Bursting of soap films. I. An experimental study

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"Bursting of soap films. I. An experimental study"
Winnie R. McEntee, & Karol J. Mysels
Journal of Physical Chemistry 73(9) 3018-3028 (1969)


Soft Matter Keywords

soap film, inertia regime

Figure 1. Apparatus for the reproducible formation of vertical films between gold electrodes.
Figure 2. Schematic arrangement for photographing the bursting film by a flash occurring at a specified delay after the initiating spark.
Figure 3. Top row shows a series of high-speed flash photographs of bursting films taken in reflected light. Delay between perforating spark and illuminating flash (film thickness): a) 75 microseconds (.28 microns), b) 250 microseconds (.43 microns), c) 200 microseconds (.01 microns). Bottom row shows scattered light images of a bursting film. Delay and film thickness: a) 900 microseconds (3.4 microns), b) 320 microseconds (.029 microns).
Figure 4. Top plot: hole radius versus time delay for several film thicknesses. Bottom plot: dependence of rim velocity on film thickness and film solution composition.

Summary

McEntee and Mysels present the data from an experimental study on bursting soap films. The films burst in an inertial regime, so we expect the viscosity of liquid film to have little impact on the dynamics of the bursting. Taking high-speed flash photographs of the burst process at different time delays from the point of perforation, the authors validate predictions on the velocity of the receding rim.

Bursting Soap Films

The authors create an elegant apparatus to observe the rapid bursting of thin soap films. A wire frame is drawn vertically out of a soap solution and brought between the tips of two gold electrodes. These electrodes are connected to an electrical circuit that discharges a spark between them. The same circuit triggers a high-speed flashbulb at a specified delay following the spark. The spark initiates rupture of the soap film, while the reflection of the flash on the film produces an image of the film while it is in the process of rupturing. The schematic drawings for the experimental apparatus and electrical circuit can be seen in Figures 1 and 2.

The entire apparatus is enclosed by light-proof material, and images are taken by a camera with a long exposure. Thus, only the spark and objects highlighted by the flash are recorded on the film. Two distinct image types were recorded for these experiments. Reflected light images show the extent of rupture as well as thickness variations in the film. Scattered light images reveal details about the receding rim. Examples of both types are shown in Figure 2.

From a series of images such as is shown in the top row of Figure 2, McEntee and Mysels are able to build up a curve hole diameter versus time. Theory proposed by Dupre in 1867 expresses a relation between the film thickness, <math>\delta</math>, its density, <math>\rho</math>, its surface tension, <math>\sigma</math>, and the velocity of the receding rim, <math>u_H</math>:

<math>u_H = sqrt(\phi \sigma / \rho \delta)</math>


written by Donald Aubrecht