Foams as viewed by small-angle neutron scattering

We report on small-angle neutron scattering measurements obtained in situ on three-dimensional aqueous foams stabilized by sodium dodecyl sulfate. Isotropic as well as anisotropic scattering data have been collected for two kinds of foams: wet foams in the steady-state regime of constant gas bubbling and dry foams under free draining conditions. Reliable scattering spectra were obtained within a few minutes, even for very dry foams. All spectra have a basic I(q) similar to q(-4) behavior on which a foam specific structure is superposed. The q-4 decrease at low q can be interpreted in terms of a Porod law from which the average bubble size is determined for both wet and dry foams. For wet foams, the intensity is modulated at high q by the structural organization of the surfactants in the liquid fraction of the foam (micelles). Remarkably, for dry foams only, the structure also appears at intermediate q, corresponding to the film thickness. This structure can be enhanced by using deuterated sodium dodecyl sulfate to suppress the micelle signal. Anisotropic measurements on dry foams reveal spikes in the 2D scattering data, suggesting an interpretation in terms of the reflection of the incident beam on the film surfaces. The isotropic scattering data may also be interpreted in terms of reflectivity instead of Porod scattering. Our experiments show the feasability of scattering measurements as a means for obtaining rapid information about average, global foam properties from the film thickness to the bubble size scale.