Soap Films

Soap film interference colours captured by Tim Durham (soap films, photography).

Tim has made an extensive project of photographing single soap films.

His films hang vertical and the area covered in these images was between 150 and 300 mm across.

The colours are as true to life as possible. There has been no enhancement or manipulation.

All images ©Tim Durham, shown with permission

The iridescent colours arise from interference between light waves reflected from the front and back film surfaces. Take the red waves at far left. Some of the incident waves are directly reflected from the front. Note that there is a 180° phase change when this happens. Some light enters the film and is internally reflected from the rear surface. The two waves combine. In the example at left the two waves are in phase and a red reflection would be seen in that direction.

Compare that with the shorter wavelength blue waves at near left. The two outgoing waves are out of phase and cancel each other. No blue would be seen.

The particular colours produced depend on viewing angle and most importantly, on the film thickness.
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Two reflected red wave crests coincide. The film looks red.

Add some soap or detergent to water and we enter a new world.

Water, instead of sitting placidly in bowl or beaker, leaps out into airbourne bubbles of iridescent hues.

It turns into foams of incredible structure that we still do not fully understand and which refuse to quickly leave the bath.

And with some care we can make large single sheet films, as here, of great delicacy and beauty.

Tim Durham - Soap films, Photography site, Exhibition Interview Video

Recipe for bubble making solution

How to make and photograph single soap films

Soap film structures - Foams

  Weaire - Phelan Foams


Detergent molecules cluster at the water film surface with their oleophilic water hating ends outwards and hydrophilic ends inwards..

No two films are the same. They are ever changing as they drain and thin. Some of the colours with sharp boundaries can be the result of multi-layer films sliding over each other.
Shorter wavelength blue waves cancel
The key to water's behavioural shift is a single layer of detergent molecules.  

Water molecules attract one another strongly. The net effect near its surface is to give it a 'surface tension' that acts to minimise the surface area.  Water's surface tension is too strong for stable bubbles or single films to exist.    Detergents considerably reduce the tension permitting water to form thin films. 

Detergent and soap molecules have one end that is water attracting - they have heads with 'polar groups'. Their other end is a hydrocarbon tail that is water hating but attractive to oils and greases.    Consequently the detergent molecules migrate to the water surface where the polar heads sit in the water and the hydrocarbon tails protrude out.  

The soap films themselves are much thicker – they are a sandwich of water molecules edged by these ultra thin detergent films. The outer detergent layers also reduce evaporation, that enemy of long lasting bubbles and dedicated bubble blowers.
Very thin film - Reflections from the front and back are out of phase and the film appears black.

Black!   Not a hole but a sign that the end is near. The film or bubble will soon collapse. Gravity has caused the vertical film to drain downwards and become thinner at the top. The black indicates that the film has become dangerously thin.

When the film is this thin there is almost no phase difference between the waves reflected from the front and back surfaces. BUT reflection from the front surface produces a 180° phase shift. As a result the two waves are completely out of phase at all wavelengths.
The film becomes black.