A foam is a state in which a large number of air bubbles are packed in a small amount of liquid. As surfactants are often used to form this state, foams are often evaluated according to the type of surfactant. However, foams are one of the states and have many physical properties that are not dependent on surfactants. This article starts with the basics of bubbles, such as surface tension, liquid film stability, and the mechanism of defoaming agents, and then we explain the fundamentals of the physical properties of foams, such as osmotic pressure, and elasticity. We also introduce the scraping of foams onto a substrate as a recent research.

Small-angle neutron scattering (SANS) is an effective technique for studying the ever-changing structure of foam. Recently, the microstructures of the foams formed by the amino acid-type novel surfactant and homogeneous EO-type surfactant with multi-branched double chains were investigated by using SANS. The information such as the size of the foam film, the presence of micelles in the foam film, and the average radius of the bubbles was clarified.

In this review, (1) the effect of the surface freezing transition of cationic surfactant – alkane mixed adsorbed film on the disjoining pressure of the foam film, and (2) the relation between the foam film stability and composition of ionic-nonionic surfactant mixed adsorbed films will be discussed. In the topic (1), it is shown that the compression of the electrical double layer of the surfactant adsorbed film occurs at the surface freezing transition, leading to an abrupt foam film thickness transition. For the topic (2), the foam stability is compared for several ionic-nonionic surfactant systems varying their composition at the surface. In general, the foam film ruptures in a short time when the adsorbed film contains 70-80% of nonionic surfactant due to the weakening of the double layer repulsion in the foam film. However, if there is a strong attraction between ionic and nonionic surfactants, the adsorbed film composition becomes almost constant independent on the bulk mixing ratio of surfactants. Using this feature, a useful strategy to design a stable foam film system will be introduced. The review ends up with the future experimental plan, including the thermodynamic analysis of the film composition above the critical micelle concentration and its application to the foam film stability under continuous dilution.

We introduce how the bubble texture forms in a glass of Guinness beer. Bubbles rise in quiescent liquid due to the buoyancy; this phenomenon can be explained by Archmedes’ principle. However, after pouring Guinness beer into a pint glass, we notice that the bubbles go down. At the same moment, we can also observe a beautiful texture motion of a number-density distribution. Such a mysterious fluid motion can be observed when several hydrodynamic conditions are fulfilled simultaneously. This review introduces the motion of a single bubble, the instability of the gravitational flow associated with the gravitational separation, and the indicator of the clarity of the bubble separation. We address why bubble texture forms only in Guinness beer; paradoxically, we present the condition under which bubbles form texture in carbonated water.