Antoine Gaillard (Univ Amsterdam) - What determines the drop size in sprays of polymer solutions?

The fragmentation of a liquid sheet or jet into droplets is a complex process known as spraying which involves different hydrodynamic instabilities. Understanding how the droplet size distribution depends on external control parameters is particularly important in agricultural crop spraying since droplets smaller than 100 micrometers do not reach the target plant and are blown away by the wind, causing environmental pollution. It has been shown that polymer additives could be used to reduce the amount of these small droplets while also increasing the efficiency of droplet deposition on hydrophobic plants by reducing the amount of bouncing and splashing during droplet impact.

In this fundamental experimental study, we investigate how viscoelasticity affects spray properties. We observe that polymer addition to water results in the formation of overall bigger droplets with a broader size distribution. We find that the median droplet size increases linearly with the extensional relaxation time of the liquid. We show that this increase can be partly attributed to an increase of the wavelength of the flapping motion responsible for the sheet breakup, resulting in the formation of thicker ligaments from which droplets are formed. We show that droplet size distributions are well described by a compound gamma distribution where the parameters encoding for the ligament corrugation and the width of the ligament size distribution saturate at sufficiently high polymer concentrations.