Electrostatic collection of jet and film drops1

Jet and film drops from bubbles bursting in the presence of an electric field will be charged by electrostatic induction. If the field is sufficiently high, the drops move rapidly upward under the influence of an electrical force to impact on any suitable collecting surface. Magnesium oxide-coated glass slides make an excellent collector for determination of the size distribution of either jet or film drops. An agar plate collector can be used to collect all of the jet drops for bacterial analysis. Combining the two types of electrostatic collectors provides evidence that suggests that the enrichment of bacteria in jet drops is highest on the top or uppermost drop and decreases progressively to a minimum value for the last few jet drops-to bc ejected-from the bubble.Each year about lOlo tons of sea salt are transferred from the oceans to the atmosphere (Blanchard 1963). Presumably much of this material enters the atmosphere as jet and film drops ejected from air bubbles bursting at the surface of the sea (Blanchard and Woodcock 1957). The film drops are produced by the collapse of the thin film of water that comprises the bubble cap. The jet drops, considerably larger than the film drops, are produced from a jet of water that rises upward from the bubble cavity. Studies of the composition of this aerosol revealed that the concentrations of many substances, both organic and inorganic, are many times higher than in the sea. A general review of much of the earlier work has been prepared by Blanchard ( 1975) ; recent work can be found in the collection of papers in issue No. 34, 1974, of the Journal de Recherches Atmosphb riques. These concentration mechanisms also appear to operate when air bubbles burst at the surface of freshwaters (Blanchard and Parker 1975) . Further understanding of these mechanisms is not likely to be attained without detailed laboratory studies of both jet and film drops, Though many studies have been done, essentially all of them considered only the top jet drop. In the few cases where the lower jet drops were collected, the film drops were also inadvertently coll This research was sponsored by National Scicnce Foundation grant GA-23413. lected. It is necessary to collect these drops separately and to determine their size distribution. We report here on a simple technique whereby this can be done and on how this can be used to help understand the mechanism of enrichment in jet drops. Design of the electrostatic collecting system If an air bubble is allowed to burst at an air-water interface in the presence of an electric field, a bound electrostatic charge will be induced on the jet as it rises into the field. This charge, which is directly proportional to the electric field strength, is transmitted to the jet drops. If the field strength is sufficiently high, the upwarddirected electrical forces on the jet drops (the product of the field strength and the drop charge) are greater than the gravity forces, and the drops move rapidly upward. This process of electrostatic induction can be demonstrated quite simply by running a comb through one’s hair and holding it above a water surface where bubbles are bursting ( Fig. 1). The potential on the comb can easily attain several thousand volts, producing field strengths of scvcral hundred volts per centimeter over a distance of about 10 cm. Under the influence of the electrical forces, the charged jet drops streak upward to collect on the comb. It makes no difference whether the charged object (electrode) producing the electric field carries a negative or positive charge. The sign of the induced charge on the jet LIMNOLOGY AND OCEANOGRAPIIY 762 SEPTEMBER 1975, V. 20 ( 5) Electrostatic drop collection 763 TO POWER SUPPLY I* 5 IOKVDC