The influence of the extensional viscosity of very low concentrations of high molecular mass water-soluble polymers on atomisation and droplet impact.

ABSTRACT

BACKGROUND: Water-soluble polymers are increasingly added to herbicide and pesticide formulations at very low concentrations (100-1000 mg L(-1)) in order to control the spray characteristics, notably to reduce spray drift and influence droplet bounce. The incorporation of polymeric adjuvants improves the efficacy of the spray solutions, thus enabling crop growers to maximise the performance of agrochemical sprays at lower dose rates of active ingredient. It is important to establish a fundamental understanding of how polymers influence the processes involved in droplet deposition. RESULTS: The shear and extensional viscosities of a series of high molecular mass (M(w)) poly(acrylamides) (M(w) approximately 10(6)-10(7)) have been determined at very low concentrations (100-1000 mg L(-1)). The polymer solutions demonstrated typical shear thinning characteristics under shear, and strain hardening behaviour under extension above a critical strain rate. The presence of the polymers was shown to increase the size of droplets produced in atomisation using an agricultural spray nozzle, as measured by laser diffraction. This was attributed to the increase in the extensional viscosity at the strain rates generated under pressure in the spray nozzle and was a function of both polymer concentration and M(w). In addition, the presence of polymer was found to have a significant influence on droplet bounce. CONCLUSIONS: The presence of very low concentrations of high molecular mass poly(acrylamides) significantly influences the size of droplets formed on atomisation and subsequent bounce characteristics. Large extensional viscosities generated above a critical strain rate are responsible for both processes.