On the long-term stability of foams stabilised by mixtures of nano-particles and oppositely charged short chain surfactants

We have studied the foaming properties of aqueous dispersions containing mixtures of silica nano-particles (Ludox TMA) and a short-chain amphiphile (n-amylamine). By combining standard hand shaking methods and microfluidic techniques we show that stable foams can be obtained at amine concentrations above approximately 0.5 wt%, which appears to be a critical concentration for cooperative association between particles and amine. In contrast to foams stabilised solely by nano-particles, these foams suffer from slow coarsening due to gas exchange between bubbles. "Superstable" foams for which coarsening is inhibited can only be produced at sufficiently high particle and amine concentrations (typically 10 and 3 wt%, respectively) for which the dispersions also gel in the continuous phase of the foam. We combine investigations of the static and dynamic properties of the particle-laden air-water interfaces in an attempt to elucidate some of the key mechanisms which control the observed behaviour.