Drying Kinetics and Particle Formation from Dilute Colloidal Suspensions in Aerosol Droplets

Abstract

Industrial processes such as spray drying of pharmaceutical and food products often involve the drying of aerosol droplets containing colloidal suspensions into powdered microparticles of desired properties. The morphology and surface properties of the final dry products/microparticles obtained after the drying process are strongly influenced by the parameters of the initial aerosol droplet composition and the drying conditions. In particular, the final dry microparticle morphology can be dependent on the dimensionless Péclet number (Pe), which expresses the relative competition between the diffusion of the dispersed particles within the droplet and the rate of solvent loss via evaporation. In this work, we examine how control over the gas phase drying conditions and initial aerosol droplet composition can be used to influence the aerosol droplet drying kinetics in the gas phase for a range of Péclet numbers. We used a single-particle levitation instrument, the electrodynamic balance, to measure the drying kinetics of colloidal silica droplets (0.10 -0.60 % v/v) under controlled gas phase drying conditions of temperature (263 -326 K) and relative humidity (0 -90 %) and obtained Péclet numbers ranging from 4.05 -184.5. We demonstrate that, for aerosol droplets with initially dilute feed colloid concentrations and within the constant evaporation regime, the starting composition does not strongly influence the solvent evaporation rate with the included nanoparticles (NPs) acting as spectators. However, the gas phase drying conditions, temperature, and relative humidity, directly influences the droplet temperature via evaporative cooling as well as the droplet drying kinetics and the final dry microparticle properties. With a priori knowledge of the droplet drying kinetics from the single droplet measurements, we further demonstrate the possibility of tailoring the morphology of the dried microparticles. Dried silica microparticles collected at Pe = 23.8 had dense spherical morphologies whiles those at the highest Pe = 180.0 had crumpled surface morphologies with a transition in morphology between these limiting Pe values. Our results extend the fundamental understanding of the mechanisms controlling the drying of aerosol droplets in colloidal suspensions across a wide range of application areas extending from spray drying, to the drying of respiratory fluid droplets containing bacteria and viruses, and the drying of atmospheric aerosol droplets.

High time resolution measurements of droplet evaporation kinetics and particle crystallisation

“…Secondly, the temperature of the droplet increases as the solute crystallises due to the enthalpy of crystallisation, also increasing the vapour pressure. 24 Previous EDB experiments have also observed this short period of increased evaporation rate, and the amount of heat released, corresponding temperature increase and elevated evaporation rate have been shown to correspond well with numerical models. 47 This suggests that the temperature increase, calculated as ~6 K, is the dominant effect increasing the evaporation rate, rather than the decrease in solution concentration.…”

“…The drying conditions can have a significant impact on the evaporation process and value of the Peclét number. 24 A comprehensive understanding of the drying process is required to predict the morphology and physical properties of the dried particles. The processes driving the evaporation, including solvent/solute interactions and drying conditions, are complex.…”

Utilizing an Electrical Low Pressure Impactor to Indirectly Probe Water Uptake via Particle Bounce Measurements

“…difficult to probe in situ (Fu et al, 2012). Varying aerosol drying kinetics, which are dependent on aerosol composition (Archer et al, 2020), likely contributed to the particle bounce results discussed here. Gregson et al (2020) recently investigated the water evaporation rates and crystallization for sodium chloride and sodium nitrate aerosol droplets and were able to observe nucleation and crystallization while also studying how drying temperature, RH, and solute concentration affect nucleation time.…”

Micro‐droplet Trapping and Manipulation: Understanding Aerosol Better for a Healthier Environment

“…They utilized this EDB to investigate the hygroscopicity, evaporation of volatile components and water transport kinetics from single aqueous organic/inorganic micro‐droplets, and these measurements allows for establishing a kinetic model to accurately predict organic aerosol evaporation [43l,96a,b,97] . Recently, this EDB was used to investigate the drying kinetics and particle formation from aerosol droplets in colloidal suspensions [98] . Besides prototype atmospheric aerosols described above, Peng et al.…”