Colloidal force measurements in the presence of a polyelectrolyte

We report investigations of the repulsive forces between monodispersed ferrofluid droplets in the presence of a weak polyelectrolyte, poly(acrylic acid) (PAA), using a magnetic chaining technique. Two interesting observations were made during our investigations. At a PAA concentration of 0.01 wt%, a long-range repulsive force profile was observed due to the adsorption of polyelectrolyte onto the droplet, without any irreversible aggregates even at very small inter-droplet spacings. Above a concentration of 0.01 wt% of PAA, formation of irreversible chaining of droplets was observed at short inter-droplet separations due to polyelectrolyte binding or bridging. The onset of binding was also independently confirmed by microscopic observation.