BUCHI Ultrasonic Package - Manual (Page 20)

4 | Description of function BÜCHI Labortechnik AG
20/46 Ultrasonic Controller
4.3 Drop size distribution
Fig. 5: Drop size distribution
X Drop diameter (microns) Y Percent of drops below given di­
ameter
1 Sauter mean diameter 2 Weight mean diameter
3 Surface mean diameter 4 Number mean diameter
5 Number median diameter
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 lognormal
distribution curve. In simple terms, this distribution has the familiar bell­shape
but on a logarithmic scale. The chart shows this distribution on a cumulative ba­
sis 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, one­half 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 av­
erage 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 num­