Flow-Focusing Monodisperse Aerosol Generator for Calibration of Spray Diagnostics Instruments

A new instrument for generating monodisperse droplets is introduced and tested with a phase Doppler interferometer. The present device is an advancement of the technology underlying Vibrating Orifice Aerosol Generator (VOAG), which has been a key tool for generating monodisperse droplets and aerosols since 1970s. In a VOAG instrument, a liquid jet is formed by forcing the liquid out of an orifice, while ultrasonic perturbation is used to break up the jet into uniform droplets. Drop diameter is typically twice the orifice diameter. This approach has been highly successful for generating relatively large drops (e.g. greater than 50 micron), but it is difficult to implement for generating fine droplets (such as droplets smaller than 25 micron), as smaller orifices are easily clogged or otherwise damaged due to the high liquid pressure needed to maintain the liquid flow. This problem is solved in the present device, referred to as Flow-focusing Monodisperse Aerosol Generator (FMAG), which does not require fine orifices for generating fine liquid jets. Instead, FMAG utilizes flow-focusing to reduce the diameter of a relatively large liquid jet (100 micron in the present realization) to the desired size. Using this approach, a final jet diameter as small as 5 microns is realized. FMAG was used to measure the transfer function of a Phase Doppler Interferometer (PDI) for standard configurations using off-axis angles of 30 and 60 degrees. Aerosols generated by FMAG were also used to validate a non-standard PDI configuration utilizing an off-axis angle of 15 degrees representing an experimental setup with limited optical access. Also, FMAG is shown to be valuable for generating aerosol of solutions of fragile molecules (such as enzymes) without damaging their chemical structure.