| Revision as of 10:21, 27 April 2013 editChiswick Chap (talk | contribs)Autopatrolled, Extended confirmed users, Page movers, New page reviewers, Pending changes reviewers, Rollbackers297,573 edits rv glitch← Previous edit | Revision as of 10:26, 27 April 2013 edit undoChiswick Chap (talk | contribs)Autopatrolled, Extended confirmed users, Page movers, New page reviewers, Pending changes reviewers, Rollbackers297,573 edits refimproveNext edit → | ||
| Line 1: | Line 1: | ||
| {{refimprove}} | |||
| '''Soap films''' are thin layers of ] (usually water-based) surrounded by air. For example, if two ]s come into contact, they merge and a thin film is created in between. Thus, ]s are composed of a network of films connected by ]. Films are used as model systems for minimal surfaces, which are widely used in mathematics. | '''Soap films''' are thin layers of ] (usually water-based) surrounded by air. For example, if two ]s come into contact, they merge and a thin film is created in between. Thus, ]s are composed of a network of films connected by ]. Films are used as model systems for minimal surfaces, which are widely used in mathematics. | ||
| == Stability of soap films == | == Stability of soap films == | ||
| ⚫ | ] | ||
| ⚫ | ] | ||
| Daily experience shows that soap bubble formation is not feasible with water or with any pure liquid. Actually, the presence of soap, which is composed at a molecular scale of ], is necessary to stabilize the film. Most of the time, surfactants are ], which means they are molecules with both a ] and a ] part. Thus, they are arranged preferentially at the air/water interface (see figure 1). | Daily experience shows that soap bubble formation is not feasible with water or with any pure liquid. Actually, the presence of soap, which is composed at a molecular scale of ], is necessary to stabilize the film. Most of the time, surfactants are ], which means they are molecules with both a ] and a ] part. Thus, they are arranged 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 from thinning and consequentially bursting. This can be shown quantitatively through calculations relating to ]. The main repulsion mechanisms are steric (the surfactants can not interlace) and electrostatic (if surfactants are charged). | Surfactants stabilize films because they create a repulsion between both surfaces of the film, preventing it from thinning and consequentially bursting. This can be shown quantitatively through calculations relating to ]. The main repulsion mechanisms are steric (the surfactants can not interlace) and electrostatic (if surfactants are charged). | ||
| Line 22: | Line 22: | ||
| == Colours of a soap film == | == Colours of a soap film == | ||
| ] | ] | ||
| The ] ] are caused by ] of (internally and externally) reflected light waves, a process called ] and are determined by the thickness of the film. This phenomenon is not the same as the origin of ] colours (caused by the ] of internally reflected light), but rather are the same as the phenomenon causing the colours in an oil slick on a wet road. | The ] ] are caused by ] of (internally and externally) reflected light waves, a process called ] and are determined by the thickness of the film. This phenomenon is not the same as the origin of ] colours (caused by the ] of internally reflected light), but rather are the same as the phenomenon causing the colours in an oil slick on a wet road. | ||
| Line 30: | Line 30: | ||
| If surfactants are well chosen and the atmosphere is controlled (humidity, air movements...) a horizontal soap film can last quite a long time (between minutes and hours). On the contrary, a vertical soap film is affected by gravity and so the liquid tends to drain, causing the soap film to thin 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. | If surfactants are well chosen and the atmosphere is controlled (humidity, air movements...) a horizontal soap film can last quite a long time (between minutes and hours). On the contrary, a vertical soap film is affected by gravity and so the liquid tends to drain, causing the soap film to thin 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== | ==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. ] 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:<ref>{{cite article |last1=Culick|first1=F.E.C. |authorlink1= |last2= |first2= |journal=Journal of Applied Physics |volume=31(6) |date= |year= 1960|month= |origyear= |publisher= |location= |isbn= |oclc= |doi= |id= |page=1128–1129 |
If a soap film is unstable, it ends by bursting. A hole is created somewhere in the film and opens very rapidly. ] 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:<ref>{{cite article |last1=Culick|first1=F.E.C. |authorlink1= |last2= |first2= |journal=Journal of Applied Physics |volume=31(6) |date= |year= 1960|month= |origyear= |publisher= |location= |isbn= |oclc= |doi= |id= |page=1128–1129}}</ref> | ||
| <math>V=\sqrt{\frac{2\gamma}{\rho h}}</math> where <math>\gamma</math> is the liquid surface tension, <math>\rho</math> is the liquid density and <math>h</math> is the film thickness. | <math>V=\sqrt{\frac{2\gamma}{\rho h}}</math> where <math>\gamma</math> is the liquid surface tension, <math>\rho</math> is the liquid density and <math>h</math> is the film thickness. | ||
| ==References== | ==References== | ||
| {{Reflist}} | {{Reflist}} | ||
| Giant film | |||
| {{Foam scales and properties}} | {{Foam scales and properties}} | ||
Revision as of 10:26, 27 April 2013
 | This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Soap film" – news · newspapers · books · scholar · JSTOR (Learn how and when to remove this message) |
Soap films are thin layers of liquid (usually water-based) surrounded by air. For example, if two soap bubbles come into contact, they merge 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.
Stability of soap films
Daily experience shows that soap bubble formation is not feasible 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 amphiphilic, which means they are molecules with both a hydrophobic and a hydrophilic part. Thus, they are arranged 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 from thinning and consequentially bursting. This can be shown quantitatively through calculations relating to disjoining pressure. The main repulsion mechanisms are steric (the surfactants can not interlace) and electrostatic (if surfactants are charged).
Moreover, surfactants make the film more stable toward thickness fluctuations due to the Marangoni effect. This gives some elasticity to the interface: if surface concentrations are not homogeneously dispersed at the surface, Marangoni forces will tend to re-homogenize the surface concentration (see figure 2).
Even in the presence of stabilizing surfactants, a soap film does not last forever. Water evaporates with time depending on the humidity of the atmosphere. Moreover, as soon as a film is not perfectly horizontal, the liquid flows toward the bottom due to gravity and the liquid accumulates at the bottom. 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. See for instance the approximate numerical method for minimal surfaces form finding at Stretched grid method. Surface tension, 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 naturally takes 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 film are caused by interfering of (internally and externally) reflected light waves, a process called thin film interference 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.
Drainage of a soap film
If surfactants are well chosen and the atmosphere is controlled (humidity, air movements...) a horizontal soap film can last quite a long time (between minutes and hours). On the contrary, a vertical soap film is affected by gravity and so the liquid tends to drain, causing the soap film to thin 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 tension 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.
where 
is the liquid surface tension, 
is the liquid density and 
is the film thickness.
References
| Foam scales and properties | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||