Foam consists of bubbles linked together. In many
applications, such as in shampoo and hand dish detergents, foam is desirable; while in many other applications, such as in auto dishwashing gel and water
treatments, low, no, or fast-collapsing foam is preferred. To control a product’s foam profile, it is helpful
to understand some principles related to foam and the
factors that impact it.
PRINCIPLES GOVERNING FOAM
GENERATION AND BREAKING
The nature of surfactants to adsorb onto water surfaces to reduce Gibbs
free energy is the driving force for foam generation. The maximum
adsorption happens at or above critical micelle concentration (CMC).
Once a bubble is formed—whether it floats under water or floats to
the surface—there is either a single or double layer of film(s) of adsorbed
surfactant molecules. Due to the bubble film curvature, there is usually a
difference between the air pressure inside and outside the bubble. This
pressure differential is the force that drives bubbles to break and, ultimately, foam to collapse.
To achieve a high initial foam height based on the Ross-Miles foam
test method, the surfactant molecule needs to move quickly enough to
reach new adsorption equilibrium on the newly increased area when a
bubble floats to the top of the liquid and its size increases (Fig. 1).
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FIG. 1. Structure and size changes of a bubble in water vs. at the surface
bubble under water bubble on or above water