spray nozzle design on the pattern and plume numbers. The whole aerosolization process can be analyzed, for instance, if larger, unwanted droplets are formed (sputtering) at the beginning or at the end of the aerosolization process. These numbers can be used both as input tools for product development and as input for quality control parameters and specifications. In Figure 3 and Figure 4, examples of spray pattern and plume geometry outputs are shown. Figure 3: Spray Pattern Results Laser Diffraction Another work horse in the aerosol characterization lab is the laser diffraction technique. Laser diffraction instruments use a laser in a lens system to form a beam that is collected on a detector ring that is perpendicular to the laser beam. The detector contains a number of laser light-sensitive rings that increase in size like an onion cut in half. When an aerosol is fired through the laser beam, the laser interacts with the individual droplets/particles in the aerosol and is diverted (diffracted) depending on the size of each aerosol droplet it interacts with. The smaller the droplets, the more diverted the laser light will be. In time (milliseconds to seconds), the laser diffraction instrument will have collected sufficient data to enable a presentation of the droplet/particle size distribution of the aerosol that protruded through the laser beam. Figure 5 and Figure 6 show an illustration of the laser diffraction set-up. The result from laser diffraction is often presented as a volume % versus particle size. From designs such as these, it is very easy to conclude if the generated aerosol droplets (or particles) are in the desired size ranges. Figure 7 shows an overlay design from two aerosol samples measured by laser diffraction. Formulation A has a volume median particle size around 50 μm and the fraction of mass contained in droplets that has the probability of being deposited in the deeper lung region (less than 10 μm) is low. On the other hand, the aerosol cloud from Formulation B has the majority of mass contained in particles smaller than 10 μm. From the characterization of these two products, it can be proposed that Formulation B has an aerosol particle size distribution that can be inhaled to a high degree, while the Formulation A has large particles that most probably will be deposited in the mouth-throat region and thus not end up in the deeper lung region. Spray pattern and plume geometry, together with laser diffraction, can give the developer a good picture of the aerosol characteristics that are essential for the application it is to be used for. The droplet/ particle size distribution information can especially tell a developer if the aerosol reaches its intended “target” or not. If the evaporation of liquid droplets in the distribution is rapid, or if the particles are 42 Spray September 2016 Figure 6: Laser Defraction Instrument Figure 4: Plume Geometry Setup Figure 5: Laser Defraction Principle (Malvern, UK) 1 HeNe laser 2 Collmating optics 3 Measurement zone 4 Fourier lens 5 Silicon diode detector array 6 Rapid data acquisition system
Spray September 2016
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