Experimental and numerical study for drop size distribution in oil-water dispersions with nonionic surfactant Tween 80

The effect of nonionic surfactant (Tween 80) concentration on the evolution of drop size distributions (DSDs) of oil/water dispersion in a stirred tank was investigated experimentally and numerically. It was found that as the surfactant concentration increases, the mean droplet size decreases, the number density of small droplets increases and the dispersion stability is enhanced. It could be attributed to the augmentation of the breakage frequency caused by a dramatic reduction of interfacial tension and the suppression of the coalescence efficiency by an immobilized interface. When applying population balance equation (PBE) to surfactant dispersions, the effect of surfactant concentration was characterized by its influence on both breakage and coalescence process. The combined effect of surfactant molecules, drop size, and the number density of droplets on the turbulence intensity was considered in the breakage model. Models of coalescence time that consider partially mobile and immobile interfaces were employed to describe the film drainage at surfactant concentrations below and above CMC (critical micelle concentration) respectively. The effect of surfactant concentration on the critical film thickness was probed by adjusting the Hamaker constant and measuring the interfacial tension, while its influence on the coalescence efficiency was investigated by further balancing the contact and coalescence time. Both the predicted Sauter mean diameter and cumulative DSD agree with the experimentally measured values at different surfactant concentrations, while the discrepancies that exist could be due to the underestimation of coalescence rate between small droplets.