Droplet Technology

With a conventional flat fan nozzle the operator can look up charts to see whether the droplets created are fine, medium or coarse. However, that is not the whole story.

Let us glance at droplet creation : the spray comes out of the flat fan nozzle - the orifice (hole) is eye shaped and because of this the spray comes out in the shape of a fan. The first part is solid water, this then within 50-100mm breaks up into lines - technical term ligaments - which then break up into droplets

This means the droplets created are generally a spectrum, with most of the droplets being grouped around one particular size.

The nozzle charts usually try to express a balance of the two ways of expressing the size of the droplets -

NMD Number Median Droplet - giving a median value at which the number of droplets larger than the median is the same as the number of droplets smaller than the median

VMD - Volume Median Droplet giving a median value where the volume of the droplets larger than the median is the same as the volume of droplets smaller than the median.

As you can appreciate from this, any nozzle will give a range of droplets and the recommendation of the manufacturer is just an approximation of spray quality - all 'medium nozzles' will produce some fine droplets, all 'medium nozzles' will produce some coarse droplets.

Now, what droplets are effective and which should we be using? When spraying, the conventional wisdom is · the coarse droplets are so large that they tend to bounce off the leaves and land on the ground - and so are only encouraged for soil acting herbicides. · the medium droplets stick on the upper surface of the leaf but gives virtually no cover on bottom of leaf - and so are usually recommended for herbicides and systemic fungicides and insecticides. · the fine droplet sticks to the top of the leaf, gives some cover on bottom of leaf and other parts, due to drift within the crop, and so are usually recommended for contact fungicides and insecticides. It is prone to drift before it enters the crop -

There is always much discussion if the coverage on the leaf is better with medium or fine droplets. This is where the technology of adjuvants - wetters and stickers, chemicals which improve the application and thus the effectiveness of a pesticide - has a critical function.

However, I would like to put forward a new idea. Our 'crop', and turfgrass is a crop, is not the common crop considered in experimental trials. Our crop is a grass. We cannot even compare it to cereals as in that crop many of the leaves are at the horizontal and there is always exposed soils between the plants

Our crop is one where the leaves are more to the vertical, and so densely packed that no soil can be seen between the plants. I suggest that there is no immediate advantage in producing a fine droplet - a medium droplet would stick just as easily, give the same coverage on both sides of leaf and have the advantage of less drift.

I have looked up many of the labels used in greenkeeping and sportsgrounds and see none that recommend fine sprays as being essential.

So my first proposition is NEVER USE A NOZZLE WHICH CREATES FINE DROPLETS The advantages are obvious - drift is cut back, and droplets stay in the air for a much shorter time - consequently operator contamination is reduced considerably.


Droplet Diameter called a Time
1 micron droplet 'smoke' 28.1 hours
10 micron droplet 'fog' 16.9 minutes
60 micron droplet 'very fine' 30 seconds
120 micron droplet 'fine' 8 seconds
220 micron droplet 'medium' 4 seconds
400 micron droplet 'coarse' 1.8 seconds

My next proposition takes it a shade further - why not use Coarse droplets ? If the droplet hits the leaf it is likely to bounce onto other leaves, shatter into smaller droplets or stick & run down the leaf, each of which would still be effective.

I have looked up many of the labels used in greenkeeping & sportsgrounds and find that they are now recommending 400 plus litres per hectare of water volume. This should be ample coverage on our 'crop' even with coarse droplets.

The advantages are significant - drift is almost non-existent at normal wind speeds, and droplets stay in the air for a much shorter time - consequently operator contamination is reduced to insignificant.

I feel that if the greenkeeping industry moved to coarser droplets with a tighter spectrum of sizes, a huge step for safety could be provided. Any cut back in efficacy, would be well balanced by the improvement on safety.

However, technology is also helping us:

LO-DRIFT NOZZLES - if the water is made to go through a round orifice at the top of the nozzle before it reaches the tip the spectrum of droplets is much reduced.

AIR BUBBLE JETS : In the past few months I have been looking very closely at the technology from Billericay Farm Services called the AIR BUBBLE JET.

Basically the idea is that the water being sprayed passes through a tapered tube at the top of the nozzle which accelerates the liquid and forms a venturi. As it emerges a vacuum is formed which causes air to be taken in from two side slots. The air & water is compressed and mixed as it passes though the chamber just before the nozzle orifice and the spray produced is made up of coarse droplets with bubbles inside - think of a Malteser!!!

The concept of this is that these 'droplets' have very low drift principal and when they reach the leaf they shatter creating stickable droplets. In tests this technology has reduced the drift by 58%. These can be used in tractor hydraulic sprayers well as knapsack sprayers. They cost about twice the normal nozzle but could be well worth the extra. Many contractors are switching to these as they can increase the 'weather window' for spraying.

I believe that as a greenkeeper or groundsman it is your duty to consider these new developments as they may make your workplace safer and you a safer person.