Foam is any substance that contains trapped air bubbles whether it be a liquid or a solid. Foams are used in many geological projects that involve the subsurface such as purifying polluted ground water and maximising the quantity of oil that is extracted from oil fields. This is because foam can displace these fluids from the high permeability ground they are stored in forcing them into the low permeability ground which causes the liquid that can not be stored to remain in its liquid state.
Modelling the mechanics of foams can be somewhat of a challenge and a key concept in the mathematics behind it is that of a limiting pressure between the air bubbles which will cause the foam to become unstable if exceeded. Through a series of experimental observations one of the assumptions made is that when the foam is within a porous media it hampers the movement of gasses but has no tangible effect on the mobility of any liquids contained in the pores. Although this assumption is commonly made there has been some speculation about the possibility the foam does have an effect on the liquids it encounters. In order to decide once and for all whether or not this effect occurs an experiment was required. Scientists took a specially carved piece of sandstone and injected it with quantities of foam. By using medical computer tomography a three dimensional map of water saturation into the foam filled rock can be seen over time. This was compared with the saturation when gasses were injected into the sandstone to see how the foam affected the two states of matter differently.
The final result was that although there was some effect on the liquid phase it was so small as to be easily ignorable and no error would result. The study suggests that previous results showing showing otherwise were due to a lack of capillary connections between bubbles as a result of a poor concentration of surfactant (a substance which reduces surface tension) in these experiments. This information allows the current models for foam action to remain without needing to be altered.