Soap films

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Contributed by Daniel Daniel

Introduction

Soap films are thin layers of liquid (usually water) surrounded by air. A soap bubble is essentially of a thin layer of water film that separates the air inside and outside of the bubble. Another example where soap films are found is foam, which consists of a network of thin water films that are connected in accordance to Plateau's laws, which will be explained in a later section. Soap films are stable due to the presence of surfactants, usually ampiphilic molecules, such as sodium dodecyl sulfate which has a hydrophilic head that interacts preferentially with water and a hydrophobic tail that interacts preferentially with air. This is schematically shown in figure 1.

Soapfilm1.png Figure 1. Schematic of how surfactants help to stabilize the soap film.

Physics of soap film

The first thing about soap film that catches the eye is irridescence (See figure 2) - how the colour of the bubble seems to change with the viewing angle. This is a very common property of colour that has a more physical rather than chemical origin. In fact, the irridescence is the result of interference of light that has been reflected off the two water-air interfaces that form the soap film, very much like the irridescence that we see in an oil slick.

Soapfilm2.jpg Figure 2. Irridescence in a soap film

Soapfilm4.jpg

Plateau's laws

Soapfilm3.jpg



http://www.soapbubble.dk/en/bubbles/geometry.php

Keyword in references:

A public study of the lifetime distribution of soap films