Reverse Engineering Shaving Cream: Physics of a Stable Foam

Although we have a fundamental understanding of foam collapse mechanisms, we are still lacking a clear insight into which foam and solution properties correlate to foam stability. For example, we know that the addition of surfactants is needed for foam generation but are not able to relate the foam stability of different surfactants to any one surfactant property. Furthermore, we know that the bulk solution rheology can be engineered to increase foam stability, but we cannot say with any certainty which rheological property maximizes foam stability. In this work, we propose a correlation between foam stability and bulk rheological properties. Here, we present an extensive rheological study of more than 15 formulations using different combinations of surfactants and viscosifiers that are typically used in commercial shaving cream formulations. All foams studied here are wet foams (φ > 10%) and were generated by using nitrous oxide as a propellant. Our findings suggest that foam stability correlates to the magnitude of elastic modulus () and the linear viscoelastic phase angle (tan δ = G”/G’) measured at low frequency ( s^-1). More specifically, the correlation between foam half-life and δ shows a better correlation than other physical parameters such as surface tension and viscosity. We also find that the half-life jumps by an order of magnitude when the modulus is larger than 10 Pa. These results present the first correlations between bulk material parameters and foam stability over a wide range of foam formulations.