Throughout his life, Belgian physicist Joseph Plateau, (1801-1883) carried out painstaking observations on soap bubbles and soap films, laying out his theories in a two volume publication, Experimental and Theoretical Statistics of Liquids subject to Molecular Forces only published in 1873, describing the general structure of touching soap bubbles, he observed:
- Soap films always meet in threes and at an angle of 120°.
- The three films form a curved triangle edge, (called a Plateau border).
- Four Plateau borders meet at angles of 109.6° and always in fours, (to form a vertex).
In relation to the general appearance:
- The surfaces of soap films are entirely smooth.
- The average curvature is always constant for part of the bubble as it is for the whole.
Although some studies into bubbles and films had been carried out beforehand these were limited to specific phenomena. The publication of Experimental and Theoretical Statistics was the most comprehensive to date. Plateau's laws set the ball rolling for serious scientific investigation thereafter.
In 1976, American mathematician Jean Ellen Taylor published The structure of singularities in soap-bubble-like and soap-film-like minimal surfaces - the first proof of Plateau's laws and noted they have a bearing on foam stability.
Foams are complex systems and the underlying physics is not straightforward. Foams are formulated to behave in particular ways, they must fulfil certain requirements to become effective depending on their final application: fire extinguishers, personal and household cleaning products, the food and drinks industry.
Some foams need be more stable than others. Fire extinguisher foams must spread over large areas and be capable of excluding air. If it collapses soon after expulsion the fire will continue its course. Compare these foam properties to those produced by the cleaning action of a toothpastes and you will begin to realise how critical mathematical and scientific investigations and conclusion is. Not all foams are in a 'liquid' state. Solid foams, (although many of which are made from liquid precursors), have a wide range of use: foam packaging and polystyrene, buoyancy aids, upholstery etc. Aluminium is a metallic foam.
In addition, foams are an unwelcome by-product in the manufacturing process: chemical, oil, paper industry, beer/wine, chemical industries and cause major problems for disposal therefore knowledge of their behaviour is essential for efficient, disposal practices to be carried out.
This is a very brief introduction to foams, foam rheology is a massive and complex area and I always encourage my audience to find out more via a web search - tap in wet foams, dry foams, metallic foams to discover why foams are vital to everyday life.