An Online Condensation-based Technique to Measure the True Volume and Density of Aerosol Particles
CYPRIEN JOURDAIN, Julie Pongetti, Jonathan Symonds, Adam M Boies, University of Cambridge
Abstract Number: 481
Working Group: Instrumentation and Methods
Abstract
The knowledge of the true volume of nanoparticles has significant implications in many fields, including spectroscopy, drug delivery, climate science, and energy harvesting. In fact, light absorption and scattering - as well as related mechanisms such as incandescence and photoacoustics - which form the base of well-established techniques in aerosol science, depend on the particle volume. However, there is no online technique to measure the true volume of nanoparticles, whose measurement instead relies on equivalent forms such as the mobility-derived volume assuming spherical particles or cumbersome ex situ analysis. Similarly, the material density of nanoparticles is commonly assumed based on bulk measurements owing to a lack of available techniques. This estimation is uncertain as aerosols are usually complex mixtures with oxidation and/or coating layers, which can drastically alter the density.
In this work, a technique is presented that allows direct measurement of the volume of nanoparticles based on the controlled condensation of heavy oils. The growth apparatus consists of a high-temperature stage used to saturate the aerosol flow with oil vapor and a low-temperature stage to force the oil molecules to diffuse towards the sample particles surface. The design, supported by computational fluid dynamics simulations, creates a high supersaturation region close to the condenser axis, promoting condensational growth while minimizing losses.
The results show that by carefully selecting the operating temperatures of the saturator and condenser, a region exists where all the sample particles are in the spherical droplet mode. From measurements of the relative changes in mass and mobility, the volume of the encapsulated nanoparticles is thus inferred. After validation with spherical PSL and sintered silver particles, for which the geometric volumes are successfully retrieved, the technique is applied to silver and soot agglomerates.