Understanding Laser Based Droplet Size Measurements for Aerosol Products Particle Diagnostic Consulting responds to the large droplets being output by a sample. If incomplete atomization occurs, the Dv(90) value may skew larger than desired and a wet or uneven product feel may result. The Dv(10) value corresponds to the smaller droplets being output by a sample. This value can correspond to material being atomized further than the desired level. This may result in failure of the product to make it to the target intended. There are a host of other parameters that may be used to monitor spray efficiency in applications, but the ones discussed here are the most commonly used in aerosol studies. Key reporting concepts While the numerical values discussed above can be valuable, they can also become quite confusing if used exclusively in early development. A very valuable tool in assessing product performance is using the droplet size distribution graphs that are included in Figure 1. These graphs usually involve two traces. One trace is related to what is called the frequency curve. This is usually represented in a histogram graphically and bins data into size categories. This frequency curve provides insight into where the primary mode of the droplet size distribution is in terms of size. The primary mode is seen as the tallest peak on the curve. The graph also displays how uniform the distribution Droplet size testing of aerosol devices has become a key component of development for product effectiveness and safety. Many of these measurements are made with laser-based systems that provide a simple-to-use measurement interface and reporting format that allow routine analysis in aerosol labs around the world. While the testing performed produces a report, the understanding of both how that report is created and what the numbers produced mean to an aerosol product is often overlooked. Key safety data parameters in droplet size testing Most analytical testing methodologies are put in place to identify qualities in products with measureable parameters that indicate product performance corresponding to user response. The concept of understanding sensory response to a product and linking that response to data provides a viable path to making more consistent and better products. Droplet size measurements are no different and several values are commonly used to identify the spray characteristics. One consistent value that is obtained via laser droplet size measurements is the percentage of spray below 10um. The 10um size corresponds to inhalation hazard of the product under normal usage. This parameter is the most straightforward value in laser-based measurements. It corresponds to safety and whether the consumer is likely to experience discomfort from the product reaching the airways. In the simplest terms, if a product under normal usage sprays a high volume of material under 10um, the likelihood of the product causing user irritation increases. The combination of product chemistry and percentage of spray volume below 10um is critical in determining safety risks in aerosol products. Key performance parameters in droplet size testing Other parameters provided on droplet size reports correspond to product efficiency and effectiveness that can be paired with user sensory response. In development, sprays may be felt as too wet or too dry for the product application. This can prompt any number of changes in the product and delivery package to obtain a more desirable response. The values most commonly involved in characterization of aerosol droplet distributions are usually reported as the Dv(10), Dv(50) and Dv(90). These values relate to the diameter (D) of droplet by volume (v) relating to the 10th, 50th and 90th percentile of the spray. The Dv(50) value is the median droplet size dispensed by volume. If the result was a perfect bell curve, the Dv(50) would match the average and the mode. However, this is very unlikely in a real world situation. Narrowing a range for the Dv(50) value is often a first step in getting the product performing ideally. The Dv(10) and Dv(90) provide a glimpse into the overall occurring atomization. The Dv(90) value cor- Kyle Butz, President Figure 1 32 Spray January 2017
Spray January 2017
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