The Action of Electrostatic Mist-Blowing

I.C.E. INAT External Centre
Foreign Trade Institute ” Institut National Chambers of Commerce
of Italy Agronomique de Tunisie” of the Regione Emilia-Romagna


I / Introduction

In the majority of pesticide applications, product distribution and its uniformity of coverage are bound to the efficiency of the treatment operation. Thus different methods have been used to evaluate the degree of dusting and the amount of product deposited.
The equipment used for tree culture includes: the “sprinkler”, the “spraying bar”, the “atomizer” and the “mist blower”. The choice of one of these methods depends on certain parameters: the cost of the investment in materials, the structure and size of the farm and the efficacy of the material itself.

At the present time, the most efficient technique is pneumatic mist blowing whereby a flow of liquid at low speed and low pressure is brought into contact with a high-speed air flow (>80 m/s). Contact of the air with the liquid causes the mixture to micronize into droplets. The droplets will become more homogeneous and their diameter smaller as the airspeed becomes faster and the amount of liquid that passes becomes less. Fine and homogeneous spraying allows the area covered to be sensibly increased while reducing the volume of the mixture used (from 100 to 200 l/ha) in relation to pressurized atomizing of the liquid which uses consistent volumes (from 500 to 1500 l/ha).
Pneumatic mist blowing can be equipped with a device for electrostatically charging the droplets. The droplets are attracted towards the vegetation by the electrostatic fields that form between the plants, which are optimum conductors (sap, mineral salts, humidity, etc.) and chemical vapor, carriers of an electrostatic charge.
Our tests were conducted to compare the action of electrostatic mist blowing with that of a classic pneumatic mist blowing process, beginning with the degree to which the droplets of the mixture over the leaves and using the image analysis method.

II / Materials and methods

Two pneumatic mist blowers, a classic mist blower, and an electrostatic mist blower were tested in an apple tree orchard.
The test conditions were: wind speed 2 m/s, 40% relative humidity and an ambient temperature of 24%.
The tests were conducted using pieces of water-sensitive paper (2.54 x 2.54 which turned blue when struck by the water droplets, thus giving rise to the relative coverage rate. The pieces of paper were affixed to the two Tunisia – field test project – final report sides of the leaves and were then pointed towards the north, south, east, west, and center of the trees.
After the implements had passed and the pieces of water-sensitive paper had been wetted, they were collected and digitized by means of an A4 color scanner whose resolution setting was 600 dpi.
The images were digitized in the red channel at 256 levels of grey. Each pixel was a square measuring 42 µm each side. The smallest droplet mark identified measured 50 µm which, using a distribution factor of 2, corresponded to droplets 25 µm in diameter.

III / Results

A four square centimeter area of digitalized image was treated. It was filtered to increase the contrast between the droplets and the background. The selection was limited to a level of grey so as to obtain an image where the pixels were black or white, depending on whether their level of grey was above or below the established threshold. The droplets of the original image were thus isolated on a white background. Using this image, the “Image Tool” logic was able to identify objects or droplet marks.

The coverage rate results (Fig. 3) show that electrostatic mist blowing obtained higher coverage rates than the classic pneumatic mist blowing in all cases. An analysis of the variant of the two treatments showed that electrostatic mist blowing was more homogeneous than classic pneumatic mist blowing.