Drying Kinetics of Salt Solution Droplets: Water Evaporation Rates and Crystallization

Abstract

The drying and crystallisation of solution droplets is a problem of broad relevance, determining the micro-structures of particles formed in spray drying, the phase of particles delivered by, for example, aerosol formulations for inhalation therapies, and the impact of aerosols on radiative forcing and climate. The ephemeral nature of free-droplets, particularly when considering the drying kinetics of droplets with highly volatile constituents, has often precluded the accurate measurement of transient properties such as droplet size and composition, preventing the robust assessment of predictive models of droplet drying rates, nucleation and crystallisation. Here, we report novel measurements of the drying kinetics of individual aqueous sodium chloride solution droplets using an electrodynamic balance to isolate and trap single aerosol droplets (radius ~ 25 µm). The initial solution droplet size and composition is shown to be highly reproducible in terms of of drying rate and crystallisation time when examined over hundreds of identical evaporating droplets. We introduce a numerical model that determines the concentration gradient across the radial profile of the droplet as it dries, considering both the surface recession due to evaporation and the diffusion of components within the droplet. Drying induced crystallisation is shown to be fully determined for this system, with nucleation and instantaneous crystallisation occurring once a critical supersaturation level of 2.04 ± 0.02 is achieved at the surface of the evaporating droplet surface. This phenomenological model provides a consistent account of the timescale and surface concentration of free-droplet crystallisation on drying for the different drying conditions studied, a necessary step in progress towards achieving control over rates of crystallisation and the competitive formation of amorphous particles.

Creating nanocrystallized chemotherapy: the differences in pressurized aerosol chemotherapy (PAC) via intracavitary (IAG) and extracavitary aerosol generation (EAG) regarding particle generation, morphology and structure

“…The introduction of nanoaerosols with superior qualities over conventional PIPAC regarding their interaction with the peritoneal surface is the latest finding in this respect [15]. Currently, numerous studies are conducted to analyze evaporation effects and nanoparticle agglomeration in physical chemistry [18,19,24]. This study is the first to analyze chemotherapeutic particles created in INAT via SEM.…”

“…Since fluid nanoaerosol particles floating within the atmosphere are subdue too many outside influences, knowing their characteristics as well as their pharmacologic and physical properties is of high importance [18,19]. To investigate these micro-and nanoparticles in their function as doxorubicin-laden chemoaerosols, we conducted a structural particle analysis during surface collision and at different stages following aerosol formation.…”

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

“…As a result, hygroscopic compounds will delay skin formation and generate smaller particles. Lowering the vapor pressure below the RH of the gas due to hygroscopy causes a reversal of the drying process, and an uptake of solvent into the drop, presenting a limit for RH above which no particle formation is possible [ 57 ]. Gregson et al investigated the effect of varying RH on the drying kinetics of NaCl droplets [ 57 ].…”

“…When increasing the RH above this value, deliquescence took place after the particle was already formed. Thus, only if the RH of the environment was below this limit could particles be obtained [ 57 ].…”

Particulate matter emitted from ultrasonic humidifiers—Chemical composition and implication to indoor air

“…The equilibration of water between a droplet and the surrounding dry air is rapid, proceeding on a time scale of a second. 15 Following water evaporation, the non-volatile solute in water will be left suspended as PM. Both the US EPA 16 and the Government of Canada 17 have raised concerns related to humidifier-generated PM 2.5 to the general public through their websites.…”