Dual-Balance Electrodynamic Trap as a Microanalytical Tool for Identifying Gel Transitions and Viscous Properties of Levitated Aerosol Particles

Abstract

2020). Dual-balance electrodynamic trap as a microanalytical tool for identifying gel transitions and viscous properties of levitated aerosol particles.ABSTRACT: The formation of gelatinous networks within an aerosol particle significantly alters the physicochemical properties of the aerosol material. Existing techniques for studying gel transitions rely on bulk rheometry, which is limited by contact with the sample, or microrheological techniques such as holographic optical tweezers, which rely on expensive equipment and high-powered lasers that can degrade light-absorbing aerosol. Here, we present a new technique to probe the microrheological characteristics of aerosol particles and explore gel formation under atmospheric conditions in a contactless environment without the need for high-power light sources. In a dualbalance quadrupole electrodynamic balance, levitated droplets of opposite polarity are trapped and equilibrated at fixed relative humidity (RH) and then subsequently merged, and the physical characteristics of the merged droplets are monitored as a function of time and RH using imaging techniques. By comparing the RHdependent characteristics of MgSO 4 (known to undergo a gel transition) to glucose and sucrose (known to remain as viscous Newtonian fluids) under fixed equilibration time scales, we demonstrate that gel phase transitions can be identified in aerosol particles, with MgSO 4 abruptly transitioning to a rigid microgel at 30% RH. Further, we demonstrate this technique can be used to also measure aerosol viscosity and identify non-Newtonian fluid dynamics in model sea spray aerosol composed of NaCl, CaCl 2 , and sorbitol. Thus, using this experimental technique, it is possible to distinguish between aerosol compositions that form viscous Newtonian fluids and those that undergo a gel transition or form non-Newtonian fluids. This technique offers a simple and costeffective analytical tool for probing gel transitions outside of bulk solubility limits, with relevant applications ranging from atmospheric science to microengineering of soft matter materials.

Ion-molecule interactions enable unexpected phase transitions in organic-inorganic aerosol

“…The upper limit to viscosity is not known. (27); see Materials and Methods for details]; RH-induced glass transitions are not typically observed for the lower-mass oxygenated molecules such as glucose (14,17,19), consistent with our observations.…”

“…S1B). As established by our work (14) and that of others (15)(16)(17)(18)26), monitoring the coalescence process of merged droplets under variable RH conditions can provide detailed information regarding RH-dependent aerosol-phase state. The typical phase behavior of binary organic and ternary organic-inorganic droplets is exemplified with individual merging events of binary gluconic acid ( Fig.…”

“…2F) demonstrates that ternary gluconic acid-CaCl 2 droplets have a much higher viscosity, relative to binary gluconic acid, despite also having a much higher water mole fraction, fig. S5B), is consistent with (hydro)gel formation, i.e., formation of a two-phase combination of liquid (water) contained within a porous solid network that assembles and disassembles within a relatively narrow range of solute concentration (1,13,14,30,31).…”

“…The phase state of levitated aerosol microdroplets was explored as a function of RH for a range of mixed organic-inorganic compositions. The organic molecules represent common atmospheric and marine aerosol proxies (2,7,9,10,14,23): glucose (the most common monosaccharide), sorbitol (a neutral sugar alcohol), gluconic acid (an aldonic acid), glucuronic acid (a uronic acid), and N-acetylneuraminic acid (a sialic acid) ( Fig. 1A).…”

“…Organic aerosol composed of relatively low-mass oxygenated organic compounds (<1000 Da), such as monosaccharides (common proxies for oxygenated aerosol material) and similar compounds, are generally assumed to remain in a liquid state or form ultraviscous or glassy Newtonian fluids at ambient relative humidity (RH) (6)(7)(8)(9)(13)(14)(15)(16)(17)(18)(19). Knowledge of aerosol viscosity is thus used to infer rates of diffusion of water or reactive species in aerosol particles through the Stokes-Einstein relationship (6).…”