18,266 research outputs found
Chiral active fluids: Odd viscosity, active turbulence, and directed flows of hydrodynamic microrotors
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 FaxĆ©nās 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
Donor-anion interactions in quarter-filled low-dimensional organic conductors
Anions have often been considered to act essentially as electron donors or acceptors in molecular conductors. However there is now growing evidence that they play an essential role in directing the structural and hence electronic properties of many of these systems. After reviewing the basic interactions and different ground states occurring in molecular conductors we consider in detail how anions influence the structure of donor stacks and often guide them toward different types of transitions. Consideration of the Bechgaard and Fabre salts illustrates how anions play a crucial role in directing these salts through complex phase diagrams where different conducting and localized states are in competition. We also emphasize the important role of hydrogen bonding and conformational flexibility of donors related to BEDT-TTF and we discuss how anions have frequently a strong control of the electronic landscape of these materials. Charge ordering, metal to metal and metal to insulator transitions occurring in these salts are considered
Discovering the hidden structure of financial markets through bayesian modelling
Understanding what is driving the price of a financial asset is a question that is currently mostly unanswered. In this work we go beyond the classic one step ahead prediction and instead construct models that create new information on the behaviour of these time series. Our aim is to get a better understanding of the hidden structures that drive the moves of each financial time series and thus the market as a whole.
We propose a tool to decompose multiple time series into economically-meaningful variables to explain the endogenous and exogenous factors driving their underlying variability. The methodology we introduce goes beyond the direct model forecast. Indeed, since our model continuously adapts its variables and coefficients, we can study the time series of coefficients and selected variables. We also present a model to construct the causal graph of relations between these time series and include them in the exogenous factors.
Hence, we obtain a model able to explain what is driving the move of both each specific time series and the market as a whole. In addition, the obtained graph of the time series provides new information on the underlying risk structure of this environment. With this deeper understanding of the hidden structure we propose novel ways to detect and forecast risks in the market. We investigate our results with inferences up to one month into the future using stocks, FX futures and ETF futures, demonstrating its superior performance according to accuracy of large moves, longer-term prediction and consistency over time. We also go in more details on the economic interpretation of the new variables and discuss the created graph structure of the market.Open Acces
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Superfluidity and Superconductivity in Body-centred-cubic and Face-centred-cubic Systems
The microscopic description of phases in strongly correlated systems such as the fullerides (A3C60) is a challenge. In particular, how these strong interactions become attraction leading to a superconducting state remains a mystery. Understanding the mechanism(s) that drive(s) unconventional superconductivity is one of the most sought-after goals in many-body physics and indeed very complex to solve.
The aim of this thesis is, firstly, to investigate the conditions in which pairing may take place between two electrons in both body-centred cubic (BCC) and face-centred cubic (FCC) systems, and secondly, to examine the possibility for the emergence of a superconducting or superfluid state from paired electrons in three-dimensional (3D) systems. Here, pair properties are studied both in the anti-adiabatic and adiabatic limits.
In the anti-adiabatic limit, we use a symmetrised approach, group theory analysis, and perturbation theory to exactly solve the two-body problem and analyse the properties of the electron pair. We also examine, using a continuous-time Monte Carlo algorithm (CTQMC), the effects of retarded electron-phonon interactions on the pair properties away from the anti-adiabatic limit. In the high-phonon frequency limit, the CTQMC also serves as a validation check for the anti-adiabatic analytic result and vice-versa (with both results showing perfect agreement).
Our result predicts that superfluidity can occur in BCC optical lattices up to a few tens of nanokelvin for fermionic lithium-6 atoms. Additionally, we found that, in the high-frequency limit, a paired state in an FCC lattice can be extremely light and small as compared to paired states on other 3D lattices. Such superlight states are expected to yield high transition temperatures under favourable circumstances. However, when the retardation effects arising from the electron-phonon interaction become important, bound pairs in the BCC lattice become lighter by orders of magnitude in a wide region of the parameter space. We also found significant long-range effects due to the vibration of the alkali ions in the cesium-doped fulleride systems leading to the creation of light pairs in its BCC structure
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Vortex identification methods applied to wind turbine tip vortices
This study describes the impact of postprocessing methods on the calculated parameters of tip vortices of a wind turbine model when tested using particle image velocimetry (PIV). Several vortex identification methods and differentiation schemes are compared. The chosen methods are based on two components of the velocity field and their derivatives. They are applied to each instantaneous velocity field from the dataset and also to the calculated average velocity field. The methodologies are compared through the vortex center location, vortex core radius and jittering zone.
Results show that the tip vortex center locations and radius have good comparability and can vary only a few grid spacings between methods. Conversely, the convection velocity and the jittering surface, defined as the area where the instantaneous vortex centers are located, vary between identification methods.
Overall, the examined parameters depend significantly on the postprocessing method and selected vortex identification criteria. Therefore, this study proves that the selection of the most suitable postprocessing methods of PIV data is pivotal to ensure robust results
Towards A Graphene Chip System For Blood Clotting Disease Diagnostics
Point of care diagnostics (POCD) allows the rapid, accurate measurement of analytes near to a patient. This enables faster clinical decision making and can lead to earlier diagnosis and better patient monitoring and treatment. However, despite many prospective POCD devices being developed for a wide range of diseases this promised technology is yet to be translated to a clinical setting due to the lack of a cost-eļ¬ective biosensing platform.This thesis focuses on the development of a highly sensitive, low cost and scalable biosensor platform that combines graphene with semiconductor fabrication tech-niques to create graphene ļ¬eld-eļ¬ect transistors biosensor. The key challenges of designing and fabricating a graphene-based biosensor are addressed. This work fo-cuses on a speciļ¬c platform for blood clotting disease diagnostics, but the platform has the capability of being applied to any disease with a detectable biomarker.Multiple sensor designs were tested during this work that maximised sensor ef-ļ¬ciency and costs for diļ¬erent applications. The multiplex design enabled diļ¬erent graphene channels on the same chip to be functionalised with unique chemistry. The Inverted MOSFET design was created, which allows for back gated measurements to be performed whilst keeping the graphene channel open for functionalisation. The Shared Source and Matrix design maximises the total number of sensing channels per chip, resulting in the most cost-eļ¬ective fabrication approach for a graphene-based sensor (decreasing cost per channel from Ā£9.72 to Ā£4.11).The challenge of integrating graphene into a semiconductor fabrication process is also addressed through the development of a novel vacuum transfer method-ology that allows photoresist free transfer. The two main fabrication processes; graphene supplied on the wafer āPre-Transferā and graphene transferred after met-allisation āPost-Transferā were compared in terms of graphene channel resistance and graphene end quality (defect density and photoresist). The Post-Transfer pro-cess higher quality (less damage, residue and doping, conļ¬rmed by Raman spec-troscopy).Following sensor fabrication, the next stages of creating a sensor platform involve the passivation and packaging of the sensor chip. Diļ¬erent approaches using dielec-tric deposition approaches are compared for passivation. Molecular Vapour Deposi-tion (MVD) deposited Al2O3 was shown to produce graphene channels with lower damage than unprocessed graphene, and also improves graphene doping bringing the Dirac point of the graphene close to 0 V. The packaging integration of microļ¬uidics is investigated comparing traditional soft lithography approaches and the new 3D printed microļ¬uidic approach. Speciļ¬c microļ¬uidic packaging for blood separation towards a blood sampling point of care sensor is examined to identify the laminar approach for lower blood cell count, as a method of pre-processing the blood sample before sensing.To test the sensitivity of the Post-Transfer MVD passivated graphene sensor de-veloped in this work, real-time IV measurements were performed to identify throm-bin protein binding in real-time on the graphene surface. The sensor was function-alised using a thrombin speciļ¬c aptamer solution and real-time IV measurements were performed on the functionalised graphene sensor with a range of biologically relevant protein concentrations. The resulting sensitivity of the graphene sensor was in the 1-100 pg/ml concentration range, producing a resistance change of 0.2% per pg/ml. Speciļ¬city was conļ¬rmed using a non-thrombin speciļ¬c aptamer as the neg-ative control. These results indicate that the graphene sensor platform developed in this thesis has the potential as a highly sensitive POCD. The processes developed here can be used to develop graphene sensors for multiple biomarkers in the future
Stratosphere-Troposphere Coupling and Oceanic Feedbacks in an Aquaplanet Model
Since the 90ās, there have been many studies looking at the impact of stratospheric temperature perturbations (STPās) on the troposphere below, and the mechanisms involved. However, it is
not well-understood how these interactions might be modified by atmosphere-ocean coupling.
This holds particular relevance to the study of e.g. ozone depletion/recovery in the Southern
Ocean region.
To investigate this, abrupt uniform, polar, and equatorial STPās ā corresponding to different
types of forcings - were applied to the atmosphere of MITgcm in an aquaplanet, double-drake configuration. Each was conducted in three different versions of the model: atmosphere-only, atmosphere + slab-ocean, and fully-coupled atmosphere + ocean. In the atmosphere-only model, atmospheric responses similar to those of previous studies were recorded, underscoring their generic nature. In the coupled models, an initial weakening (~decades), and ā in the fully coupled model - subsequent strengthening and poleward-shift (~centuries) of the underlying
atmospheric response was produced, in polar/equatorial heating experiments. Uniform heating saw the latter process occur initially too. Sea surface temperature (SST) anomalies were found to drive these changes, with extratropical/tropical anomalies controlling the former/latter process.
These were in turn attributable to changes in the zonal winds, causing anomalies in the turbulent
and ocean heat fluxes, although uniform STPās saw SST changes that were more controlled by
large, near-uniform anomalous downwelling longwave fluxes.
Our results highlight the importance of incorporating atmosphere-ocean coupling when
studying the effects of STPās, especially over longer timescales (&100 years). With respect
to ozone depletion in the Southern Ocean, they suggest an amplified poleward jet shift which
- as greenhouse gases continue to rise ā may continue and amplify further into the 21st/22nd
century.Open Acces
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