were fabricated by interleaving between the plies of the laminate the piezoelectric elements. The problem of negatively impacting on the mechanical properties of the hosting structure was addressed by shaping the piezoelectric materials in appropriate ways. In the case of polymeric piezoelectric materials, the electrospinning technique allowed to produce highly-porous nanofibrous membranes which can be immerged in the hosting matrix without inducing delamination risk. The flexibility of the polymers was exploited also for the production of flexible tactile sensors. The sensing performances of the specimens were evaluated also in terms of lifetime with fatigue tests. In the case of ceramic piezo-materials, the production and the interleaving of nanometric piezoelectric powder limitedly affected the impact resistance of the laminate, which showed enhanced sensing properties. In addition to this, a model was proposed to predict the piezoelectric response of the self-sensing composite materials as function of the amount of the piezo-phase within the laminate and to adapt its sensing functionalities also for quasi-static loads. Indeed, one final application of the work was to integrate the piezoelectric nanofibers in the sole of a prosthetic foot in order to detect the walking cycle, which has a period in the order of 1 second. In the end, the energy harvesting capabilities of the piezoelectric materials were investigated, with the aim to design wearable devices able to collect energy from the environment and from the body movements. The research activities focused both on the power transfer capability to an external load and the charging of an energy storage unit, like, e.g., a supercapacitor

    Methodological document on statistical use of economic administrative records

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    This methodological document is intended to furnish the countries of the region with a set of guidelines and recommendations for producing innovative and timely economic statistics, and to provide greater benefits by making statistical use of administrative records on economic issues

    Anion Photoelectron Spectroscopic Studies: Antioxidants, Actinide Clusters, and Molecular Activation

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    Gas phase anion photoelectron spectroscopy is uniquely suited to study chemistry at the molecular level, as atoms, molecules, and clusters are isolated and thus unperturbed by confounding environmental effects which often complicate analyses carried out in the liquid or solid state. Photoelectron spectroscopy provides information about the electronic structure of anions, as well as the geometry of the anions and corresponding neutral species when combined with theoretical calculations. A variety of ion sources were employed to generate the anions in these studies: electrospray ionization (ESI), laser vaporization (LVS), and pulsed arc cluster ion source (PACIS). Using these techniques, two antioxidants, a range of actinide containing clusters, and multiple activation reactions were studied. Additionally, a new double rod laser vaporization source was designed and constructed to generate single atom catalyst (SAC) mimics. Chapter III presents the studies of this thesis and is divided into three major sections based on ion source: ESI, LVS, and PACIS. ESI brought the water-soluble antioxidants (ascorbate, deprotonated ascorbate, propyl gallate, and gallate) into the gas phase. LVS ablated uranium and thorium rods to generate gas phase atoms and actinide containing clusters, as well as highlighted the reaction between iridium and hydroxylamine and the phenomenon of intramolecular electron-induced proton transfer. Finally, using PACIS, two thorium clusters were generated, and CO2 activation with two metal hydrides were studied
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