Oscillations soap film

Chapter 5 investigates the shape of liquid drops, bubbles, and the liquid surface in the vicinity of a solid surface, using the Laplace-Young equation. The last chapter. Chapter 6, contains a number of interesting properties and applications, such as the vibrational oscillations of soap film membranes and the application of soap films to the analogue solutions of the differential equations of Poisson and Laplace. 

A jet of water issuing from a circular orifice, or tap, does not retain its cylindrical form. The jet of water will break up into droplets. A careful examination of the jet will show that oscillations appear in the surface of the jet which grow in amplitude along the trajectory of the jet, and finally cause the jet to break up into droplets (Fig. 5.14(a)). Similar phenomena are to be observed in closed stationary cylinders of soap film. The surface develops oscillations and eventually breaks up into soap bubbles if the cylinder is sufficiently long (Fig. 5.14(b)). However if the length of the cylinder of soap film is small enough any oscillations that occur in the surface will decay away. 

The behaviour of an unstable jet of fluid, X lrra, is shown in Fig. 5.14(a). A chance disturbance imposed on the jet causes its profile to undergo oscillations which build up with time and eventually lead to the break up of the jet into separate droplets which tend to oscillate due to their initial distortion. In the case of a cylindrical soap film (Fig. 5.14((>)) a similar sequence occurs. The oscillations grow with time and eventually cause the soap film to break up into individual bubbles. 

Soap Film Oscillations, 3 min. 20 sec., 8 mm Film-loop, Ealing Scientific, Watford WD2 4PW. 

The lower frequency modes can often be demonstrated by hand. A wire frame of any shape can be oscillated or vibrated at the appropriate normal mode frequency to produce a vibrating film with the characteristic nodal line pattern, such as shown in Figs. 6.4 and 6.5. Frames with the dimensions of order 30 cm are suitable for this purpose. A large low dish, or photographic developing tray, provides a simple container for the soap solution. It is not necessary to have large quantities of soap solution as one has only to... 

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