(263c) Bimodal Aerosol Size Distributions from Laboratory Atomization/Evaporation

Bi-modal aerosol size distributions have been observed in laboratory studies using atomized droplet generation followed by diffusion drying. While there is evidence of multi-component distributions forming from various component separations within droplets, our research has observed similar effects from single-component aerosols. The bi-modal effect is observed via Scanning Mobility Particle Sizing (SMPS) analysis. This effect is controllable by altering the concentration of the initial atomized solution between 0.1 and 10 grams per liter, by altering the flowrate of the aerosol stream through a series of dryers, and by altering the drying amount and rate via the number of and saturation of water within diffusion dryers. In this research, we have investigated both single- and multi-component solutions using both organic and inorganic constituents dissolved in water. We have done a series of experiments generating aerosols by varying the above factors such that the final aerosol size distribution can be predictably controlled for single- or bi-modality. The resulting aerosols have been collected with both PIXE impactors and Nano-Aerosol Samplers and have been imaged with both Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) in order to understand their true particle shapes and dimensions. Furthermore, we have used X-ray Diffraction (XRD) to analyze the collected aerosols for crystal polymorphic structures and compared the XRD graphs against known structures to determine the extent to which meta-stable polymorphic forms of organic crystals are occurring. Results of all of these experiments and analysis will be presented and explained. One key conclusion from these studies is that aerosol atomization and evaporation producing particles on the order of 100 nanometers in diameter actually produces crystal structures and can be modified to produce both stable and metastable polymorphs of these crystals.