Physico‐chemical studies on agricultural sprays VIII.—Viscosity and spray drop size of water‐in‐oil emulsions

The effect of the viscosity of the spray fluid on the size of drops produced from fan‐jet nozzles is complex; an increase in viscosity can lead to either an increase or a decrease in the drop size. With water‐in‐oil emulsions the value of the viscosity governing spray behaviour is the limiting value measured under conditions of very high shear. The factors which influence the limiting value of the viscosity include the disperse phase ratio, the method of mixing and the degree to which the two phases are mixed, the nature and concentration of the emulsifying agents used and the type of toxicant incorporated in the emulsion. Within certain limits, it is shown that the disperse phase ratio provides a convenient means of varying the viscosity of the emulsions and that the viscosity may also be varied by variations in the emulsifying agents and toxicants. Although the method of mixing has a considerable effect on the viscosity of the emulsion, the further mixing that takes place in the spray nozzle governs the effective viscosity of the spray fluid that is emitted from the nozzle. The practical application of the theoretical expressions which describe the formation of drops from fanjet nozzles has been discussed and the relationship between disperse phase ratio, viscosity, nozzle size and operating pressures has been illustrated for several typical water‐in‐oil emulsions.