Transitions in oscillatory structural forces between drops and bubbles

We have used the collisions between two micro-droplets or micro-bubbless measured with atomic force microscopy to observe transitions in solution structure of high concentration surfactant or polyelectrolyte solutions. Previously, we have observed significant differences in the behavior structural forces between deformable interfaces compared to the periodic oscillatory structural forces commonly observed between rigid interfaces[1]. In addition, quantitative comparison between these system types indicates that the deformable nature of droplets or bubbles allows them to act as far more sensitive probes than solid spheres. We use this enhanced sensitivity of deformable interfaces to probe changes in structural forces in two different systems. First, using micro-droplets in the AFM, we obverse the transition from sphere to rod for high concentration of cetyltrimethylammonium bromide micelles through the addition of sodium salicylate[2]. The structural force observed between drops in the presence spherical micelles wanes as rods are formed and a depletion force is observed from both osmotic and entropic origins. Second, we explore the force behavior between to micro-bubbles in response to changes in the molar mass dispersity of a polyelectrolyte, sodium poly(styrene sulfonate) (NaPSS). A depletion force is observed in the presence of poly-disperse NaPSS system where the evolution of a oscillatory structural force was seen as the polydispersity of the molar mass is reduced via dialysis[3]. Comparisons were also made between mixtures of mono-disperse molar mass samples to confirm the dispersity effect on the interaction force.