Whether the spray droplets actually hit the target is determined by the size, energy and trajectory of the droplets, and the size and shape of the target.

Droplet movement near the target

Small droplets (very fine or fine spray qualities) have low mass and energy, so gravity has little effect on them.

The movement of small droplets results from being carried by prevailing air flows, which means they have the potential to move with the flow of air around leaves and may not hit the target, particularly if the leaf is large and broad.

Larger droplets (medium to coarse spray quality), usually have enough momentum (mass and velocity) to overcome air resistance and hit the target. Very large droplets (Very Coarse or larger) have high mass and as a result, high velocity. While they are very good for drift management, they can shatter or bounce off the weed target and leaf retention may be compromised.

Some targets, such as cereals, stubble and larger grasses, are relatively easy to hit, while other small, thin targets, such as two-leaf grass seedlings, are difficult to hit.

Large, broad leaves can be hard to hit with small droplets due to air flow around the leaf, while they are easier to hit with larger droplets.

Find out more

The impact of nozzle choice on droplet retention is further discussed in Module 3: Nozzle design and function.

Retention of droplets – impact of leaf surface

The retention of droplets on a leaf surface is determined by the physical and chemical properties of the droplet (for example, momentum and surface tension), as well as the characteristics of the leaf surface easy or hard-to-wet.

Whether a leaf is hard or easy to wet is determined by a combination of leaf surface characteristics, such as the presence of a cuticle with water-repellent waxes and/or trichomes (microscopic hairs), as well as the size and angle of the leaf. Cuticle type varies greatly between species.

Hard-to-wet surfaces are more likely to cause large droplets to bounce off.

Find out more

For more information on the process of droplet formation and factors affecting deposition and retention go to Module 3: Nozzle design and function.