In situ Raman and X-ray scattering of a single supersaturated aqueous Mg(NO3)2 droplet ultrasonically levitated

A single liquid droplet in the air generated by ultrasonic levitation provides such analytical advantages as a small sample volume (~ μL) for expensive proteins, container-free condition for deeply supercooling and supersaturation, time-dependent observation, and homogeneous rapid mixing. The investigation of the properties and structure of a droplet at a molecular level is highly needed for understanding the physicochemical behaviors of a droplet and an underlying mechanism of processes in the droplet. We develop in situ Raman and synchrotron X-ray scattering methods of a single liquid droplet of ~ 1 mm size ultrasonically levitated. The composition of a supersaturated Mg(NO₃)₂ droplet and speciation in the droplet are determined by analyzing the nitrate N-O and the water O-H stretching vibrational Raman bands. The X-ray interference function of an supersaturated Mg(NO₃)₂ droplet is subjected to an empirical potential structure refinement modeling to reveal the ion solvation, association, and solvent water structure. Furthermore, crystallization of Mg(NO₃)₂⋅nH₂O from a saturated droplet is observed and identified.

Liquid-Liquid Phase Separation of Single Optically Levitated Water-Ionic Liquid Droplets in Air

In this study, to investigate the equilibrium morphology of liquid-liquid phase-separated droplets in air, a temperature-responsive ionic liquid (IL) showing lower critical solution temperature behavior was employed. ILs have negligible vapor pressure and do not evaporate from aerosol droplets during dehumidifying processes. We demonstrated that the liquid-liquid phase separation of single optically levitated aqueous droplets containing the temperature-responsive IL can be induced by controlling the air relative humidity. The formation of liquid-liquid phase-separated droplets of partially engulfed morphology was successfully observed under an optical microscope, and their configurations were compared with those calculated by a thermodynamic model based on interfacial tensions and relative volume ratios of two immiscible phases.

Optical-Trapping Laser Techniques for Characterizing Airborne Aerosol Particles and Its Application in Chemical Aerosol Study

We present a broad assessment on the studies of optically-trapped single airborne aerosol particles, particularly chemical aerosol particles, using laser technologies. To date, extensive works have been conducted on ensembles of aerosols as well as on their analogous bulk samples, and a decent general description of airborne particles has been drawn and accepted. However, substantial discrepancies between observed and expected aerosols behavior have been reported. To fill this gap, single-particle investigation has proved to be a unique intersection leading to a clear representation of microproperties and size-dependent comportment affecting the overall aerosol behavior, under various environmental conditions. In order to achieve this objective, optical-trapping technologies allow holding and manipulating a single aerosol particle, while offering significant advantages such as contactless handling, free from sample collection and preparation, prevention of contamination, versatility to any type of aerosol, and flexibility to accommodation of various analytical systems. We review spectroscopic methods that are based on the light-particle interaction, including elastic light scattering, light absorption (cavity ring-down and photoacoustic spectroscopies), inelastic light scattering and emission (Raman, laser-induced breakdown, and laser-induced fluorescence spectroscopies), and digital holography. Laser technologies offer several benefits such as high speed, high selectivity, high accuracy, and the ability to perform in real-time, in situ. This review, in particular, discusses each method, highlights the advantages and limitations, early breakthroughs, and recent progresses that have contributed to a better understanding of single particles and particle ensembles in general.

Impact of the particle mixing state on the hygroscopicity of internally mixed sodium chloride-ammonium sulfate single droplets: a theoretical and experimental study

Sodium chloride (NaCl) is the main constituent of sea-salt aerosols. During atmospheric transport, sea-salt aerosols can interact with gases and other particles including secondary aerosols containing ammonium sulfate ((NH4)2SO4). This paper reports on the deliquescence relative humidity (DRH) of internally mixed sodium chloride-ammonium sulfate (NaCl/(NH4)2SO4) coarse particles by means of an acoustic levitation system fitted with a confocal Raman microscope (CRM). The chemical composition and physical state of individual levitated particles of different initial NaCl mole fractions were monitored during the deliquescence cycle by CRM. Experimental results were compared to the data predicted by the thermodynamic model E-AIM (Extended-Aerosol Inorganics Model). We demonstrated that NH4Cl, Na2SO4 and NH4NaSO4·2H2O are formed in recrystallized particles and coexist with NaCl and (NH4)2SO4. All these products are randomly distributed within the particles. Deliquescence curves described two or three-stage phase transitions depending on the initial composition of the droplet. Significant discrepancies between the model and the laboratory experiments were observed for NaCl mole fractions varying between 0.40 and 0.77 due to a divergence between the predicted and the truly present products in the particles' solid fraction during the humidification cycle.