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Drop size in an ultrasonically produced spray is governed by the frequency at which the nozzle vibrates, and by the surface tension and density of the liquid being atomized. However, frequency is the predominant factor. Median drop size is inversely proportional to frequency to the 2/3 power. Thus, the higher the frequency the smaller the median drop size. Typically, the drop size distribution from ultrasonic nozzles follows a log-normal distribution curve. In simple terms, this distribution has the familiar bell-shape but on a logarithmic scale. The accompanying chart shows this distribution on a cumulative basis for several nozzle frequencies for water.
Various parameters can be used to characterize the mean and median drop size of a particular drop distribution. The number median diameter defines the 50% point in drop size - that is, one-half of the number of drops in the spray have diameters larger than this value while the other half have diameters smaller than this value. The number mean and weight mean diameters are average diameters. The number mean diameter is obtained by adding together the diameter of each drop in a spray sample and dividing that sum by the number of drops in the sample. The weight mean diameter is obtained by adding together the volume of each drop in a spray sample (volume is proportional to diameter cubed), taking the cube root of this sum, and finally dividing by the number of drops. The Sauter mean diameter is a specialized parameter used primarily in combustion applications. It measures the effective ratio of drop volume to surface area.
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