How is Foam Created?
Methods of Testing Foam

How is Foam Created?

Everyone has got something to do with foam everyday: showering, shaving, washing, and let's not forget the occasional beer. All these different foams have their specific characteristics. For example, shaving foam should foam very well and be stable, in contrast a washing agent should foam little so that the washing machine does not overflow and the laundry within is protected. That is why the choice and combination of foam generating, stabilizing and reducing substances has to be exactly adjusted and tested for the application.

The requirements for the development of foam in liquids are first the reduction of the surface tension and second the input of air into the liquid. The surface tension can be lowered through surfactants. In doing so the surfactants attach to the boundary between the liquid and the air. The surfactants also attach themselves to the air bubbles which have been pumped into the liquid. The air bubbles rise to the liquid surface and form a double layer or lamella consisting of two boundaries (air - liquid - air) with attached surfactants. These lamella are the main element of foam. Through gravity the liquid gradually runs out of the foam, i.e. the foam decays.

There are two different kinds of foam: ball and polyhedron foam. Ball or wet foam consists of many singular bubbles in the form of balls, between which much liquid is situated. When the liquid drains out of the lamella so that the single bubbles move closer to each other they form into polyhedrons. This is called polyhedron foam, or dry foam. Both types occur in a foaming liquid, whereby the polyhedron foam is situated above the ball foam.

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Methods of Testing Foam

In developing new products it is essential to exactly differentiate properties. Thus, the foaming behaviour of liquids has to be tested particularly with regard to the application. This is done in two steps. First, foam has to be produced and, second, the foam height has to be measured.

Conventional methods of foam generation are based on mechanical principals such as agitation (ASTM D3601-88 (Re-approved 1997)), beating (DIN 53902-1), Ross-Miles Method (DIN 53902-2), shower (DIN 51395-1) or insertion of air.

The following methods are used to measure the height of the foam:

Visual
By means of a vessel with a measuring scale, the foam height is visually determined by the user in specific intervals. This demands great care because the foam surface is not even and consequently, subjective mistakes are not eliminated.
Transparency
The foam height is measured with a light source and a light sensor. The foam refracts the emitted light and hence shades the light sensor. The measurement result can be corrupted through the uncleanness of the vessel. Unevenness of the foam height is not recorded.

In using these methods of testing foam there is the disadvantage that either the measurement results are not reproducible or subjective influences effect the results.

By means of the patented Rotor Method, resulting in defined foam generation, and a special sensor which records the surface profile of foam, the new SITA Foam Testing System gives reproducible measurement results without subjective influences.

For further information about methods of testing foam you may request publications from us.