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

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

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 ambient 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 infrared 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 in 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

Core-Level Photoelectron Angular Distributions at the Liquid–Vapor Interface

ConspectusPhotoelectron spectroscopy (PES) is a powerful tool for the investigation of liquid-vapor interfaces, with applications in many fields from environmental chemistry to fundamental physics. Among the aspects that have been addressed with PES is the question of how molecules and ions arrange and distribute themselves within the interface, that is, the first few nanometers into solution. This information is of crucial importance, for instance, for atmospheric chemistry, to determine which species are exposed in what concentration to the gas-phase environment. Other topics of interest include the surface propensity of surfactants, their tendency for orientation and self-assembly, as well as ion double layers beneath the liquid-vapor interface. The chemical specificity and surface sensitivity of PES make it in principle well suited for this endeavor. Ideally, one would want to access complete atomic-density distributions along the surface normal, which, however, is difficult to achieve experimentally for reasons to be outlined in this Account. A major complication is the lack of accurate information on electron transport and scattering properties, especially in the kinetic-energy regime below 100 eV, a pre-requisite to retrieving the depth information contained in photoelectron signals.In this Account, we discuss the measurement of the photoelectron angular distributions (PADs) as a way to obtain depth information. Photoelectrons scatter with a certain probability when moving through the bulk liquid before being expelled into a vacuum. Elastic scattering changes the electron direction without a change in the electron kinetic energy, in contrast to inelastic scattering. Random elastic-scattering events usually lead to a reduction of the measured anisotropy as compared to the initial, that is, nascent PAD. This effect that would be considered parasitic when attempting to retrieve information on photoionization dynamics from nascent liquid-phase PADs can be turned into a powerful tool to access information on elastic scattering, and hence probing depth, by measuring core-level PADs. Core-level PADs are relatively unaffected by effects other than elastic scattering, such as orbital character changes due to solvation. By comparing a molecule's gas-phase angular anisotropy, assumed to represent the nascent PAD, with its liquid-phase anisotropy, one can estimate the magnitude of elastic versus inelastic scattering experienced by photoelectrons on their way to the surface from the site at which they were generated. Scattering events increase with increasing depth into solution, and thus it is possible to correlate the observed reduction in angular anisotropy with the depth below the surface along the surface normal.We will showcase this approach for a few examples. In particular, our recent works on surfactant molecules demonstrated that one can indeed probe atomic distances within these molecules with a high sensitivity of ∼1 Å resolution along the surface normal. We were also able to show that the anisotropy reduction scales linearly with the distance along the surface normal within certain limits. The limits and prospects of this technique are discussed at the end, with a focus on possible future applications, including depth profiling at solid-vapor interfaces

Photoelectron Spectroscopy from a Liquid Flatjet - data

Data set pertaining to the article "Photoelectron spectroscopy from a liquid flatjet", published in J. Chem. Phys. 158, 234202 (2023). Files with extension .h5 are hdf5-files structured according to the NeXus standard v2020.10, see https://www.nexusformat.org/ https://fairmat-experimental.github.io/nexus-fairmat-proposal/50433d9039b3f33299bab338998acb5335cd8951/mpes-structure.html NeXus data files can be opened with any software capable of opening hdf5-structured files. The following viewers are adapted to the specifics of the NeXus data format: * nexpy (distributed with python) * https://h5web.panosc.eu/h5wasm (web-based NeXus viewer maintained by the European Photon and Neutron Open Science Cloud-consortium) Additionally, some properties of our liquid jet sample environment are described by extensions to standard NeXus explained in a notes-section in each file. In each NeXus file-entry, two types of spectra are shown: 1. Sweep-averaged spectra, integrated over the non-dispersive coordinate of our detector ('data'). 2. As-measured data ('raw'). Files with extension .txt are tab-separated ascii-files. The following files are provided: Measured spectra underlying the articles' figures: Figure2NEXUS.h5 Figure3NEXUS.h5 Figure4NEXUS.h5 Figure5NEXUS.h5 Figure6NEXUS.h5 Figure7NEXUS.h5 Numeric representation of the results shown in graphical form: 'Figure 6.txt'. In case you have any questions regarding this data set please contact: Uwe Hergenhahn, [email protected] .Funding acknowledgements: Deutsche Forschungsgemeinschaft (Wi 1327/5-1), Max-Water initiative of the Max-Planck-Gesellschaft, JSPS KAKENHI Grant No. JP20K15229

Photoelectron spectroscopy from a liquid flatjet

We demonstrate liquid-jet photoelectron spectroscopy from a flatjet formed by the impingement of two micron-sized cylindrical jets of different aqueous solutions. Flatjets provide flexible experimental templates enabling unique liquid-phase experiments that would not be possible using single cylindrical liquid jets. One such possibility is to generate two co-flowing liquid-jet sheets with a common interface in vacuum, with each surface facing the vacuum being representative of one of the solutions, allowing face-sensitive detection by photoelectron spectroscopy. The impingement of two cylindrical jets also enables the application of different bias potentials to each jet with the principal possibility to generate a potential gradient between the two solution phases. This is shown for the case of a flatjet composed of a sodium iodide aqueous solution and neat liquid water. The implications of asymmetric biasing for flatjet photoelectron spectroscopy are discussed. The first photoemission spectra for a sandwich-type flatjet comprised of a water layer encapsulated by two outer layers of an organic solvent (toluene) are also shown

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

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 stars2,3,4. 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 characterized6,7,8,9, the explosive interaction between alkali metals and water10,11 has so far only permitted the preparation of aqueous solutions with low, submetallic electron concentrations12,13,14. 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 × 10$^{21}$ electrons per cubic centimetre, is confirmed using optical reflection and synchrotron X-ray photoelectron spectroscopies