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The vertically ejected jet drops from bubbles smaller than 1 mm in diameter are formed from a thin superficial layer of liquid accelerated inward by surface forces. The drops are easily collected and offer a novel nonmechanical surface microtome.Small gas lbubbles, breaking at a liquid surface as shown in Figure 1, eject droplets several centimeters into the air.2 These jet droplets are composed of surface material and are easily collected by various means (e.g., a filter, an aerosol impactor,a or simply an inclined plate draining into a collection vessel). The thickness of the surface thus collected is not adequately known: it lies between 0.5 and 20 pm for a 600-pm bubble, with indirect evidence4 suggesting 10 pm. This is many times thinner than the surface collected by McBain and 1HumphreysJ6 delicate and complex surface microtome, yet requires no more apparatus than a source of uniform bubbles, which may be generated in large numbers by an oscillating capillary tip.6 Uniformity of bubble size may be necessary if a uniform thickness of surface cut is desired.In contrast to foam-collection methods,' which sample the surface by forming stable film caps, the bubble microtome depends upon foam instability and is suited for surface concentrations far lower than those which can be sampled by foam methods.The smaller the thermodynamic surface excess Ti (ions/cmz), the thinner the slice 6' (cm) required to see it, sincewhere di is the concentration of i in the collected drops (ions/cm3) and ci is the concentration in the bulk liquid. The detection limit of (dici) is set by the analytic method used, and further sensitivity must come from a reduction in 6'. Using the bubble microtome, Fits from lo9 down to lo7 ions/cm2 (or 10-7 monolayer) have been estimated using radiotracers for assay;* these surface concentrations (although 100-fold higher than thebulk fluid) were toosmall to give measurable surface tension (6) changes to evaluate ri independently as -(&/a In a)/RT. The energy for drop production comes from the surface free energy dG = adA, where dA is the change in area. (See Figure 2.) Gravity (hydrostatic pressure)contributes only about 1% of the total energy. Surface tension produces a pressure P given by Laplace's familiar formula P = u(K1 + K z ) , where the K's are the principal curvatures. Since P is exerted normal to the surface, it might seem that the jet drop could be composed of material from the nadir of the bubble through a simple radial displacement. However, there are several arguments for boundary-layer-like flow down the cavity walls. Blanchard2 has observed that monolayer material from the top surface of the liquid (not from the interior bubble surface) appears to cover 200% of the drop surface (for drops from 3-mm bubbles) -that is, there must be some mechanism acting to collect material from outside the cavity, bring it to the center, and compress it. He further notes complete removal of oleic acid surface films by prolonged bubbling. The spreading velocity of surfactants (which falls to zero whe...

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Preliminary interpretation of pH and sodium concentration data for marine rains collected during February and July, 1981 at American Samoa (14°15′S, 170°34′W) as part of the Sea/Air Exchange (SEAREX) Program indicate that rain is usually slightly more acidic than would be expected on the basis of atmospheric CO2‐pure water equilibrium, in this remote region of the tropical South Pacific Ocean. The data are consistent with calculations of pH using the alkalinity equation for "model" solutions containing only sea‐salt and traces of strong acids in equilibrium with atmospheric CO2. Alkalinity due to sea‐salt appears to affect pH significantly only when the salinity of the rain is greater than 0.01 ‰ or 3000 µg Na/kg. The observed range of pH values is much smaller than has been observed recently at subtropical marine sites: this, along with the relatively high average pH value, suggests that climatic factors influence levels of acidity in rain over remote ocean areas.