Measuring electrostatic charge on pneumatically generated spray drops

Activities such as the suppression of coal dust would benefit from an improved understanding about how the presence of surfactant alters the charge distribution of spray drops. However, there is no standard method to measure charge and size distributions of nozzle-generated spray drops. To fill this gap, a new spray drop charge measurement system was developed to measure a broad range of sizes and electrostatic charges for spray drops based on the principle that the free-falling drops can be separated according to their electrostatic mobility in an electrical field with known strength. High and low concentration (1 × 10⁻⁴ and 1 × 10⁻⁶ M, respectively) anionic, cationic, and nonionic surfactant sprays and water spray were tested. Nonionic and cationic surfactant-containing sprays and water spray carried net positive charge on average, while anionic surfactant-containing sprays tended to carry net negative charge on average. Increasing surfactant concentration did not increase the magnitude of drop charge among the tested surfactants. Drop charge level was significantly higher among larger size drops, with the average charge per particle on 320 μm drops 1000 to 10,000 times higher than on 20 μm drops. The spray drop charge measurement system developed in this study can be used to measure spray drops with an electrostatic charge ranging from 0 to 960,000 charges per drop and diameter ranging from 20 to 270 μm. Potential design improvements described in this study and additional computing power could improve measurement accuracy and precision, as well as the capacity of the system.