Stability of thin liquid films containing polydisperse particles

We present experimental results and theoretical calculations for the effect of polydispersity in particle size on the step-wise thinning (i.e. stratification) of thin liquid films and thereby on the film stability. This study is aimed at understanding the basic mechanisms of foam lamella stability in three-phase foam systems containing liquid, gas and colloidal particles without any surfactants or polymers. The film thinning phenomenon was experimentally observed using a reflected light microinterferometric technique. Polydispersity in particle size leads to a weakening of particle layering (i.e. structural barrier) and thereby to a reduction in foaminess. Our experiments using a monodispersed (8% v/v) 8 nm sized hydrophilic silica particles showed that with addition of just 2% v/v of 100 nm particles, the foaminess was reduced drastically. Light scattering experiments were conducted to determine the effects of polydispersity on inter-particle interactions. The second virial coefficient was found to be lower for polydisperse systems thereby corroborating our results on decrease in film stability.