Thermal radiation and thickness fluctuations in freely suspended liquid films

Abstract

Thermal convection in a vertically suspended soap film subjected to a vertical temperature gradient is marked by intense density fluctuations ␦ 2 , uncommon to laboratory Rayleigh-Bénard convection ͑RBC͒. Such large fluctuations result from stratification in the film under the influence of gravity. Herein we present the first direct measurement of two-dimensional density fluctuations in a free-standing soap film using a single-point infrared detector. The radiation densitometer is nonintrusive and responds instantaneously to local density variations. Measurements of the power spectrum ⌫͑f͒ of ␦ 2 are carried out using a variety of sample geometries of the aspect ratio of unity.In all cases, ⌫͑f͒ scales with the frequency as f −1.4±0.1 in the low frequency regime, which is in good agreement with the Bolgiano's theoretical prediction for a stably stratified fluid. Combining thermal imaging and particle tracking, we also show that it is feasible to measure the full-field density flux j͑x , y͒ = 2 ͑x , y͒v͑x , y͒ in the film. Despite that individual snapshots of j͑x , y͒ appear random, the time-averaged flux forms a close loop similar to the large-scale circulation commonly seen in RBC, when a sufficiently large temperature gradient is present.

Effects of rotation on temperature fluctuations in turbulent thermal convection on a hemisphere

“…To further check these results we have carried out additional experiments to measure the velocity structure functions. These results show that the velocity structure functions display Bolgiano Obukhov scaling consistent with an exponent of 3n/5 as expected and observed experimentally before 12 , 13 , 16 . Under rotation, the scaling range diminishes and the structure function exponents become smaller, contrary to 3D experiments where the exponent increases.…”

Surface tension profiles in vertical soap films

“…This fact attests to both the existence of a nonuniform thickness profile in the soap film and also to the evolution of that profile with time. From a mechanical point of view, this thickness profile implies that (i) as proposed by Gibbs in 1878 [3], for any value H of the height above the bottom edge of the frame, the weight of the part of the film which lies beneath H must be counterbalanced by the local surface tension forces and (ii) since the fringe pattern in the film stretches from bottom to top, implying that the thickness gradient in the film gets smaller as H increases, the sustaining surface tension profile must both increase and saturate with H. Due to their highly interfacial nature, soap films have been in the center of numerous studies during the past few decades [1,[4][5][6][7][8][9][10][11][12][13], Early works by Mysels and co-workers [1,13,14] showed that their behaviors strongly depend on parameters such as viscosity [13] but also on the chemicals used to produce them. Specific surfactant-linked phenomena such as marginal regeneration [1,7] have also been noticed to account in phenomena such as drainage, leading to unexpected film lifetimes [1], Those studies of isolated soap films have been widely used to model global behaviors of more complex systems such as foams [15].…”

Velocity Field in a Vertical Foam Film

“…However only the component normal to the thickness gradient is available. An alternative is to use tracers [18,19], but they slightly deform the film which, in a vertical film, leads to artefactual gravity induced motion. Here, we propose a novel technique, based on fluorescent labeling of the flow, which overcomes these drawbacks.…”