Nozzle design can impact on droplet velocity. Generally, increasing orifice size or operating pressure will result in increased droplet velocities as the droplets leave the nozzle.

Consider the impact of nozzle design on droplet velocity (close to the point where the sheet breaks up to form droplets): for nozzles of equivalent size and operating pressure, solid droplets such as those produced by extended range flat fans normally produce the highest droplet velocities, followed by pre-orifice nozzles and then air-induction nozzles.

Small droplets tend to slow down rapidly after leaving the nozzle. Most will reduce their velocity by more than 90 percent before traveling less than half a metre. Provided slow moving droplets actually reach the target, they will typically be well retained on most capturing surfaces (leaves and stubble). Lighter air-filled droplets will generally have a slower terminal velocity than a similar size solid droplet. Their slower speed further aids retention of the droplet on the catching surface. The downside of droplets with slower velocity is that small droplets may remain suspended in the atmosphere and hence be more prone to drift.

Higher droplet velocities can be useful when using fine-to-medium spray qualities. The velocity can increase travel distances from the nozzle for smaller droplets (before losing momentum) and improve droplet impact and retention (provided the nozzle height is not too far from the target).

Larger droplets tend to have higher velocities when they arrive at the target, and may only lose 50 per cent of their velocity before reaching the target. This means they could still be travelling at 20 to 30 kilometres per hour when they hit the target.

High droplet velocities can be hard to retain in some situations (target and product combinations) if using coarse to very coarse spray qualities or larger.

Large droplets travelling at high speeds tend to have a lot of energy when they hit the target and may bounce or shatter on impact. The extent to which a droplet will bounce or shatter depends on the formulation type and the characteristics of the leaf or target surface e.g. a large droplet may bounce or shatter off a 'difficult to wet' leaf surface, however the same droplet traveling at the same speed may be retained on an 'easy to wet' leaf surface. A droplet that bounces or shatters becomes unpredictable and may, or may not, subsequently be retained on the treated leaf surface.

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For more information on how droplet properties affect leaf retention and uptake, see Module 2 Product requirements.

Generally the risk of droplet bounce increases for droplets over about 250 micron, while the incidence of shattering on impact increases significantly for droplets above 300-400 micron in diameter. The magnitude of bounce or shatter is dependent on leaf surface i.e. there is less bounce/shatter on plants that are 'easy to wet'. Including a low surface tension / high spreading surfactant, either in the formulation or as a tank mix, will typically reduce the extent of droplet bounce and shatter. However, reducing the surface tension also reduces average droplet size and therefore increases the drift risk of the smaller droplets being produced by the same nozzle.

Some formulation types will tend to shatter more on hairy leaf surfaces, while others will tend to bounce more on smooth leaf surfaces, particularly once the droplet size becomes too large, too fast, or both.

Droplet bounce or shatter may not produce a poorer outcome if the crop canopy can recapture the satellite droplets, which have less energy on impact. This is generally easier to achieve on a large, prostrate broadleaf surface than a small, upright grass leaf. Droplet bounce and shatter are more likely to cause problems with efficacy for herbicide applications in fallow situations or where there is limited foliage to recapture droplets not initially retained by the target. Conversely, having large, fast moving droplets that may bounce off stubble and weed surfaces and not be recaptured may result in increased deposition onto the ground, which may be beneficial for pre-emergent herbicides.