What determines the drop size in sprays of polymer solutions?

The effect of viscoelasticity on sprays produced from agricultural flat fan nozzles is investigated experimentally using dilute aqueous solutions of polyethylene oxide (PEO). Measurements of the droplet size distribution reveal that polymer addition to water results in the formation of overall bigger droplets with a broader size distribution. The median droplet size is found to increase linearly with the extensional relaxation time of the liquid and to be well captured by a simple empirical expression in terms of non-dimensional Weber and Deborah numbers. We show that the increase in droplet size with increasing viscoelasticity can be partly attributed to an increase of the wavelength of the flapping motion responsible for the sheet breakup, resulting in the formation of larger ligaments from which droplets are formed. We also show that droplet size distributions, rescaled by the average drop size, are well described by a compound gamma distribution with parameters n and m encoding for the ligament corrugation and the width of the ligament size distribution, respectively. These parameters are found to saturate to values n = 4 and m = 4 at sufficiently high polymer concentrations.