Foams are air dispersions into water. In the every day life, foams can also change the food and cosmetics textures or be an undesirable substance in the industrial production. To control their stability for each of their numerous applications, it is capital to know how to master the mechanisms at the origin of the foam short life. The main machanisms involved:
- drainage (water flow between the bubbles);
- coalescence (merging of two bubbles);
- maturing (gas transfer because of the gas pressure difference between two neighbouring bubbles).
In order to study the maturing and the coalescence free from the drainage, experiments were performed in microgravity during parabolic flights and in the International Space Station (ISS). Investigators compared the foams' stability and the possibility to build them on the ground and in microgravity.
The picture on the right shows the experimental device which was used to create foams: a piston forces the solution through a brutal section constriction to burst bubbles.
The device triggered an unexpected result which has been explained by the scientific team: the pure water foam existence in microgravity.
On the ground, to build foam we systematically add a surfactant (molecule like soap) which protects bubbles against maturing and coalescence.
On one hand, in the absence of surfactants (picture a), air inclusions merge instantaneously and there is no foam of water.
On the other hand, in microgravity, a bubbling liquid was observed after agitating pure water (picture b).This absence of drainage ensures a certain quantity of water between the bubbles, preventing them to be too close and coalesce.
This result is very promising because it shows that it is possible to observe foams at high liquid contents ("wet foam") during long times aboard the ISS.
It will be possible to study the maturing phenomenon (very slow, so it cannot be observed during parabolic flights) with wet foams and thanks to a much more elaborated device which is going to be installed in the FSL (Fluid Science Laboratory) aboard the ISS (International Space Station) by 2015. This maturing should prove to be very different from the one of dry foams observed on the ground.