In the 1600s, when the phenomenon of electrostatic force was discovered, the simple explanation assigned to it was the attraction forces between two objects having different values of electric potential. At that time, there was no notion how to apply it in practical applications, so nobody understood its usefulness. In fact, the first attempts at industrial applications did not happen for another three hundred years, and these were to do with the paint spraying of grounded objects in the 1950s.
The term electrospray, when the phenomenon of electrostatic force was discovered, the simple explanation assigned to it was the holds for doing so. The first is when the liquid becomes atomized by some mechanical or aerodynamic means, and then the droplets are charged, a process called Post Atomization Charging (PAC). It is widely used in industrial and commercial applications of which crop spraying is one example. The second method is when the charged droplets are produced by Electro Hydrodynamics Atomization (EHDA). This is caused by (EHD) related instabilities being produced at the surface of the liquid by an electric field. EHDA depends on liquid conductivity, which has resulted in the division of liquids into three categories: conductive, semi-conductive and insulating according to the charge relaxation time τ [6] Atomisation will occur based on conductivity ơ and permittivity ɛ of the liquid as illustrated in the table [7]:
Table 2.1: Classification of liquids, Huneiti [7].
σ ˂ 18-8 Sm-1 10-8 ˂ σ ˂ 10-4 Sm-1 σ > 10-4 Sm-1
Insulating Liquids Semiconducting Liquids Conducting Liquids
• Long relaxation time, so charge carriers take a long time to reach the surface when an electric field is applied.
• Cannot be sprayed by induction, due to lack of charge carriers.
Charge injuction necessary to atomize these liquids by EHDA.
Example: Corn oil.
σ = 5x10-11Sm-1 τ = 4.8 s ɛ = 2.7
• Moderate relaxation time.
• Relaxation time and droplet formation times are comparable.
• Ideal for EHDA.
• Stable jet obtained as a result of tangential stress and surface charge induced by the (radial) electrostatic field.
Example: Ethylene glycol
σ = 1x10-6 Sm-1 τ = 0.4 ms ɛ = 41
• Short relaxation time.
• Charge relaxation time must be shorter than the droplet formation time.
• Surface becomes an equipotential when subjected to an electric field.
• Harder to attain EHDA.
• Ideal for charging by induction.
• Example: Tap water
σ = 5x10-2 Sm-1 τ =21ns
ɛ =80
Electrospray is defined as the process of simultaneous droplet generation and charging, which happens when the electric force is stronger than the surface tension force. It consists of charged droplet generation, conveyance of particles to the target object and target deposition [8]. Liquid coming out from the nozzle at high pressure is subjected to an electric field, with this pressure causing elongation of the meniscus so as to form a jet spindle. This process allows for the generation of fine droplets of charge of magnitude close to one-half of the Rayleigh limit, which is the magnitude of charge on droplets that overcomes the surface tension force, thereby resulting in their fission [9]. The interactions that happen between charges on the surface of the liquid and the applied electric field give out two results: the first being the acceleration of the liquid and subsequent disruption into droplets, whilst the second is the build-up of the charges on the droplets [6].
Agricultural applications began after the success in the industrial field, with the first pertaining to the development of pesticide materials in powder form. Subsequently, electrostatic techniques were applied to liquid agricultural pesticides and herbicides, because
there are a number of benefits in using the electrostatic principle in spraying technology, which include: good distribution of droplets/particles, more uniform deposits on the surfaces of the intended targets as well as low or even no drift. Furthermore, pollination and biomaterial in agriculture have delivered satisfactory results through electrostatic application. However, the direct application of electrostatic paint spraying equipment in orchards is not feasible for three reasons, as explained [10].
1- The object in electrostatic painting to be coated is located relatively close to the paint gun (usually < 50 cm), which results in a significant increase in deposition efficiency on the object surface due to the high voltage applied to the gun. However, in orchard spraying the object to be coated is often much further away from the sprayer and hence, the technique is not effective.
2- In electrostatic painting, the process takes place in controlled environment with the conditions characterised as having minimum outside influences. But in the case of agricultural spraying, this takes place under a range of environmental conditions, such as temperature, relative humidity, wind velocity, and vibration, which can hinder spraying.
3- There are different skills requirements between operators working in the two different fields. Regarding electrostatic painting, the equipment is usually maintained by a full-time technician and the spraying is carried out by a skilled operator. While in the context of agriculture, pesticide spraying is only one of a number of operations that a farmer must carry out and thus, the equipment must be simple to operate, extremely robust, and reliable.
In general, electrostatic applications have been applied in several domains regardless of the percentage of the efficiency of this technique.