Assembly, Elasticity, And Structure Of Lyotropic Chromonic Liquid Crystals And Disordered Colloids

This dissertation describes experiments which explore the structure and dynamics in two classes of soft materials: lyotropic chromonic liquid crystals and colloidal glasses and super-cooled liquids. The first experiments found that the achiral LCLCs, sunset yellow FCF (SSY) and disodium cromoglycate (DSCG) both exhibit spontaneous mirror symmetry breaking in the nematic phase driven by a giant elastic anisotropy of their twist modulus compared to their splay and bend moduli. Resulting structures of the confined LCLCs display interesting director configurations due to interplay of topologically required defects and twisted director fields. At higher concentrations, the LCLC compounds form columnar phases. We studied the columnar phase confined within spherical drops and discovered and understood configurations of the LC that sometimes led to non-spherical droplet shapes. The second experiments with SSY LCLCs confined in hollow cylinders uncovered director configurations which were driven in large measure by an exotic elastic modulus known as saddle-splay. We measured this saddle-splay modulus in a LCLC for the first time and found it to be more than 50 times greater than the twist elastic modulus. This large relative value of the saddle-splay modulus violates a theoretical result/assumption known as the Ericksen inequality. A third group of experiments on LCLCs explored the drying process of sessile drops containing SSY solutions, including evaporation dynamics, morphology, and deposition patterns. These drops differ from typical, well-studied evaporating colloidal drops primarily due to the LCLC\u27s concentration-dependent isotropic, nematic, and columnar phases. Phase separation occurs during evaporation, creating surface tension gradients and significant density and viscosity variation within the droplet. Thus, the drying multiphase drops exhibit new convective currents, drop morphologies, deposition patterns, as well as a novel ordered crystalline phase. Finally, experiments in colloidal glasses and super-cooled liquids were initiated to probe the relationship between structure and dynamics in their constituent particles. The displacements of individual particles in the colloids can be decomposed into small cage fluctuations and large rearrangements into new cages. We found a correlation between the rate of rearrangement and the local cage structure associated with each particle. Particle trajectories of a two-dimensional binary mixture of soft colloids are captured by video microscopy. We use a machine learning method to calculate particle ``softness\u27\u27, which indicates the likelihood of rearrangement based on many radial structural features for each particle. We measured the residence time between consecutive rearrangements and related probability distribution functions (PDFs). The softness-dependent conditional PDF is well fit by an exponential with decay time decreasing monotonically with increasing softness. Using these data and a simple thermal activation model, we determined activation energies for rearrangements

Dynamics of shallow flows with and without background rotation

Large-scale oceanic and atmospheric flows tend to behave in a two-dimensional way. To further understand such flows, a large scientific effort has been devoted to the study of perfect two-dimensional flows. For the last 30 years, there has been a large interest in experimentally validating the results from numerical and theoretical studies concerning two-dimensional flows, particularly twodimensional turbulence and spatially periodic two-dimensional flows. Inspired by geophysical flows, experimentalists have used stratification, shallow fluid layer configurations, and background rotation to enforce the two-dimensionality of flows in the laboratory. However, as all these methods have shortcomings, it is difficult to achieve a perfectly two-dimensional flow in the laboratory. The work presented in this thesis focuses on two of the common methods used to enforce the two-dimensionality of flows: the shallow layer configuration and background rotation. To further understand the effect of these methods on the two-dimensionality of flows, we studied the dynamics of generic elementary vortical structures in a shallow fluid layer with and without background rotation. Through the analytical and numerical study of a decaying axisymmetric monopolar vortex, we revised the usual argument for considering shallow flowsas two-dimensional. This argument is based on the continuity equation, and it states that the vertical velocity can be neglected if the ratio of vertical to horizontal length scales of the flow is small. By performing numerical simulations and a perturbation analysis for shallow flows, it was shown that this argument is not valid in general, and that the two-dimensionality of the flow does not depend exclusively on the aspect ratio. Instead, it also depends on the dynamics of the flow; particularly, a shallow flow behaves in a two-dimensional way if the flow evolution is dominated by bottom friction over the whole fluid depth. These results were supported by the numerical and experimental study of a more complex flow structure, namely a dipolar vortex, in a shallow fluid layer. For the study of decaying dipolar vortices, numerical simulations were performed using a finite element code. The flow was initialized with a Lamb–Chaplygin dipolar vortex with a Poiseuille-like vertical profile, after which it was left to evolve freely. The 3D structure of the vortex was obtained using the 2 vortex detection criterion. Using this tool, it was observed how the vortex is gradually distorted due to the secondary 3D motions. An experimental investigation of an electromagnetically forced dipolar vortex, where Particle Image Velocimetry (PIV) was used to calculate the velocity field in a horizontal cross-section of the flow, supports the numerically obtained results. It is assumed that flows subjected to strong background rotation behave like two-dimensional flows due to the reduction of gradients in the direction parallel to the rotation axis, as stated by the Taylor–Proudman theorem. This phenomenon results in the formation of columnar structures. In the current work, it was found that the flow can behave in a two-dimensional way as long as the rotation rate is fast enough, irrespective of the aspect ratio. In other words, this is true even if the fluid depth is of the same order as the thickness of the Ekman boundary layer, for which case no columnar structures are formed. This is attributed to the linear coupling between primary and secondary motions. From the study of decaying vortical structures, it was concluded that neither adding background rotation to a shallow flow nor decreasing the depth of a rotating flow necessarily increases the degree of two-dimensionality of the flow. The last two chapters of this thesis are dedicated to the study of a shallow dipolar structure that is continuously driven by time-independent electromagnetic forcing. For a shallow structure without background rotation, it was observed that for weak forcing the flow can be considered indeed as twodimensional. However, every shallow flow, even for very small fluid depths, becomes three-dimensional for a sufficiently high forcing magnitude. An equivalent result was obtained for a similar flow subjected to background rotation. The change in behavior is associated with a change in the vertical profile of the horizontal velocity, which is clearly absent in perfectly two-dimensional flow. The results presented in this thesis confirm that under certain conditions shallow flows and flows subjected to background rotation can behave as a twodimensional flow. However, more importantly, it is shown that there are clear limits to this behavior. This work presents a better understanding of the basic dynamics of shallow flows with and without background rotation and of the extent to which these flows can be considered as quasi-two-dimensional

Data Assimilation in high resolution Numerical Weather Prediction models to improve forecast skill of extreme hydrometeorological events.

The complex orography typical of the Mediterranean area supports the formation, mainly during the fall season, of the so-called back-building Mesoscale Convective Systems (MCS) producing torrential rainfall often resulting into flash floods. These events are hardly predictable from a hydrometeorological standpoint and may cause significant amount of fatalities and socio-economic damages. Liguria region is characterized by small catchments with very short hydrological response time, and it has been proven to be very exposed to back-building MCSs occurrence. Indeed this region between 2011 and 2014 has been hit by three intense back-building MCSs causing a total death toll of 20 people and several hundred million of euros of damages. Building on the existing relationship between significant lightning activity and deep convection and precipitation, the first part of this work assesses the performance of the Lightning Potential Index, as a measure of the potential for charge generation and separation that leads to lightning occurrence in clouds, for the back-building Mesoscale Convective System which hit Genoa city (Italy) in 2014. An ensemble of Weather Research and Forecasting simulations at cloud-permitting grid spacing (1 km) with different microphysical parameterizations is performed and compared to the available observational radar and lightning data. The results allow gaining a deeper understanding of the role of lightning phenomena in the predictability of back-building Mesoscale Convective Systems often producing flash flood over western Mediterranean complex topography areas. Despite these positive and promising outcomes for the understanding highly-impacting MCS, the main forecasting issue, namely the uncertainty in the correct reproduction of the convective field (location, timing, and intensity) for this kind of events still remains open. Thus, the second part of the work assesses the predictive capability, for a set of back-building Liguria MCS episodes (including Genoa 2014), of a hydro-meteorological forecasting chain composed by a km-scale cloud resolving WRF model, including a 6 hour cycling 3DVAR assimilation of radar reflectivity and conventional ground sensors data, by the Rainfall Filtered Autoregressive Model (RainFARM) and the fully distributed hydrological model Continuum. A rich portfolio of WRF 3DVAR direct and indirect reflectivity operators, has been explored to drive the meteorological component of the proposed forecasting chain. The results confirm the importance of rapidly refreshing and data intensive 3DVAR for improving first quantitative precipitation forecast, and, subsequently flash-floods occurrence prediction in case of back-building MCSs events. The third part of this work devoted the improvement of severe hydrometeorological events prediction has been undertaken in the framework of the European Space Agency (ESA) STEAM (SaTellite Earth observation for Atmospheric Modelling) project aiming at investigating, new areas of synergy between high-resolution numerical atmosphere models and data from spaceborne remote sensing sensors, with focus on Copernicus Sentinels 1, 2 and 3 satellites and Global Positioning System stations. In this context, the Copernicus Sentinel satellites represent an important source of data, because they provide a set of high-resolution observations of physical variables (e.g. soil moisture, land/sea surface temperature, wind speed, columnar water vapor) to be used in NWP models runs operated at cloud resolving grid spacing . For this project two different use cases are analyzed: the Livorno flash flood of 9 Sept 2017, with a death tool of 9 people, and the Silvi Marina flood of 15 November 2017. Overall the results show an improvement of the forecast accuracy by assimilating the Sentinel-1 derived wind and soil moisture products as well as the Zenith Total Delay assimilation both from GPS stations and SAR Interferometry technique applied to Sentinel-1 data

Solidification and Gravity VII

International audienc

Direct Numerical Simulation of Transition and Turbulence in Magnetohydrodynamic Flows in Rectangular Ducts

In this work, a flow of an electrically conducting fluid is driven through a rectangular duct by a constant pressure gradient in the presence of a transverse, externally applied magnetic field: the flow is studied using the method of Direct Numerical Simulation (DNS). This particular Magnetohydrodynamic (MHD) flow investigation is important in the development of liquid metal blankets design, which is the proposed cooling system within nuclear fusion reactors. The duct walls parallel to the magnetic field are ideally electrically insulating, while the walls perpendicular to the magnetic field are ideally electrically conducting. This flow is referred to as a Hunt’s flow. In this work the emergence of time dependent flow and its transition to a fully developed turbulent regime is explored. By fixing the strength of the magnetic field and increasing the fluid velocity, a number of time-dependent flow regimes have been observed in the side layers, which includes Ting-Walker vortices, elongated vortical structures, fully turbulent side-wall jets, as well as singular and multiple side-wall jet detachments. It has been found that at low velocities, the time-dependant flow takes the form of TingWalker vortices, which develop in the side layers of the duct. For all but the lowest magnetic fields studied, the Ting-Walkers vortices completely disappear after a short initial transient time, being replaced by new, higher energy, complex, anisotropic vortical structures. Additionally, a number of new flow regimes involving jet detachment have been identified. This study also demonstrates that Hunt’s flow exhibits hysteresis behaviour, where different unsteady states are possible for the same flow parameters

Strong disorder RG approach of random systems

There is a large variety of quantum and classical systems in which the quenched disorder plays a dominant r\^ole over quantum, thermal, or stochastic fluctuations : these systems display strong spatial heterogeneities, and many averaged observables are actually governed by rare regions. A unifying approach to treat the dynamical and/or static singularities of these systems has emerged recently, following the pioneering RG idea by Ma and Dasgupta and the detailed analysis by Fisher who showed that the Ma-Dasgupta RG rules yield asymptotic exact results if the broadness of the disorder grows indefinitely at large scales. Here we report these new developments by starting with an introduction of the main ingredients of the strong disorder RG method. We describe the basic properties of infinite disorder fixed points, which are realized at critical points, and of strong disorder fixed points, which control the singular behaviors in the Griffiths-phases. We then review in detail applications of the RG method to various disordered models, either (i) quantum models, such as random spin chains, ladders and higher dimensional spin systems, or (ii) classical models, such as diffusion in a random potential, equilibrium at low temperature and coarsening dynamics of classical random spin chains, trap models, delocalization transition of a random polymer from an interface, driven lattice gases and reaction diffusion models in the presence of quenched disorder. For several one-dimensional systems, the Ma-Dasgupta RG rules yields very detailed analytical results, whereas for other, mainly higher dimensional problems, the RG rules have to be implemented numerically. If available, the strong disorder RG results are compared with another, exact or numerical calculations.Comment: review article, 195 pages, 36 figures; final version to be published in Physics Report