The significance of the Foam in a liquid and the requirements for foam properties vary greatly. Foam is desirable in shower gels, shaving creams, or beverages like beer and sodas, whereas in industrial processes, foam often causes problems and must be avoided. Therefore, foam properties, from foam formation to Foam decay, are set and tested during product development.
Foam and Types of Foam
Foam is a dispersion of a gas in a liquid or solid. The volume of the gas is relatively larger than that of the other phase. Foams in liquids are usually stabilized by amphiphilic substances, such as Surfactants, particles, or other compounds.
Spherical foam or wet foam is formed when there is a lot of dispersing agent or liquid between the gas bubbles. The liquid causes the gas bubbles to assume a spherical shape (smallest surface area for the largest volume). In polyhedral foam or dry foam, the distance between the individual gas bubbles decreases, and the dispersing agent contracts into lamellae.
Spherical foam and Polyhedral foam often occur simultaneously, with polyhedral foam situated above the spherical foam. Gravity causes the liquid to flow down from the foam lamellae.
Methods
The methods for foam characterization and testing are diverse and generally follow an application-oriented approach. For example, in the evaluation of dishwashing detergents, the plate test is widespread, where dirty plates are washed until the foam layer of the washing liquid collapses. In this and other manual tests, the foam is generated by manual shaking, stirring, or pouring, and the stability and amount of foam are visually assessed. The reliability of these tests heavily depends on the training and experience of the personnel.
Instrumental methods mimic manual tests but provide reproducible processes and quantitative, user-independent evaluations through technical implementation. Foam generation methods also rely on mechanical principles such as stirring, shaking, beating, pouring, spraying, or air supply.
SITA FoamTester
The SITA FoamTester generates foam by stirring with a mechanical stirring disc. Two optical, contactless measurement systems provide data for characterizing foam properties such as foam volume, foam stability, liquid drainage, and foam structure.
Foam volume describes the Total volume of the generated foam as the sum of its gaseous and liquid components. The calculation of foam volume with the SITA FoamTester is based on measurement data of the lower foam boundary (upper liquid boundary) and the upper foam boundary.
The upper foam boundary is determined by the Foam Surface Scanner using structured light. The foam surface is illuminated with a sequence of stripe patterns of defined shape, and the resulting sectional images are captured with a camera. From the image data, the coordinates of the foam surface can be precisely calculated, and the total volume in the measuring vessel accurately determined.
The Foam Interface Scanner captures the lower foam boundary as total reflection on the container wall. Total reflection occurs in media with different optical densities. When light hits the air behind the container wall, it is reflected and detected at an angle by a light-sensitive sensor. This allows an accurate separation of the gas phase from the liquid phase and the determination of the remaining liquid volume.
Foam Formation, Foam Decay, and Liquid Drainage
Foam formation and decay are dynamic processes. Foam formation represents foam volume as a function of time (stirring time), while foam decay measures the foam volume in a resting state. Parallel to foam decay, the lower foam boundary provides information on liquid drainage and thus foam drying.
Foam structure is also captured with the Foam Interface Scanner as total reflection. For foam structure measurement, the light-sensitive sensor and the light source are moved up to 50 mm along the measuring vessel. This allows the entire range of (130 x 50) mm of the foam structure to be recorded. Using image processing algorithms, various geometric properties of all captured bubbles are determined and statistically evaluated in the PC software SITA FoamLab.
The foam structure at interfaces, such as the container wall, differs from the structure in the free foam volume because the foam interacts with the interface. Initially, three cases are distinguished:
- The interface to the liquid
- The interface to the surrounding atmosphere
- The interface to the wall of the measuring vessel
At the liquid interface, the foam structure is determined by the high local liquid content, typically resulting in spherical foam.
The foam bubbles at the interface to the surrounding atmosphere, the upper foam boundary, are not influenced by neighboring bubbles. To minimize the foam surface, the lamellae form domes. As the foam dries through Drainage, polyhedral foam forms. Therefore, the structural influence at the interface to the surrounding atmosphere is high.
At the interface to the container wall, Wetting effects come into play. Surface-active substances such as surfactants enhance the wetting of the vessel wall by the foam. This leads to a deformation of the Bubble shape, and pseudo-Plateau borders settle between the deformed bubbles. Such a border consists of only one lamella.
For metrological analysis, the influence of foam structure at an interface is significant. Due to the curvature of the pseudo-Plateau borders, these lamellae are not visible in the observation. Due to the optical properties of thin layers, light is only reflected or refracted at a certain thickness. Therefore, the measurable bubble diameter is not the distance between two pseudo-Plateau borders but lies in between. The difference between the actual and the measurable Bubble size is a systematic measurement deviation.
