10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Exploring the Impacts of Drying Dynamics on Final Particle/Granule Morphology Using a New Droplet Chain Instrument
JIM WALKER, Rachael E.H. Miles, Jonathan P. Reid, University of Bristol
Abstract Number: 1332 Working Group: Materials Synthesis
Abstract The evaporative drying of droplets dispersed in a gas phase to form solid granules is a key process in a wide range of industrial applications, from spray drying to produce functional nano- and micro-particles, the delivery of pharmaceuticals for inhalation, spray painting, and food processing. For example, liquid milk, which has a short shelf-life, relatively high mass and requires refrigeration, is often spray dried to a powder prior to storage or transportation.
The structure and density adopted by a granule as the solvent evaporates can vary significantly depending on the dynamics of the drying process. For example, by varying the Peclet number (Pe), a measure of the relationship between the evaporation rate and diffusion rate of colloidal particles within the droplet, structures as diverse as crystals, encapsulated solids, hollow spheres and buckled/deformed/folded shapes can be formed. The desired granular structure and density varies depending on the application and an unexpected change in structure, such as buckling, can have unfavourable consequences for the efficacy of the product. The drying conditions which control granule morphology are still poorly understood.
In this work we use a new falling droplet chain instrument to resolve the rapid drying dynamics of individual droplets containing particle dispersions over their full evaporative lifetimes and compare with the resulting granule morphologies. We examine the conditions, such as initial droplet size, drying rate and the composition and concentration of the colloidal dispersion, required to form granules of different morphologies with particular attention paid to the conditions which result in granule buckling.
The instrument creates a continuous train of droplets, dispensed using a droplet-on-demand generator, which appears as a static chain when stroboscopically illuminated at the dispensing frequency [1]. The uniformity of the dispensed droplets and the reproducibility of the drying events means each droplet in the chain represents a subsequent snapshot in the lifetime of an individual droplet. The exceptional temporal resolution at which measurements can be made (10 ms) is calculated from the reciprocal of the dispensing frequency. High resolution imaging allow the drying dynamics to be resolved by measuring properties including the volume equivalent diameter (density independent), the aerodynamic diameter (density dependent) and optical diameter with < 1 um accuracies. The initial droplet diameter, typically 30 – 80 µm, can be controlled by the dispenser and the evaporation rate controlled with a temperature controlled gas flow. The final granule morphologies are examined using offline analysis tools such as scanning electron microscopy (SEM).
[1] Baldelli et al. (2015) Mol. Pharm. 12:2562-2573.