Evaluation of emulsion stability by monitoring the interaction between droplets

Emulsion stability is a key physical characteristic in the preparation and use of emulsions. In this study, the interaction forces between two droplets in different solutions were measured by a modified DCAT21 tensiometer equipped with a high-frequency digital camera which can capture the droplets coalesce process and correspondingly force curves at the same time. The results indicate that for pure surfactant solution, only when the concentration exceeds the critical micelle concentration, sufficient surfactant molecules are adsorbed on the oil-water interface to form a high-strength interface film, so as to prevent droplet coalescence and improve the emulsion stability. However, for pesticide solution, the active ingredient with a certain surface activity will reduce the interfacial tension of the solution. We also found that the active ingredient and surfactant molecules exhibit competitive adsorption in the same phase and mixed adsorption in different phases, which have different effects on the emulsion stability. This study compared the stability of the pure surfactant solution and pesticide solution through microscopic observed the change of force and the coalescence phenomenon in the dynamic process of droplet contact which can advance our understanding of the mechanism of the emulsion stability.