Evaporation and crystallization of NaCl-water droplets suspended in air by acoustic levitation

An investigative study into an oscillatory reaction in acoustically levitated droplets

For the first time we have studied an oscillatory chemical reaction (the well-known Belousov-Zhabotinsky (BZ) reaction) in acoustically levitated droplets. Acoustically levitated droplets allow wall-less reaction studies, reduce consumption of sample/reagents, offer high throughput measurements, and enable environmentally friendly chemistry by significantly reducing plastic waste. In this work, microdroplets of the BZ reactants were mixed at the central axis of a low-cost acoustic levitator. The chemical reaction observed in acoustically levitated droplets proceeded in the same way as that in both stirred and unstirred vials where the volume of droplets was 750-fold lower than the solutions in vials. The observed oscillation frequency in droplets was lower than that observed in vials, possibly as a result of evaporative cooling of the droplets. This work has shown that oscillatory reactions can be successfully carried out in acoustically levitated droplets, which allows the application of this technique to areas such as analysis, synthesis and actuation of smart materials and studies of the origins of life.

On the Chemistry of Aqueous Ammonium Acetate Droplets during Native Electrospray Ionization Mass Spectrometry

Ammonium acetate (NH4Ac) is a widely used solvent additive in native electrospray ionization (ESI) mass spectrometry. NH4Ac can undergo proton transfer to form ammonia and acetic acid (NH4+ + Ac- → NH3 + HAc). The volatility of these products ensures that electrosprayed ions are free of undesired adducts. NH4Ac dissolution in water yields pH 7, providing "physiological" conditions. However, NH4Ac is not a buffer at pH 7 because NH4+ and Ac- are not a conjugate acid/base pair (Konermann, L. J. Am. Soc. Mass Spectrom. 2017, 28, 1827-1835.). In native ESI, it is desirable that analytes experience physiological conditions not only in bulk solution but also while they reside in ESI droplets. Little is known about the internal milieu of NH4Ac-containing ESI droplets. The current work explored the acid/base chemistry of such droplets, starting from a pH 7 analyte solution. We used a two-pronged approach involving evaporation experiments on bulk solutions under ESI-mimicking conditions, as well as molecular dynamics simulations using a newly developed algorithm that allows for proton transfer. Our results reveal that during droplet formation at the tip of the Taylor cone, electrolytically generated protons get neutralized by Ac-, making NH4+ the net charge carriers in the weakly acidic nascent droplets. During the subsequent evaporation, the droplets lose water as well as NH3 and HAc that were generated by proton transfer. NH3 departs more quickly because of its greater volatility, causing the accumulation of HAc. Together with residual Ac-, these HAc molecules form an acetate buffer that stabilizes the average droplet pH at 5.4 ± 0.1, as governed by the Henderson-Hasselbalch equation. The remarkable success of native ESI investigations in the literature implies that this pH drop by ∼1.6 units relative to the initially neutral analyte solution can be tolerated by most biomolecular analytes on the short time scale of the ESI process.

Evaporation issues of acoustically levitated fuel droplets

Fuel droplet evaporation is essential to the generation of flammable mixtures in thermal engines. Generally, liquid fuel is injected directly into the hot, high-pressure atmosphere to form scattered droplets. Many investigations on droplet evaporation have been conducted with techniques involving the influence of boundaries, such as suspended wires. Ultrasonic levitation is a non-contact and non-destructive technology that can avoid the impact of hanging wire on droplet shape and heat transfer. Besides, it can simultaneously levitate multiple droplets and allow them to associate with each other or be used to study droplet instability behaviors. This paper reviews the influences of the acoustic field on levitated droplets, the evaporation characteristics of acoustically levitated droplets, and the prospects and limitations of ultrasonic suspension methods for droplet evaporation, which can serve as references for relevant studies.