Droplet density affects droplet mass, which then influences the rate of droplet sedimentation. When mass is combined with droplet velocity it can also affect the energy of the droplet at impact and the ability of the droplet to be retained on the target.

Increased specific gravity (weight per unit volume of the spray solution) increases droplet density, which can then increase the rate of sedimentation and the energy at impact.

This can result in reduced drift and can also lead to reduced retention of larger droplets, depending on the properties of the formulation and the characteristics of the leaf surface.

Droplet density can also be reduced with some formulation types (for example, crop oils) or may be reduced when air is included in the droplet. Reduced droplet density leads to a reduced rate of sedimentation and can lessen the energy at impact. This potentially improves retention – but an increased risk of spray drift is the trade-off. 

For these reasons, air-filled (air inducted) nozzles are often chosen for foliar herbicides, where the application objective is retention onto the leaf surface. Generally an air-filled droplet will be lighter and traveling slower than the same size solid droplet. Additionally, there will be more air-filled droplets produced at the same spray volume.

For pre-emergent herbicides, where the soil is the target, solid droplets are typically preferred.

Notes

Increased specific gravity can also impact the measurement of flow rate, which subsequently impacts operating pressure in some spray systems.  Module 8 discusses when adjustments should be made to the rate controller or calibration calculations for tank mixes with increased specific gravity (such as foliar fertilisers).