Factors influencing agricultural spray deposit structures on hydrophobic surfaces

The properties of dried deposits formed by placing drops of a commercial turf grass fungicide spray formulation on smooth, waxy PARAFILM® M surfaces were measured as functions of formulation variables. The deposit structure reveals the extent to which dispersed active ingredient particles are bound to oil droplets in agricultural spray dispersions, before drying. The dilute spray dispersion (herein called Green Emulsion) was a suspoemulsion, which is a mixed dispersion of large (2–50 μm) mineral oil drops and small (∼430 nm) polychlorinated Cu (II) phthalocyanine (PG7) pigments, also known as Green 7. The distribution of PG7 pigment in the dried spray drop deposit depended upon the partitioning of PG particles in the suspension before drying. Individually dispersed PG7 particles in the aqueous phase mainly ended up in the annulus (coffee ring), whereas PG7 particles adsorbed on the oil emulsion droplets ended up as a central oily deposit (the dome). After most of the water evaporated, some oil migrated beyond the dome and slowly beyond the annulus. Increasing the oil-soluble surfactant concentration inhibited PG7 binding to the oil/water interface, giving a very small dome area and a wide annulus after drying. The deposit structure is sensitive to the mixing intensity when spray concentrates are diluted in a mix-tank. Increased intensity of mixing the suspoemulsion gives smaller oil droplets, more PG7 bound to the oil/water interface and a smaller dome. The ASTM E2044 “Standard Test Method for Spreading of Liquid Agricultural Spray Mixtures” involves placing a drop of crop spray dispersion on PARAFILM® and measuring the dimensions of the resulting sessile drop. This work demonstrates that allowing the sessile drops on PARAFILM® to dry and analyzing the resulting deposit structure gives a direct measure of the extent to which active ingredient particles, initially dispersed in water, are transferred on or into oil drops in suspoemulsions.