98 research outputs found
High-Speed Imaging and Electrochemical Studies during the Freezing of Supercooled Aqueous Droplets
In der vorliegenden Arbeit werden Wassertropfen mit einem Durchmesser von etwa 2-3 mm akustisch levitiert
oder zwischen sehr dünnen Drahtringen positioniert, um Randeffekte zu vermeiden,
und um bis zu 24 K unterkühlt. Weil die Schmelzenthalpie nicht schnell genug an die Umgebung abgegeben
werden kann und nur teilweise im Tropfen gespeichert werden kann, spaltet sich der Gefrierprozess
in zwei Phasen auf. Per Hochgeschwindigkeitskamera wird in einem für diese Arbeiten entwickelten Kühlsystem
in unterkühlten Wassertropfen ein Eiswachstum mit konstanter, schneller Geschwindigkeit
beobachtet. Sie liegt in der Größenordnung von 0,1 m/s und wächst mit steigender Unterkühlung zunächst
linear bis quadratisch an. Dagegen wird für stark unterkühlte Tropfen eine Tendenz zu einem
Geschwindigkeitsmaximum beobachtet. Es wird ein neues Modell zur Beschreibung der Gefriergeschwindigkeit
vorgestellt, welches in gutem Einklang mit den experimentellen Befunden steht.
Um die komplexen Wärmeproduktions- und -transportprozesse beim Gefrieren der Tropfen
zu erfassen, wurden zwei Modelle im Rahmen von FEM-Simulationen entwickelt und damit die
Evolution der Verteilungen von Wärme und dendritischem Eis für viele Kombinationen von Radius,
Unterkühlung, Gefriergeschwindigkeit und relativer Tropfengeschwindigkeit berechnet. Es
wird gezeigt, dass erst für sehr kleine Tropfen die Oberfläche groß genug ist,
um einen signifikanten Anteil der Schmelzwärme an die Umgebung abzugeben und der kritische Radius,
der ein einstufiges Gefrieren ermöglicht, zwischen 0,1 und 1 Mikrometern zu erwarten
und hauptsächlich von der Unterkühlung sowie der Gefriergeschwindigkeit abhängig ist.
Diese Grenze liegt innerhalb der Größenverteilung der unterkühlten Tropfen in der Atmosphäre.
Als ein wesentliches Ergebnis dieser Arbeit werden erstmalig jeweils ein elektrischer Effekt für
die erste und die zweite Gefrierstufe an unterkühlten Tropfen beobachtet und untersucht. Der
Effekt in der ersten Gefrierstufe zeigt einen charakteristischen Doppelpeak mit einer Amplitude
von bis zu 3 V, ist abhängig von der Art der Ionen und der Unterkühlung. Während der Effekt
für höhere Konzentrationen abrupt einbricht, verschwindet er nicht mit abnehmender Ionenkonzentration.
Der beobachtete Effekt der zweiten Gefrierstufe ähnelt in seiner Gestalt dem bekannten
Workman-Reynolds-Effekt und ist in seiner Stärke und Polarität von der vorausgehenden dendritischen Phase,
insbesondere von ihrer Gefrierrichtung, abhängig.In this work water droplets with a diameter of about 2-3 mm are levitated acoustically or
positioned between thin wire loops to minimize wall effects and cooled down up to 249 K.
Because the heat cannot be released to the environment quickly enough and can only be partially stored in the system,
the freezing process splits in two stages. In this work fast and constant freezing speeds of
supercooled water droplets are measured with a high-speed camera in a newly developed cooling chamber.
The ice grows roughly planar through the droplet. Furthermore, details of the dendritic structure are noted
in some cases. The freezing speed is in the order of 0.1 m/s and increases with supercooling linearly
to quadratically, but for the strongest supercoolings the freezing speed tends to reach a maximum.
Based on the theory of Wilson and Frenkel a new model is presented, which predicts the freezing speed
as a function of supercooling under consideration of the dendritic freezing stage and is in good agreement
with the experimental data. Two new finite element models are have been developed to unravel the complex
heat production and transfer processes during the whole freezing of the supercooled droplets.
So, the evolution of heat and ice portion are computed for many combinations of droplet radii, supercoolings,
freezing speeds and relative droplet speeds. It is shown, that only for very small droplets a significant
portion of the freezing enthalpy is released to the environment. As a further important result
the critical radius, which allows a one step freezing, is estimated to exist between 0.1 and 1 micrometers.
This critical radius depends mainly on the supercooling temperature and the freezing speed and
meets well the size distribution of droplets in the atmosphere.
Further results are the observation of two electric effects during both freezing steps. The
effect in the first step shows a characteristic double peak with an amplitude of 3 V and depends
on sort and concentration of the ions as well as on the supercooling. Whereas the effect vanishes
for high concentrations, it persist to exist even for very low concentrations in contrast to the
Workman-Reynolds-freezing-potential. The characteristics of this effect in the second freezing
step is similar to the WRFP and the polarity as well as the strength of the effect are depending on
the direction of proceeding dendritic freezing step.
The findings are relevant in particular for atmospheric physics and chemistry
Imaging Temperature and Thickness of Thin Planar Liquid Water Jets in Vacuum
We present spatially resolved measurements of the temperature of a flat
liquid water microjet for varying pressures, from vacuum to 100% relative
humidity. The entire jet surface is probed in a single shot by a
high-resolution infrared camera. Obtained 2D images are substantially
influenced by the temperature of the apparatus on the opposite side of the IR
camera; a protocol to correct for the thermal background radiation is
presented. In vacuum, we observe cooling rates due to water evaporation on the
order of 105 K/s. For our system, this corresponds to a temperature decrease of
approximately 15 K between upstream and downstream positions of the flowing
leaf. Making reasonable assumptions on the absorption of the thermal background
radiation in the flatjet we can extend our analysis to infer a thickness map.
For a reference system our value for the thickness is in good agreement with
the one reported from white light interferometry.Comment: The following article has been submitted to Structural Dynamics.
After it is published, it will be found at Lin
Observation of Intermolecular Coulombic Decay and Shake-up Satellites in Liquid Ammonia
We report the first nitrogen 1s Auger–Meitner electron spectrum from a liquid ammonia microjet at a temperature of ~223 K (–50 °C) and compare it with the simultaneously measured spectrum for gas-phase ammonia. The spectra from both phases are interpreted with the assis- tance of high-level electronic structure and ab initio molecular dynamics calculations. In addition to the regular Auger–Meitner-electron features, we observe electron emission at kinetic energies of 374–388 eV, above the leading Auger–Meitner peak (3a12). Based on the electronic structure calculations, we assign this peak to a shake-up satellite in the gas phase, i.e., Auger–Meitner emission from an intermediate state with additional valence excitation present. The high-energy contribution is significantly enhanced in the liquid phase. We consider various mechanisms contributing to this feature. First, in analogy with other hydrogen-bonded liquids (noticeably water), the high-energy signal may be a signature for an ultrafast proton transfer taking place before the electronic decay (proton transfer mediated charge separation). The ab initio dynamical calculations show, however, that such a process is much slower than electronic decay and is, thus, very unlikely. Next, we consider a non-local version of the Auger–Meitner decay, the Intermolecular Coulombic Decay. The electronic structure calculations support an important contribution of this purely electronic mechanism. Finally, we discuss a non-local enhancement of the shake-up processes
Spectroscopic evidence for a gold-coloured metallic water solution
Insulating materials can in principle be made metallic by applying pressure. In the case of pure water, this is estimated1 to require a pressure of 48 megabar, which is beyond current experimental capabilities and may only exist in the interior of large planets or stars2–4. Indeed, recent estimates and experiments indicate that water at pressures accessible in the laboratory will at best be superionic with high protonic conductivity5, but not metallic with conductive electrons1. Here we show that a metallic water solution can be prepared by massive doping with electrons upon reacting water with alkali metals. Although analogous metallic solutions of liquid ammonia with high concentrations of solvated electrons have long been known and characterized6–9, the explosive interaction between alkali metals and water10,11 has so far only permitted the preparation of aqueous solutions with low, submetallic electron concentrations12–14. We found that the explosive behaviour of the water–alkali metal reaction can be suppressed by adsorbing water vapour at a low pressure of about 10−4 millibar onto liquid sodium–potassium alloy drops ejected into a vacuum chamber. This set-up leads to the formation of a transient gold-coloured layer of a metallic water solution covering the metal alloy drops. The metallic character of this layer, doped with around 5 × 1021 electrons per cubic centimetre, is confirmed using optical reflection and synchrotron X-ray photoelectron spectroscopies
\AA ngstrom depth resolution with chemical specificity at the liquid-vapor interface
The determination of depth profiles across interfaces is of primary
importance in many scientific and technological areas. Photoemission
spectroscopy is in principle well suited for this purpose, yet a quantitative
implementation for investigations of liquid-vapor interfaces is hindered by the
lack of understanding of electron-scattering processes in liquids. Previous
studies have shown, however, that core-level photoelectron angular
distributions (PADs) are altered by depth-dependent elastic electron scattering
and can, thus, reveal information on the depth distribution of species across
the interface. Here, we explore this concept further and show that the
anisotropy parameter characterizing the PAD scales linearly with the average
distance of atoms along the surface normal. This behavior can be accounted for
in the low-collision-number regime. We also show that results for different
atomic species can be compared on the same length scale. We demonstrate that
atoms separated by about 1~\AA~along the surface normal can be clearly
distinguished with this method, achieving excellent depth resolution.Comment: Submitted to Phys. Rev. Let
Deeply cooled and temperature controlled microjets Liquid ammonia solutions released into vacuum for analysis by photoelectron spectroscopy
A versatile, temperature controlled apparatus is presented, which generates deeply cooled liquid microjets of condensed gases, expelling them via a small aperture into vacuum for use in photoelectron spectroscopy PES . The functionality of the design is demonstrated by temperatureand concentration dependent PES measurements of liquid ammonia and solutions of KI and NH4I in liquid ammonia. The experimental setup is not limited to the usage of liquid ammonia solutions solel
Photoelectron spectra of alkali metal–ammonia microjets: From blue electrolyte to bronze metal
Experimental studies of the electronic structure of excess electrons in liquids—archetypal quantum solutes—have been largely restricted to very dilute electron concentrations. We overcame this limitation by applying soft x-ray photoelectron spectroscopy to characterize excess electrons originating from steadily increasing amounts of alkali metals dissolved in refrigerated liquid ammonia microjets. As concentration rises, a narrow peak at ~2 electron volts, corresponding to vertical photodetachment of localized solvated electrons and dielectrons, transforms continuously into a band with a sharp Fermi edge accompanied by a plasmon peak, characteristic of delocalized metallic electrons. Through our experimental approach combined with ab initio calculations of localized electrons and dielectrons, we obtain a clear picture of the energetics and density of states of the ammoniated electrons over the gradual transition from dilute blue electrolytes to concentrated bronze metallic solutions
Compression of a Stearic Acid Surfactant Layer on Water Investigated by Ambient Pressure X-ray Photoelectron Spectroscopy
We present a combined Langmuir–Pockels trough and ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the compression of stearic acid surfactant layers on neat water. Changes in the packing density of the molecules are directly determined from C 1s and O 1s APXPS data. The experimental data are fit with a 2D model for the stearic acid coverage. Based on the results of these proof-of-principle experiments, we discuss the remaining challenges that need to be overcome for future investigations of the role of surfactants in heterogeneous chemical reactions at liquid–vapor interfaces in combined Langmuir–Pockels trough and APXPS measurements
Hydrolysis of oligosaccharides over solid acid catalysts: a review
Mild fractionation/pretreatment processes are becoming the most preferred choices for biomass processing within the biorefinery framework. To further explore their advantages, new developments are
needed, especially to increase the extent of the hydrolysis of poly- and oligosaccharides. A possible way forward is the use of solid acid catalysts that may overcome many current drawbacks of other common methods. In this Review, the advantages and limitations of the use of heterogeneous catalysis for the main groups of solid acid catalysts (zeolites, resins, carbon materials, clays, silicas, and other oxides) and their relation to the hydrolysis of model soluble disaccharides and soluble poly- and oligosaccharides are presented and discussed. Special attention is given to the hydrolysis of hemicelluloses and hemicellulose-derived saccharides into monosaccharides, the impact on process performance of potential catalyst poisons originating from biomass and biomass hydrolysates (e.g., proteins, mineral ions, etc.). The data clearly point out
the need for studying hemicelluloses in natura rather than in model compound solutions that do not retain the relevant factors influencing process performance. Furthermore, the desirable traits that solid acid catalysts must possess for the efficient hemicellulose hydrolysis are also presented and discussed with regard to the design of new catalysts
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