Rheology of foams
Foams - Formation, formulation and properties

Under low stress, foam behaves like a solid. It is assigned an elastic modulus G, which is a function of bubble size and humidity. For example, for detergent foam, this modulus is of the order of 10 Pa, whereas it is 8 · 10 ¹⁰ Pa for steel. The value of the elastic modulus for detergent foam is low, as it is a surface property due to surface tension.

If sufficiently high stresses are applied, topological changes occur which are no longer immediately reversible. The foam gradually becomes plastic. Beyond a stress threshold τ _y , the foam behaves like a liquid, "flowing" (figure