Ice crystal | EPFL Graph Search

Ice crystals are solid ice in symmetrical shapes including hexagonal columns, hexagonal plates, and dendritic crystals. Ice crystals are responsible for various atmospheric optic displays and cloud formations. Formation At ambient temperature and pressure, water molecules have a V shape. The two hydrogen atoms bond to the oxygen atom at a 105° angle. Ice crystals have a hexagonal crystal lattice, meaning the water molecules arrange themselves into layered hexagons upon freezing. Slower crystal growth from colder and drier atmospheres produces more hexagonal symmetry. Depending on environmental temperature and humidity, ice crystals can develop from the initial hexagonal prism into many symmetric shapes. Possible shapes for ice crystals are columns, needles, plates and dendrites. Mixed patterns are also possible. The symmetric shapes are due to depositional growth, which is when ice forms directly from w

Spatial and Morphological Control of Cyanine Dye Thin Films

Nicolas Alexandre Serge Leclaire

Cyanine dyes are organic semiconductor compounds with light absorption and emission properties useful for emerging technologies such as solar cells and light-emitting devices. The characteristics of these materials in the solid state depend on their organization of the constituting building blocks. This thesis focuses on controlling the morphology of cyanine dye thin films at different length scales and clarifying the resulting properties. When microstructures present features whose size matches visible light wavelengths, new properties may arise from light-matter interactions. Here the properties resulting from the light-matter interactions of cyanine droplet films cast from solution are studied. Based on experimental evidence, it is shown that dye droplet ensembles scatter light with different efficiencies and wavelength ranges depending on their dimensions. FDTD simulations are used to show that this effect results fromscattering enhancement at the absorption edge of the dye where the refractive index varies considerably. Simulations also provide a better understanding of individual dropletsâ interaction with light. While earlier work had hypothesized that the observed scattering phenomenon were due to crystalline clusters within the droplets, this work highlights the contribution of the dye filmmorphology. Cyanines also form single crystals whose fabrication induces molecular-scale order in the material. Previous work demonstrated that thin single crystals could be grown by solvent vapor annealing of dye droplets. Here it is shown that in uncontrolled conditions, cyanine single crystals destabilize to formdendritic crystals. In-situ microscopy observations highlight the solute reservoir role of the droplet distribution surrounding a growing crystal; when the distance between droplets and the crystal front is large, the solute supply is diffusion-limited. Moreover variations in local pressure equilibrium between the droplets and crystal front lead to advection fluxes which perturb the crystal growth. These observations help design configurations to either prevent crystal destabilization or take advantage of the dendritic growth in a controlled manner. In addition, the patterning of crystals on a substrate is relevant for their application in devices. A practical challenge is to induce single crystal growth at specific locations. Here, surfaces patterned with SAMs of hydrophilic and hydrophobic thiols are used to create dye droplet arrays from which crystals can be grown. This method is shown to yield local crystallization of the dye and to prevent crystal destabilization through better control of the droplet distribution. By varying the dye solution concentration, partial control over crystal density is achieved, however it proved difficult to control the number of nuclei per droplet. A more controlled evaporation and solvent vapor annealing system might be necessary to master the nucleation process. Finally the structure and optical properties of cyanine single crystals are addressed. The crystal structure was determined by X-ray diffraction. Structural aspects are shown to lead to excitonic couplings, which are evidenced by orientation-dependent spectroscopic measurements of single crystals. Although further investigation of the absorption band structure is necessary, the results are promising for photovoltaic devices as they might improve exciton transport compared to amorphous layers.

EPFL2018

Aerosol Transport and Distribution

Quentin Bourgeois

The fourth report of the Intergovernmental Panel of Climate Change (IPCC) unambiguously indicates that the climate is changing. Climate change is induced by changes in atmospheric abundances of greenhouse gases and aerosols, solar radiation and land surface properties, which all alter the radiative balance of the Earth. Among them, greenhouse gases and aerosols are likely the compounds affecting the climate the most. While greenhouse gases warm the Earth, aerosols influence its radiative balance by scattering and absorbing solar radiation (the direct effect), and by modifying cloud amount and properties (the indirect effect). The radiative forcing of aerosols on the climate is however the less understood among the various forcings currently considered in the IPCC assessment. This is likely related to the lack of comprehensive and accurate observations of aerosols (and in particular of their vertical distribution) which thus makes global aerosol models difficult to constrain. Recent model intercomparisons have indicated that different assumptions regarding aerosol emissions, formation and growing properties, and removal processes generate large diversities within models. These large diversities are found in particular in the Arctic and the upper troposphere region where extreme surrounding conditions take place, limiting instrument sensitivity and accuracy. The objectives of this thesis are to better quantify the processes driving the aerosol distribution in regions where the uncertainties are the largest, including the Arctic region and the upper troposphere, and to assess the quality of CALIOP observations. For this purpose, we used the fully coupled global climate-aerosol-chemistry ECHAM5-HAMMOZ model conjointly with a suite of remote sensing observations including the CALIOP satellite instrument, which provides the vertical distribution of aerosols. The model predicts the size distribution and composition of aerosols as well as the number concentration of cloud droplets and ice crystals. We first investigated the processes that drive the transport of soluble and insoluble compounds toward the Arctic in the ECHAM5-HAMMOZ model. Recognizing that scavenging processes may be an issue in global models, we "re-visited" their properties in ECHAM5-HAMMOZ. We find that the model better reproduces the aerosol vertical distribution in the northern mid- and high-latitudes, especially in the free troposphere, by decreasing aerosol scavenging coefficients in the model. Using smaller aerosol scavenging coefficients results in an increase of aerosol burden and lifetime, especially in the Arctic. In addition, the Arctic haze phenomenon is better represented in the simulation using decreased aerosol scavenging coefficients. The relative contributions of different processes governing the transport of aerosols from the midlatitudes toward the Arctic together with the relative contributions of different geopolitical source regions were then quantified. We find that Europe and Siberia contribute the most to the Arctic pollution even though Asian anthropogenic emissions are much larger. The model suggests that BC emitted in Siberia and Europe is ten times more efficiently deposited onto the Arctic than BC emitted in Asia. We next investigated an aerosol layer that forms in the vicinity of the tropical tropopause layer (TTL) over the southern Asian and Indian Ocean region. Aerosols in this layer are observed with the CALIOP instrument even though ice clouds partly mask their signal. The aerosol layer follows the Inter Tropical Convergence Zone (ITCZ), which indicates that these aerosols are likely transported during convective processes. The ECHAM5.5-HAM2 model reproduces such an aerosol layer in the TTL over the same region but clearly overestimates CALIOP data. Model results suggest that aerosols in this layer are mostly sulfate particles transported during convective processes and originating from natural sources. The shortwave forcing of this aerosol layer is estimated. Finally, we assessed the accuracy of aerosol extinction retrievals from the CALIOP satellite instrument by comparing them with remote sensing observations such as AERONET, MODIS and MISR over the last four years. Overall, standard CALIOP products underestimate by 50% observed aerosol extinctions. However, this comparison is substantially improved by screening CALIOP data. The comparison of the CALIOP screened product with the ECHAM5.5-HAM2 model shows large disparities in model results. The model reproduces quite well the observed interannual variability and vertical distribution of aerosols in Europe and North Africa while it underestimates them in America and Asia. This is likely due to an underestimate of dust and anthropogenic emissions in North America and Asia.

EPFL2012

Monte Carlo Modeling of Crystal Channeling at High Energies

Philippe Jean Schoofs

Charged particles entering a crystal close to some preferred direction can be trapped in the electromagnetic potential well existing between consecutive planes or strings of atoms. This channeling effect can be used to extract beam particles if the crystal is bent beforehand. Crystal channeling is becoming a reliable and efficient technique for collimating beams and removing halo particles. At the European Organization for Nuclear Research (CERN), the installation of silicon crystals in the Large Hadron Collider (LHC) is under scrutiny by the UA9 collaboration with the goal of investigating if they are a viable option for the collimation system upgrade. This thesis describes a new Monte Carlo model of planar channeling which has been developed from scratch in order to be implemented in the FLUKA code simulating particle transport and interactions. Crystal channels are described through the concept of continuous potential taking into account thermal motion of the lattice atoms and using Moliere screening function. The energy of the particle transverse motion determines whether or not it is trapped between the crystal planes while single Coulomb scattering on lattice atoms can lead to dechanneling. The volume capture and reflection applying to quasi-channeled particles are also modeled. Analogously to dechanneling, single scattering is used to determine the occurrence of volume capture. The parameters of the crystals, such as torsion or miscut, are described as well. For channeled particles, the suppression of electromagnetic and nuclear collisions is implemented. It is stronger for particles oscillating close to the center of the channel and is crucial for a correct evaluation of the rate of dechanneling without having recourse to the use of a macroscopic dechanneling length. The UA9-H8 experiment conducted at CERN aims at investigating new crystal physics as well as characterizing crystals that can be of interest in view of the implementation of crystal collimation at CERN, including in the LHC. This experiment uses silicon strip detectors situated on both sides of the crystal. Putting together upstream and downstream tracks in coincidence and matching at an identical fitted location on the crystal, it yields information about the deflections given to the beam population. Several runs from the UA9-H8 experiment are analyzed and compared to the model results. Channeling and dechanneling rates, as well as angular distributions at crystal exit are shown to be in a very encouraging agreement both for strip and quasi-mosaic crystals.

EPFL2014