Raman spectroscopy of optically levitated supercooled water droplet

Strengthening Aqueous Electrolytes without Strengthening Water

Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions-25 wt.% LiCl and 62 wt.% H3 PO4 -cooled to -78 °C exhibit a significantly widened stability window. Integrated experimental and simulation results reveal that, upon cooling, Li+ ions become less hydrated and pair up with Cl- , ice-like water clusters form, and H⋅⋅⋅Cl- bonding strengthens. Surprisingly, this low-temperature solvation structure does not strengthen water molecules' O-H bond, bucking the conventional wisdom that increasing water's stability requires stiffening the O-H covalent bond. We propose a more general mechanism for water's low temperature inertness in the electrolyte: less favorable solvation of OH- and H+ , the byproducts of hydrogen and oxygen evolution reactions. To showcase this stability, we demonstrate an aqueous Li-ion battery using LiMn2 O4 cathode and CuSe anode with a high energy density of 109 Wh/kg. These results highlight the potential of aqueous batteries for polar and extraterrestrial missions.

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.

Optical trapping and light scattering in atmospheric aerosol science

Aerosol particles are ubiquitous in the atmosphere, and currently contribute a large uncertainty to climate models. Part of the endeavour to reduce this uncertainty takes the form of improving our understanding of aerosol at the microphysical level, thus enabling chemical and physical processes to be more accurately represented in larger scale models. In addition to modeling efforts, there is a need to develop new instruments and methodologies to interrogate the physicochemical properties of aerosol. This perspective presents the development, theory, and application of optical trapping, a powerful tool for single particle investigations of aerosol. After providing an overview of the role of aerosol in Earth's atmosphere and the microphysics of these particles, we present a brief history of optical trapping and a more detailed look at its application to aerosol particles. We also compare optical trapping to other single particle techniques. Understanding the interaction of light with single particles is essential for interpreting experimental measurements. In the final part of this perspective, we provide the relevant formalism for understanding both elastic and inelastic light scattering for single particles. The developments discussed here go beyond Mie theory and include both how particle and beam shape affect spectra. Throughout the entirety of this work, we highlight numerous references and examples, mostly from the last decade, of the application of optical trapping to systems that are relevant to the atmospheric aerosol.

Temperature measurements of liquid flat jets in vacuum

Sub-μm thin samples are essential for spectroscopic purposes. The development of flat micro-jets enabled novel spectroscopic and scattering methods for investigating molecular systems in the liquid phase. However, the temperature of these ultra-thin liquid sheets in vacuum has not been systematically investigated. Here, we present a comprehensive temperature characterization using optical Raman spectroscopy of sub-micron flatjets produced by two different methods: colliding of two cylindrical jets and a cylindrical jet compressed by a high pressure gas. Our results reveal the dependence of the cooling rate on the material properties and the source characteristics, i.e., nozzle-orifice size, flow rate, and pressure. We show that materials with higher vapor pressures exhibit faster cooling rates, which is illustrated by comparing the temperature profiles of water and ethanol flatjets. In a sub-μm liquid sheet, the temperature of the water sample reaches around 268 K and the ethanol around 253 K close to the flatjet's terminus.

Temperature-Dependent Liquid Water Structure for Individual Micron-Sized, Supercooled Aqueous Droplets with Inclusions

Herein, we measure the water structure for individual micron-sized droplets of water, salt water, and water containing biologically and marine relevant atmospheric inclusions as a function of temperature. Individual droplets, formed on a hydrophobic substrate, are analyzed with micro-Raman spectroscopy. Analysis of the Raman spectra in the O-H stretching region shows that the equilibrium of partially and fully hydrogen-bonding water interactions change as temperature decreases up until there is a phase transition to form ice. Using these temperature-dependent measurements, the thermodynamic parameters for the interchange between partially and fully hydrogen-bonded water (PHW ⇄ FHW) for different supercooled droplets (water, salt water, and water containing biologically and marine relevant atmospheric inclusions) have been determined.

Low-Temperature Water Uptake of Individual Marine and Biologically Relevant Atmospheric Particles Using Micro-Raman Spectroscopy

The interaction of water vapor and the water uptake behavior of atmospheric particles are often investigated as a function of relative humidity (0-100% RH) at ambient temperature. However, lower temperature studies are important to understand how atmospheric particles nucleate ice through various mechanisms including immersion freezing. Immersion freezing requires the formation of a condensed water droplet at lower temperatures prior to freezing. To better understand low-temperature water uptake behavior of marine and biologically relevant atmospheric particles, we have investigated water uptake of single atmospheric particles using a micro-Raman spectrometer coupled to an environmental cell for measurements at lower temperatures and as a function of relative humidity. These particles include sodium chloride, sucrose, Snomax, lipopolysaccharide, and laminarin. Particles range in size from 2 to 3 μm in diameter and can be monitored by using optical microscopy and Raman spectroscopy as a function of relative humidity at temperatures between 253 and 298 K. From the Raman spectra collected, we can determine a Raman growth factor defined as an increase in the intensity of the O-H stretch as a measure of the integrated water content of a particle compared to the dry particle. These data show that for lipopolysaccharide, laminarin, and Snomax, unlike simple saccharides such as sucrose and other soluble organics, as temperature decreases, water uptake begins at lower relative humidity and does not follow a solubility temperature dependence. This suggests that at lower temperatures the particles are adsorbing water on the surface rather than dissolving and absorbing water. Furthermore, repeated water uptake cycles cause a change in the morphology of some of these particles, which is shown to promote water uptake at lower relative humidity. These results give new insights into water uptake of these different marine and biologically relevant particles at low temperature at subsaturation relative humidity prior to droplet formation and immersion freezing.

Micro-droplet Trapping and Manipulation: Understanding Aerosol Better for a Healthier Environment

Understanding the physicochemical properties and heterogeneous processes of aerosols is key not only to elucidate the impacts of aerosols on the atmosphere and humans but also to exploit their further applications, especially for a healthier environment. Experiments that allow for spatially control of single aerosol particles and investigations on the fundamental properties and heterogeneous chemistry at the single-particle level have flourished during the last few decades, and significant breakthroughs in recent years promise better control and novel applications aimed at resolving key issues in aerosol science. Here we propose graphene oxide (GO) aerosols as prototype aerosols containing polycyclic aromatic hydrocarbons, and GO can behave as two-dimensional surfactants which could modify the interfacial properties of aerosols. We describe the techniques of trapping single particles and furthermore the current status of the optical spectroscopy and chemistry of GO. The current applications of these single-particle trapping techniques are summarized and interesting future applications of GO aerosols are discussed.

Unraveling Particle Formation: From Single Droplet Drying to Spray Drying and Electrospraying

Spray drying and electrospraying are well-established drying processes that already have proven their value in the pharmaceutical field. However, there is currently still a lack of knowledge on the fundamentals of the particle formation process, thereby hampering fast and cost-effective particle engineering. To get a better understanding of how functional particles are formed with respect to process and formulation parameters, it is indispensable to offer a comprehensive overview of critical aspects of the droplet drying and particle formation process. This review therefore closely relates single droplet drying to pharmaceutical applications. Although excellent reviews exist of the different aspects, there is, to the best of our knowledge, no single review that describes all steps that one should consider when trying to engineer a certain type of particle morphology. The findings presented in this article have strengthened the predictive value of single droplet drying for pharmaceutical drying applications like spray drying and electrospraying. Continuous follow-up of the particle formation process in single droplet drying experiments hence allows optimization of manufacturing processes and particle engineering approaches and acceleration of process development.

Optical deformation of single aerosol particles

Advancements in designing complex models for atmospheric aerosol science and aerosol-cloud interactions rely vitally on accurately measuring the physicochemical properties of microscopic particles. Optical tweezers are a laboratory-based platform that can provide access to such measurements as they are able to isolate individual particles from an ensemble. The surprising ability of a focused beam of light to trap and hold a single particle can be conceptually understood in the ray optics regime using momentum transfer and Newton's second law. The same radiation pressure that results in stable trapping will also exert a deforming optical stress on the surface of the particle. For micron-sized aqueous droplets held in the air, the deformation will be on the order of a few nanometers or less, clearly not observable through optical microscopy. In this study, we utilize cavity-enhanced Raman scattering and a phenomenon known as thermal locking to measure small deformations in optically trapped droplets. With the aid of light-scattering calculations and a model that balances the hydrostatic pressure, surface tension, and optical pressure across the air-droplet interface, we can accurately determine surface tension from our measurements. Our approach is applied to 2 systems of atmospheric interest: aqueous organic and inorganic aerosol.

Shrinking of Rapidly Evaporating Water Microdroplets Reveals their Extreme Supercooling

The fast evaporative cooling of micrometer-sized water droplets in a vacuum offers the appealing possibility to investigate supercooled water-below the melting point but still a liquid-at temperatures far beyond the state of the art. However, it is challenging to obtain a reliable value of the droplet temperature under such extreme experimental conditions. Here, the observation of morphology-dependent resonances in the Raman scattering from a train of perfectly uniform water droplets allows us to measure the variation in droplet size resulting from evaporative mass losses with an absolute precision of better than 0.2%. This finding proves crucial to an unambiguous determination of the droplet temperature. In particular, we find that a fraction of water droplets with an initial diameter of 6379±12  nm remain liquid down to 230.6±0.6  K. Our results question temperature estimates reported recently for larger supercooled water droplets and provide valuable information on the hydrogen-bond network in liquid water in the hard-to-access deeply supercooled regime.

Lidar to determine the fractions of ice, liquid and water vapor in polar tropospheric clouds

A new Lidar combining Raman spectroscopy and linear polarization analysis is presented. This new instrument identifies the fraction of ice, liquid, and water vapor in low level polar tropospheric clouds and provides the polarimetric S and P state of the backscattering 532 nm Lidar signal. An overview of the research applications is given followed by a theoretical estimation of the Lidar returns. The instrument concept and optical characteristics are discussed. First Lidar profiles and instrument evaluations will be provided during the conference.

Quantifying Raman OH-band spectra for remote water temperature measurements

Remote water temperature measurements by Raman scattering is a perspective tool for in situ and/or real-time studies for inaccessible areas such as the Arctic region. State-of-the-art laser remote temperature detection techniques are based on temperature-dependent transformation of the Raman OH stretching vibration band. This study compared different approaches quantifying Raman OH-band spectra transformation with temperature: the two-color technique, deconvolution procedure, Raman difference spectroscopy, and centroid technique. Distilled water was probed remotely by compact Raman LIDAR, and the results demonstrated that the Raman OH-band centroid technique achieved the best temperature measurement accuracy (±0.15°C).

Low-temperature Bessel beam trap for single submicrometer aerosol particle studies

We report on a new instrument for single aerosol particle studies at low temperatures that combines an optical trap consisting of two counter-propagating Bessel beams (CPBBs) and temperature control down to 223 K (-50 °C). The apparatus is capable of capturing and stably trapping individual submicrometer- to micrometer-sized aerosol particles for up to several hours. First results from studies of hexadecane, dodecane, and water aerosols reveal that we can trap and freeze supercooled droplets ranging in size from ~450 nm to 5500 nm (radius). We have conducted homogeneous and heterogeneous freezing experiments, freezing-melting cycles, and evaporation studies. To our knowledge, this is the first reported observation of the freezing process for levitated single submicrometer-sized droplets in air using optical trapping techniques. These results show that a temperature-controlled CPBB trap is an attractive new method for studying phase transitions of individual submicrometer aerosol particles.

Density and Phonon-Stiffness Anomalies of Water and Ice in the Full Temperature Range

The specific-heat difference between the O:H van der Waals bond and the H-O polar-covalent bond and the Coulomb repulsion between electron pairs on adjacent oxygen atoms determine the angle-length-stiffness relaxation dynamics of the hydrogen bond (O:H-O), which is responsible for the density and phonon-stiffness oscillation of water ice over the full temperature range. Cooling shortens and stiffens the part of relatively lower specific-heat, and meanwhile lengthens and softens the other part of the O:H-O bond via repulsion. Length contraction/elongation of a specific part always stiffens/softens its corresponding phonon. In the liquid and in the solid phase, the O:H bond contracts more than the H-O elongates, hence, an O:H-O cooling contraction and the seemingly "regular" process of cooling densification take place. During freezing, the H-O contracts less than the O:H elongates, leading to an O:H-O elongation and volume expansion. At extremely low temperatures, the O:H-O angle stretching lowers the density slightly as the O:H and the H-O lengths change insignificantly. In ice, the O-O distance is longer than it is in water, resulting in a lower density, so that ice floats.

Raman Spectra of Liquid Water from Ab Initio Molecular Dynamics: Vibrational Signatures of Charge Fluctuations in the Hydrogen Bond Network

We report the first ab initio simulations of the Raman spectra of liquid water, obtained by combining first principles molecular dynamics and density functional perturbation theory. Our computed spectra are in good agreement with experiments, especially in the low frequency region. We also describe a systematic strategy to analyze the Raman intensities, which is of general applicability to molecular solids and liquids, and it is based on maximally localized Wannier functions and effective molecular polarizabilities. Our analysis revealed the presence of intermolecular charge fluctuations accompanying the hydrogen bond (HB) stretching modes at 270 cm(-1), in spite of the absence of any Raman activity in the isotropic spectrum. We also found that charge fluctuations partly contribute to the 200 cm(-1) peak in the anisotropic spectrum, thus providing insight into the controversial origin of such peak. Our results highlighted the importance of taking into account electronic effects in interpreting the Raman spectra of liquid water and the key role of charge fluctuations within the HB network; they also pointed at the inaccuracies of models using constant molecular polarizabilities to describe the Raman response of liquid water.