Fluorocarbon Vapors Slow Down Coalescence in Foams

Abstract

can be reduced or even fully stopped. [1][2][3][4] Coarsening, for instance, is commonly prevented by adding fluorocarbon vapors to the foaming gas. [1,4] Since fluo rocarbons are quasi insoluble in water, [5] their transport through the aqueous films separating neighboring bubbles is hampered. As a result, an osmotic pressure difference between neighboring bubbles is created, which counteracts the destabilizing Laplace pressure difference. Fluorocarbon vapors are thus widely used in foam science and biomedical applications, e.g., to stabilize microbubbles. [6][7][8][9] However, until now, fundamental and applied research completely neglected the possible influence of fluorocarbon vapors on other foam properties in general, and on foam coalescence in particular. We show here that fluorocarbon vapors can very effectively hinder coalescence even at very low concentrations. We hypothesize that this is due to the formation of a mixed fluorocarbon/surfactant layer at the gas/water interface. The complex properties of surfactant monolayers in the presence of fluorocarbons have been investigated in depth in recent years, [10][11][12][13][14][15] yet without establishing a link to foam properties. Our findings open new possibilities of controlling foam stability by introducing a "coadsorbate" from the gas phase.

Neutron radiography of liquid foam structure near a vertical wall

“…The air was mixed with approximately 2 g L −1 of perfluorohexane C 6 F 14 , yielding better foam stability. 19,20 By means of vertical plates the foam was confined to a region of 150 mm width (see Fig. 1(b)) which coincides with the visible area in neutron radiography.…”

Foam‐based cleaning of surfaces contaminated with mixtures of oil and soot

“…While the bubble size of the stable PFH‐containing foams remains nearly constant within 60 min, the mean radius of the bubbles of the unstable PFH‐free foams increases by a factor of five within 30 min (Schad et al, 2021). Note that it was shown only recently that this important difference in foam stability results from the simultaneous suppression of foam coarsening and bubble coalescence by PFH (Steck et al, 2021).…”

“…The same cleaning tests were repeated with very stable PFH-containing foams with the same liquid fractions. In these foams, coarsening and coalescence of the foam bubbles is efficiently prevented by the PFH (Andrieux et al, 2018;Gandolfo & Rosano, 1997;Höhler et al, 2008;Meagher et al, 2015;Schad et al, 2021;Steck et al, 2021;Weaire & Pageron, 1990), leading to a nearly constant bubble size during the experiment (Figure 1, right). Looking at Figure 3 (top and middle), one sees that the contaminated area is split into many small areas with foam in between.…”

Minimal‐Invasive Method for the Evaluation of Liquid Fractions in Foams with a Point Level Sensor

“…Foam was produced by pneumatically introducing air at the bottom of the column by means of a sparger with a flow rate of d V air+C6F14 /d t = 1.7 × 10 −5 m 3 s −1 (1000 sccm min −1 ), resulting in a superficial gas velocity in the empty column of 0.22 cm s −1 . The effect of coarsening during the experiments was reduced by adding a small amount of perfluorohexane (C 6 F 14 ) to the air before entering the cell 20.…”