Bubbles. Boundary-layer "microtome" for micronthick samples of a liquid surface

Abstract

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...

High-Speed Jet Formation after Solid Object Impact

“…5(c) is of the order of the disc radius, its maximum thickness is only about 0:01R 0 . Thus far, a similar surface layer has only been observed when jetting is directly caused by surface waves [12]. In the present case, the thinness of the layer is even more remarkable as it does not arise from a surface phenomenon but from the collapsing motion of the entire bulk liquid.…”

“…Instead, this jet formation is shown to depend crucially on the kinetic energy contained in the entire collapsing wall of the cavity even far above the pinch-off singularity. This is in contrast to jets observed in many other situations where narrow confining cavity walls are not present, e.g., for bubbles bursting on a free surface or near a solid wall [9][10][11], wave focusing [12,13], or jets induced by pressure waves [14]. In addition, surface tension in our case turns out to be irrelevant in contrast to capillary-driven scenarios as suggested for Faraday waves [7,8].…”

Note on minimum wetting rates under isothermal conditions.

“…The purpose of this note is to complementthe paper published in this journal2) (hereafter referred to as [2]) and to make answer to Ponter and Vijayan( hereafter referred to as [1]) whopointed out theoretical weakness of our equation proposed in [2].…”

Adsorption and Wetting Mechanisms at the Surface of Aqueous Hydrocarbon Solutions as a Possible Source of Atmospheric Pollution

“…But, the contribution of the atmospheric dissolution of such species through vapour pressure equilibrium or surface adsorption and bubbling has been rarely considered in atmospheric pollution mechanisms [2][3][4]. However, in the vicinity of wrecked tankers with huge amounts of oil progressively leaking from their tanks and dissolving in the seawater, the passing of hydrocarbons from water to the atmosphere through surface mechanisms is worth being taken into account.…”