Electrostatic Charging Effect On The Droplet Characteristics And Volumetric Distribution Of Surfactant-Agrochemical Composite Spray

The application of electrostatic principle in pesticide spraying is an innovative technology to enhance spray droplets characteristics,in-situ deposition and wettability from surfactant-pesticide formulation on leaf surfaces in plant protection.It is capable of improving wraparound deposition of spray droplets on adaxial（topside）and abaxial（underside）leaf orientation irrespective of the anisotropic roughness level.The degree of impaction behaviour on leaf surfaces is dependent on system parameters,chargeability and concentration of surfactant-pesticide formulation.At constant liquid flow rate of 250 mL/min,travel speed of 2 m/s and pump air flow pressure of 60psi,the variable applied voltages,liquid flow pressures,spraying heights,nozzle spacing and concentration of surfactant-pesticide formulation were investigated.The formulation properties and multiplier effect of charged spray droplets were monitored.Hydrophobic leaves of Brassica campestris（Shanghaicai）plant were used as droplet deposition target in the experiment.The specific findings are outlined.（1）Optimum parameters combination for electrostatic spraying of agrochemicals:The system parameters influenced spray droplets characteristics.Investigation of optimum parameters combination for electrostatic spraying of pesticides was undertaken from 2-12 kV applied voltages,2,3,4 bar liquid flow pressures and 20-60 cm spraying height regimes using a 110^°flat fan nozzle type.In all,the combined parameters of 12 kV applied voltage,4 bar liquid flow pressure and 60cm spraying height produced maximum charged spray swath and fine particulate droplet sizes.Also,an applied voltage of 12 kV,liquid flow pressure of 4 bar at spraying height of 20 cm yielded maximum chargeability.However,Tukey pairwise comparison test showed no significant differences in charge-mass ratio,spray swath and droplet sizes for spraying at 8-12 kV applied voltages,50-60 cm spraying height regimes and 3-4 bar liquid flow pressures.The combined optimum parameters took reference from a uniform plant height surface during spraying.（2）Charged pesticide spray swath and volumetric droplets distribution at variable nozzle types,spacing and orientation:A combination of nozzle types,nozzle spacing and spraying heights influenced charged spray swath（W_n）and volumetric droplets（Q_n）distribution from a developed inductive nozzle cap.Both flat fan（FF）and hollow cone（HC）nozzle tips were independently slotted into the cap and mounted on a telescopic boom to discharge pesticide spray jets.The FF-nozzle produced relatively smaller charged droplet sizes D_V0.5（67.4μm-79.2μm）,wider spray W_n（R²=98.37%;max.185 cm）and wider Q_n distribution(C₃₇ grooves,15.86 mL/s),than HC-nozzle of D_V0.5（70.5-92.5μm）,spray W_n（R²=85.45%;max.115cm）but lower Q_n distribution(C₂₃ grooves,15.72 mL/s)on patternator test bench respectively.A spray Uniformity Index（UI）allowed permissible spraying range of FF-nozzles at 0.4≤UI≤0.57 and HC-nozzles at 0.3≤UI≤0.42 on the boom.The overlap swath UI=0.33 produced 100%overlap at FF-nozzle spacing of75 cm by height of 50 cm,and non-overlap swath gap from HC-nozzles occurred at UI≤0 in 100-120 cm spacing by 40-60 cm heights.Spraying from FF-nozzles at 0.12≤UI≤0.18 yielded uniformity of Q_n distribution,while that of HC-nozzles was effective at UI=0.25,corresponding to 25%-50%spray overlap.Therefore,to reduce pesticide spray repeatability from overlap nozzle spraying,FF-nozzle should be mounted at 100 cm spacing by 50 cm height or 120 cm spacing by60 cm height while HC-nozzle at 75 cm spacing by 60 cm height on the boom.Spraying above100%overlap（UI>0.33）increased volumetric droplets repeatability and application rate of two or more nozzles on the boom,while below 25%overlap（UI<0.08）decreased efficiency of pesticide spraying.（3）Charged spray droplets characteristics and deposition on leaf surfaces from surfactant-pesticide formulation:Superposition of electric charges to surfactant L-77 and pesticide glyphosate（EC）formulations enhanced spray droplet sizes and chargeability,inhibited droplets rebounding and improved deposition on hydrophobic abaxial-adaxial leaf surfaces.Droplet sizes of EC,L-77 and EC+L-77 decreased with an increased applying voltage.The Charge-Mass Ratio（CMR）of EC+L-77 composite solution superseded that of sole L-77 and EC solution.Based on ImageJ analysis of droplets density per leaf area,maximum exposure of adaxial leaf surfaces intercepted more charged spray droplets than abaxial surfaces.From linear regression model,the quantity of charged spray deposition from EC+L-77 solution was highest on both adaxial（approx.27.44）and abaxial（approx.5.57）hydrophobic leaf surfaces than EC and L-77 solutions.Also,contact angles of charged spray droplets were smaller on adaxial than abaxial surfaces due to direct flight and high impact force than those droplets coiling and pinning at underside leaf surfaces.Surfactant-pesticide composite spraying is therefore considered suitable for electrostatic application in plant protection technology.（4）Charged monosized droplet behaviour on leaf surface from surfactant-pesticide spray:Finally,different concentrations of surfactant and pesticide in aqueous solution affected monosized droplet chargeability and impaction behaviour on hydrophobic leaf surfaces.With an exception of water（control）,the conductivity and chargeability of surfactant-pesticide formulations increased with an increasing concentration,whereas surface tension（γ）decreased with an increasing concentration,which consequently minimized static contact angle（θ）on hydrophobic surfaces of Brassica campestris leaves.A reduction inγ-andθ-values enhanced deposition and wetted area of L-77,EC and L-77+EC formulations.The wetted area of L-77+EC concentration increased from 9.26 mm² atθ-value of 65.29°in 2.5 mL/L+7.5 mL/L to 13.29mm² atθ-value of 43.1°in 10 mL/L+15 mL/L due to the combined effect of surfactant concentration in the solution.Additionally,the degree ofγ-andθ-values reduction was proportional to charged droplet sizes than uncharged droplets.For each 1μL raise in droplet volume,γ-value of L-77,EC and L-77+EC decreased by approx.5.2,4.6 and 3.3 respectively,whereasθ-value reduced by approx.28.45%,28.24%and 27.23%respectively.The outcome of the experiment is useful for wraparound deposition of droplets from surfactant-pesticide composite spraying by induction charging principle on difficult-to-wet leaf surfaces in plant protection.In conclusion,the superposition of electric charges to surfactant-pesticide spray droplets at optimum parameters combination（applied voltage,spraying height,nozzle type and spacing,liquid flow pressure）enhanced droplets characteristics（chargeability,droplet sizes,volumetric distribution）and impaction behaviour（deposition,retention and wettability）on hydrophobic adaxial-abaxial leaf surfaces of Brassica campestris plant.