AN ATOMIZER TO GENERATE MONODISPERSE DROPLETS FROM HIGH VAPOR PRESSURE LIQUIDS

This work describes an atomizer capable of generating monodisperse droplets of high vapor pressure liquids such as HFA227ea and HFA134a. This atomizer is expected to be a useful tool in future fundamental explorations of the mechanics of these highly dynamic aerosols. Hydrofluoroalkanes (HFAs) are the most common propellants in metered dose inhalers (MDIs), which are themselves the most common delivery method for pulmonary disease medications. As a result of the high vapor pressure of these fluids, generation of monodisperse droplets and their study are not possible with previously existing monodisperse atomizers due to fluid flashing when exposed to ambient conditions. The new atomizer uses a piezoelectric transducer to disintegrate the propellant jet, a cooling circuit with a low-temperature limit of -30 degrees C to reduce propellant vapor pressure, and a high-pressure feed system (up to 2.7 MPa) to keep the propellant in the liquid phase, thereby avoiding flash atomization. In the current setup, the diameter of the generated droplets was monitored with a laser scattering system (using Fraunhofer diffraction theory) to optimize the operating parameters of the setup (for example, the actuation frequency and amplitude of the piezoelectric transducer). This monitoring system allowed for the measurement of droplet geometric diameter, spacing, and velocity. The capabilities of the atomizer for generating monodisperse droplets are demonstrated by a case study in which monodisperse dry particles of beclomethasone dipropionate (BDP) are generated using a solution of BDP, ethanol co-solvent, and HFA134a propellant.