Abstract. The parameters describing the birth of film droplets originating from bubbles bursting on seawater surfaces are presented. Results are given for bubble sizes D b from 2 to 14.6 mm equivalent volume diameter. It is shown, contrary to earlier reports, that the films of all bubbles with Db up to at least 14.6 mm burst in an orderly manner in which a hole appears at a well-defined location, usually the film's edge, and propagates from there gathering up the film's mass into a toroidal ring as it progresses. This process is enabled because surface tension provides the force required to sustain the centripetal accelerations. Film drops are created when beads, of sufficient size, form along the length of the toroidal ring and surface tension is insufficient to maintain the centripetal accelerations at these accumulation points. Pieces of the ring break loose and leave the toroidal ring along paths tangential to the bubble's cap. It is shown that only bubbles larger than 2.4 mm diameter can launch film droplets by this means and that this begins when the film has rolled up through an angle of about 31 ø independent of both bubble size and (theoretically) surface tension. Film drop spray patterns recorded on MgO-coated cylindrical shells surrounding the burst bubbles yield film drop numbers and trajectories. In addition, film drop size distributions, their speed of launch, and the speed at which the film opens have been determined as a function of bubble size. The droplet sizes cited here are substantially larger than most previous estimates, and with a high probability, these droplets follow downward trajectories which lead them to impact the surface. A strong inference may be drawn that these impacts give birth to secondary droplets that are smaller than their parents and which have upward velocity components. IntroductionBubbles bursting on water surfaces generate aerosols by at least two distinct mechanisms. The first droplets created come from the cap, or film, which separates the bubble's interior from the air above. These droplets are called film drops. The second set of droplets is born somewhat later when the hole, left after the cap has gone, collapses, generating an unstable jet thrusting upward from the bottom of the cavity. These drops are referred to as jet drops. This paper is concerned with film drops.It has long been noted that some film drops travel in a horizontal direction, while others are apparently entrained upward by the escaping gas. In this paper, evidence is provided that for bubble sizes up to at least 14.6 mm diameter, Blanchard's description prevails. It is also argued that the horizontal droplets result when surface tension is insufficient to keep bits of the rapidly advancing toroid from tearing loose. It is further suggested that the socalled vertical cloud of droplets, observed by many workers, is created when the horizontal droplets impact the water's surface with sufficient force, and dose enough to the bubble's edge, to give birth to droplets from splashing which are then en...
The size distributions of the jet drops produced by individual air bubbles bursting on a fresh water surface are presented. The bubbles studied ranged in size from 349 to 1479 μm radius. The probability that a bubble of radius rb produces at least n drops, p(rb, n), is given for n up to 7. The underwater sound made by collapsing bubbles is discussed briefly.
An estimate of dF0/dr, the sea spray aerosol production per increment droplet radius per unit time per unit area of the ocean, has recently been obtained from a term‐by‐term evaluation of the following expression: dF0/dr = dE/drτ−1W(U). The differential whitecap aerosol productivity, dE/dr, the number of aerosol droplets per increment droplet radius produced during the decay of a unit area of whitecap, has been redetermined, for the size range 0.8 μm < r < 8 μ, from the results of recent experiments in the University College, Galway, whitecap simulation tank in which Particle Measuring Systems aerosol spectrometers were used. The characteristic decay time for whitecaps, τ, and the initial whitecap area were in the present study determined from the frame‐by‐frame analysis of cine‐film recordings of whitecaps generated in the simulation tank. The fraction of the sea surface covered by whitecaps W at wind speed U was taken from Monahan and O'Muircheartaigh (1980). The resulting expression, representing the local sea surface aerosol source function in units of m−2 s−1 μm−1 appropriate for inclusion in any model for the prediction of the vertical profile of aerosols within the marine boundary layer, is given by dF0/dr = 5.77 × 10−6U3.41 (ΔdN/dr), where the last term represents the change in aerosol concentration per increment droplet radius, expressed in m−3 μ−1, that occurs in the air within the hood over the tank when an individual whitecap decays.
The parameters of the births of jet droplets originating from bubbles bursting on water surfaces are presented. Results are given for bubble sizes Rb from 350‐ to 1500‐μm equivalent volume radius in both sea and fresh waters. The ejection speeds of the jet droplets generated by a collapsing bubble and the height above the surface, as well as the time, at which the top drop breaks off the ascending jet have been measured. The dependence of the average droplet speed se on bubble size is shown to be an exponential for at least the top two drops. For the top drop this dependence is given by se = 10.7 exp (−0.00127 Rb) m s−1 for Rb in micrometers which is a fit to measurements between 350‐ and 1500‐μm radius. Extrapolated to small bubble size, this equation projects that the maximum droplet speed of ejection will approach 10.7 m s−1 as bubble size vanishes. Extrapolated to larger bubbles, on the other hand, this equation projects that a bubble with Rb = 3000 μm will emit a droplet with a speed of only 0.2 m s−1. The time t that the top drop separates from the rising jet is given by t = 2.45×10−5 Rb1.65, where Rb is in micrometers and t is in milliseconds.
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