Foam Physics

 

A foam is a dispersion of a gas in a liquid, semi-solid or a solid continuous phase. Examples of foams are cappuccino foam, ice cream, toppings, whipped creams, chocolate mousses, cast-iron, aerated concrete, polyurethane foam, beer foam, etc. These examples have the gas dispersed as bubbles in a continuous phase. There are also examples that are called foams but that are in reality sponges. A sponge structure differs from a foam structure by having two continuous phases. In a sponge structure besides the continuous liquid, semi-solid or solid phase also the gas phase is continuous and examples of such systems are many bakery products such as bread, sponge cake and waffle.

 

Foams can be generated by, for instance, agitation at the gas-liquid interphase, introducing gas bubbles into the liquid phase through a glass sinter or a grit, by nucleation of gas bubbles in a supersaturated liquid, by excessive pressure gradients or by microbial activity as can be observed in fermented products.

 

Foaming is mainly a physical phenomenon in which, of course, the chemical composition of the system plays a crucial role. The lifetime of a foam is for different reasons of great importance. Undesired foam can create serious problems, ranging from occasional annoyance to major disruption on production processes. For example, foam can interfere with process instruments, sensors, pumps and filters, slow down drying of products as a result of the slower drainage of the liquid to be removed from the product, create bubbles in coatings or separation and segregation of process ingredients. Controlling the instability of a foam can be a functional part in production processes as, for instance, textile printing, distillation processes, gas washing or in products such as beer and champagne.

 

For the understanding of controlling foams it is a prerequisite to be aware of the factors that contribute to foam stability. The thin liquid films between the gas bubbles in a foam play a crucial role in the stability of a foam. In the stability of the thin liquid films the surface rheological properties play a major role and depend on the surface active components present in the continuous phase. Foaming is a dynamical process and not all of the physical parameters in a foam are in complete equilibrium, hence the relation between foaming behaviour and the surface rheological properties of the foaming liquid can only be found if the dynamic surface properties are taken into account. The three main physical processes that contribute to foam stability are drainage, coalescence and disproportionation.

 

As a result of drainage the foam becomes dryer because the Plateau borders and liquid films become thinner. A distiction can bemade between two drainage processes. The first one is drainage that mainly takes place through the Plateau borders which is governed by gravity and capillary forces. The second is drainage of the thin liquid films in a foam in which additionally to gravity forces the Plateau border suction is the driving force for viscous flow through the interior of the films. The presence of particles (i.e. Pickering stabilisation), high film elasticity, a high surface dilational viscosity, a high bulk viscosity, electrostatic double layer repulsion (i.e. ionic surfactants) or entropic repulsion (i.e. non-ionic macromolecular surfactants and proteins) function as foam stabilising factors by acting in the thin liquid films between the bubbles in a foam. These chemical-physical properties slow down or even stop the film thinning process caused by drainage. This way the liquid films stay thicker and will be more stable against rupture compared to thinner ones.

 

Coalescence is a foam coarsening process that often can be observed in very liquid foams such as beer foam. Coalescence is the merge of two bubbles as a result of the rupture of the thin liquid film between the bubbles. The thin liquid films between bubbles are the most fragile structures in a foam. As a result of drainage and in absence of a disjoining pressure, such as electrostatic or entropic repulsion, a film can become very thin without resistance. When the films become thinner Van der Waals attractive forces between the two film surfaces become more and more pronounced and will lead to enhanced film thinning by squeezing the liquid out of the film towards the Plateau borders. When the film reaches a critical thickness it may rupture spontaneously due to thermal or mechanical disturbances. Therefore, a high enough disjoining pressure may prevent film rupture due to this mechanism.

 

When an aqueous foam is made with a gas that is well soluble in the water phase a second foam coarsening process that occurs is disproportionation which is an iso-thermal distillation process that is called Ostwald ripening in the case of, for example crystals instead of bubbles. Disproportionation refers to the interbubble gas diffusion as a result of gas pressure differences between bubbles. The rate of disproportionation increases with decreasing film thickness and increasing surface tension and is therefore directly related to drainage. On the other hand disproportionation enhances foam drainage as a result of the disappearance of bubbles, i.e. liquid films and Plateau borders, leading consequently to an excess of liquid that will drain. Furthermore, a coarser foam will drain faster than a finer foam. The reason for this phenomenon can be found in the surface properties. When the surfaces of the bubbles in a foam are motionless during drainage, which is the case with small bubbles, the drainage will be decelerated to the maximum. When the bubbles become larger there is a treshold value beyond which the surface tension gradient is not large enough to keep the bubble surface motionless resulting in faster drainage.

 

Drainage, coalescence and disproportionation are interrelated and occur simultaneously in a foam. They are determined by the surface and bulk properties of the foaming system. Amongst other parameters, temperature, pH, the concentration and type of surface active material all affect the properties of the surface of the films in a foam and with that foam stability.

 

With the understanding of these three main physical processes Foams.nl can not only offer the knowledge to control foam stability, foam texture and foam appearance but also can provide advice on avoiding foam formation or the destuction of undesired foams that occur during production processes.

 

Foams.nl offers not only solutions to foam related issues in the food and non-food industry but also to issues related to dispersions and emulsions, processes such as drying and freezing, recipe optimization and structuring.