Non-aqueous foams: Current understanding on the formation and stability mechanisms

The most common types of liquid foams are aqueous ones, and correspond to gas bubbles dispersed in an aqueous liquid phase. Non-aqueous foams are also composed of gas bubbles, but dispersed in a non-aqueous solvent. In the literature, articles on such non-aqueous foams are scarce; however, the study of these foams has recently emerged, especially because of their potential use as low calories food products and of their increasing importance in various other industries (such as, for instance, the petroleum industry). Non-aqueous foams can be based on three different foam stabilizers categories: specialty surfactants, solid particles and crystalline particles. In this review, we only focus on recent advances explaining how solid and crystalline particles can lead to the formation of non-aqueous foams, and stabilize them. In fact, as discussed here, the foaming is both driven by the physical properties of the liquid phase and by the interactions between the foam stabilizer and this liquid phase. Therefore, for a given stabilizer, different foaming and stability behavior can be found when the solvent is varied. This is different from aqueous systems for which the foaming properties are only set by the foam stabilizer. We also highlight how these non-aqueous foams systems can easily become responsive to temperature changes or by the application of light.