Vapor Phase Transmission Electron Microscopy: Visualization Tool for In-Situ Aerosol Phenomena
DEWANSH RASTOGI, Yuhang Wang, Kotiba A. Malek, Akua Asa-Awuku, Taylor J. Woehl,
University of Maryland College Park Abstract Number: 217
Working Group: Instrumentation and Methods
AbstractBulk particle measurements using aerosol instrumentation have been the backbone of cloud condensation studies and are used widely to understand the collective behavior of particles. Another emerging field of interest has been Single Particle Measurements (SPM); these measurements provide high-resolution data from individual particles under observation. SPMs are especially effective when coupled with bulk aerosol measurements by providing valuable insights about phase, morphology, and time-resolved data.
In this research, we build the groundwork for a visualization tool for in situ imaging of the process of water condensation on a Silicon Nitride (SiN) surface. In order to directly record the nanoscale condensation dynamics of sessile water droplets in electric fields, we use vapor-phase transmission electron microscopy (VPTEM). Sessile water nanodroplets were condensed by VPTEM imaging of saturated water vapor and grew to a size of about 500 nm before dissipating over the course of a minute. According to simulations, electron beam charging of the silicon nitride microfluidic channel windows produced electric fields of ~108V/m, which decreased the water vapor pressure and accelerated the nucleation of liquid water droplets of nanoscale. A mass balance model revealed that droplet evaporation was consistent with radiolysis-induced evaporation via the conversion of water to hydrogen gas, whereas droplet expansion was consistent with electric field-induced condensation. With the use of the model, we were able to quantify several electron beam-sample interactions and vapor transport characteristics, establish that electron beam heating was negligible, and show that the literature values considerably overestimated water vapor diffusivity and underestimated radiolytic hydrogen production. These experimental methods were refined to perform water condensation visualization on submicron salt particles (both organic and inorganic) under atmospheric conditions.