Soap film
Encyclopedia
Soap films are thin layers of liquid (usually water based) surrounded by air. For example, if two soap bubble
s enters in contact, they merged and a thin film is created in between. Thus, foams
are composed of a network of films connected by Plateau borders
. Films are used as model systems for minimal surfaces, which are widely used in mathematics.
Surfactants stabilize films because they create a repulsion between both surfaces of the film, preventing it to thin and burst. This can be made quantitative thanks to disjoining pressure
. The main repulsion mecanisms are steric (the surfactants can not interlace) and electrostatic (if surfactants are charged).
Moreover, surfactants make the film more stable toward thickness fluctuations thanks to Marangoni effect
. They indeed conferes some elasticity to the interface: if surface concentration are not homogeneously dispersed at the surface, Marangoni forces will tend to re-homogenize the surface concentration (see figure 2).
Even in presence of stabilizing surfactants, a soap film does not last forever. Water evaporates with time and this depends on the humidity of the athmosphere. Moreover, as soon as a film is not perfectly horizontal, the liquid flows toward the bottom thanks to gravity and the liquid accumulates at the botom. At the top, the film thins and bursts.
, which measures the energy needed to create a surface indeed acts as a physical surface minimizer: since energy is proportional to the soap film surface, the film deforms to minimize its surface and, thus its energy.
Depending on the support of the soap film, the latter then takes naturally the minimal surface. On a flat support, the soap film is usually flat. But, on more complicate support, it takes the minimal achievable surface.
are caused by interfering of (internally and externally) reflected light waves and are determined by the thickness of the film. This phenomenon is not the same as the origin of rainbow
colours (caused by the refraction
of internally reflected light), but rather are the same as the phenomenon causing the colours in an oil slick on a wet road.
indeed leads to surface minimization and, thus, to film disappeance. The hole operture is not instantaneous and is slowed down by the liquid inertia. The balance between both forces (inertia and surface tension) leads to the opening velocity:
where 
is the liquid surface tension, 
is the liquid density and 
is the film thickness.
Soap bubble
A soap bubble is a thin film of soapy water enclosing air, that forms a hollow sphere with an iridescent surface. Soap bubbles usually last for only a few seconds before bursting, either on their own or on contact with another object. They are often used for children's enjoyment, but they are also...
s enters in contact, they merged and a thin film is created in between. Thus, foams
Foam
-Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...
are composed of a network of films connected by Plateau borders
Plateau's laws
Plateau's laws describe the structure of soap films. These laws were formulated in the 19th century by the Belgian physicist Joseph Plateau from his experimental observations.Plateau's laws state:# Soap films are made of entire smooth surfaces....
. Films are used as model systems for minimal surfaces, which are widely used in mathematics.
Stability of soap films
Before blowing a bubble, a kid plunges his plastic frame in a soapy solution. Daily experience shows that this is not feaisable with water or with any pure liquid. Actually, the presence of soap, which is composed, at a molecular scale of surfactants is necessary to stabilize the film. Most of the time, surfactants are amphiphile, which means they are schizophrenic molecules with an hydrophobic and an hydrophilic part. So they go preferentially at the air/water interface (see figure 1).Surfactants stabilize films because they create a repulsion between both surfaces of the film, preventing it to thin and burst. This can be made quantitative thanks to disjoining pressure
Disjoining pressure
Disjoining pressure , in surface chemistry, according to an IUPAC definition, arises from an attractive interaction between two surfaces...
. The main repulsion mecanisms are steric (the surfactants can not interlace) and electrostatic (if surfactants are charged).
Moreover, surfactants make the film more stable toward thickness fluctuations thanks to Marangoni effect
Marangoni effect
The Marangoni effect is the mass transfer along an interface between two fluids due to surface tension gradient...
. They indeed conferes some elasticity to the interface: if surface concentration are not homogeneously dispersed at the surface, Marangoni forces will tend to re-homogenize the surface concentration (see figure 2).
Even in presence of stabilizing surfactants, a soap film does not last forever. Water evaporates with time and this depends on the humidity of the athmosphere. Moreover, as soon as a film is not perfectly horizontal, the liquid flows toward the bottom thanks to gravity and the liquid accumulates at the botom. At the top, the film thins and bursts.
Importance of surface tension: minimal surfaces
From a mathematical point of view, soap films are considered as minimal surfaces. Surface tensionSurface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
, which measures the energy needed to create a surface indeed acts as a physical surface minimizer: since energy is proportional to the soap film surface, the film deforms to minimize its surface and, thus its energy.
Depending on the support of the soap film, the latter then takes naturally the minimal surface. On a flat support, the soap film is usually flat. But, on more complicate support, it takes the minimal achievable surface.
Colours of a soap film
The iridescent colours of a soap filmColours of a soap film
The iridescent colours of soap bubbles are caused by interfering of reflected light waves and are determined by the thickness of the film...
are caused by interfering of (internally and externally) reflected light waves and are determined by the thickness of the film. This phenomenon is not the same as the origin of rainbow
Rainbow
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
colours (caused by the refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
of internally reflected light), but rather are the same as the phenomenon causing the colours in an oil slick on a wet road.
Drainage of a soap film
If surfactants are well chosen and athmosphere is controlled (humidity, air movements...) an horizontal soap film can last during quite a long time (between minutes and hours). On contrary, a vertical soap film, is submitted to gravity so the liquid tend to drain. The soap film first thins at the top. Since the colour depends on the thickness, it accounts for the interference fringes,that can be seen at the top of figure 4.Bursting of a soap film
If a soap film is unstable, it ends by bursting. A hole is created somewhere in the film and opens very rapidly. Surface tensionSurface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
indeed leads to surface minimization and, thus, to film disappeance. The hole operture is not instantaneous and is slowed down by the liquid inertia. The balance between both forces (inertia and surface tension) leads to the opening velocity:
where is the liquid surface tension, is the liquid density and is the film thickness.