PDA and High Speed Image Analysis of HFA/Ethanol pMDI Aerosols: New Findings

B Myatt, R Newton, D Lewis, T Church, G Brambilla, G Hargrave, H Versteeg, B Gavtash, & E Long

The pressurised metered dose inhaler is one of the most popular devices for producing a respirable cloud of droplets or particles for inhaled therapy of respiratory conditions. The primary atomisation mechanism of a pMDI is relatively poorly understood. This work aims to further develop this fundamental understanding by conducting and analysing phase Doppler anemometry (PDA) and high speed imaging studies.
We report the findings of a PDA study of aerosol plumes generated by a commercial actuator of HFA134a placebo formulations containing ethanol up to 20% w/w. Data for the temporal variation of aerosol velocity and droplet size 25 mm from the spray orifice exit is presented alongside internal flow images of the expansion chamber obtained from custom made transparent models. With increasing ethanol concentration, the spray plume velocity remained broadly constant, but the number mean diameter increased. Internal flow imaging has revealed marked differences in the flow regimes within the expansion chamber between formulations. Previous studies have suggested that the aerosol velocity and droplet size are strongly linked with the formulation vapour pressure, a function of ethanol concentration. The variation of flow regime with ethanol concentration and mean droplet diameter increase observed in this study suggest that other physical properties (e.g. surface tension and/or viscosity) play more significant roles in governing the drop size than previously acknowledged.
This study demonstrates that increasing the concentration of ethanol in the formulation increases the droplet size produced. However, the ethanol concentration does not have a discernible effect on the aerosol plume velocity.

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