How surfactants influence the drop size in sprays from flat fan and hollow cone nozzles

Spraying is a widely used method to produce a liquid sheet that breaks up into droplets of a certain size distribution. When spraying simple liquids, it is known which experimental parameters determine the droplet size distribution. For many applications, however, surfactants are added, producing a hitherto unknown effect on the droplet size distribution. Using two generic types of spraying nozzles, we sprayed solutions of different types of aqueous surfactants and measured the droplet size distribution of the sprays. We find that the breakup of surfactant solutions is similar to that of pure water but results in droplets that are smaller. The resulting droplet size distribution can be well described using the predictions for simple liquids provided that we replace the parameter of the equilibrium surface tension with the dynamic surface tension of the surfactant solution at a surface age of 15 ms, which is the characteristic time for destabilization and breakup of the liquid sheet. By rescaling with the mean droplet size, the droplet size distributions of water and sprays with different concentrations of surfactants all collapse onto a single curve that is given by the compound Gamma function found previously for pure liquids.