Foaming mechanisms in surfactant free particle suspensions

Physical mechanisms for foam formation in the presence of colloidal particles are poorly understood. This study is aimed at enhancing the basic understanding of the mechanisms that produce foaming in a three-phase system, containing solid, liquid, and gas, and identifying key parameters that aggravate foaming in the absence of surface-active agents. The foaming ability of a system ("foaminess) was investigated by aerating a suspension of nanosized hydrophilic silica particles. We observed that the foaminess was directly proportional to particle concentration and inversely proportional to particle size. However, even a small degree of bidispersity in particle size drastically reduced foaminess. To explain these observations, the drainage of a single foam lamella containing nanosized particles was studied using a novel capillary force balance apparatus, which incorporates the microinterferometric technique. The single foam lamella thins in a stepwise manner, as the colloidal particles self-organize into a layered structure between the gas bubbles. This layer provides a barrier against coalescence of bubbles, thereby stabilizing the foam lamella. The number of stepwise film thickness transitions increased with particle concentration and decreased with the increase in the particle size, consistent with the results from foaming experiments. Particle bimodal distribution greatly decreases the foam-lamella stability, consistent with the theoretical prediction.