ADHESION OF SPRAY DROPLETS TO FOLIAGE - THE ROLE OF DYNAMIC SURFACE-TENSION AND ADVANTAGES OF ORGANOSILICONE SURFACTANTS

Measurements of the dynamic surface tension of seven surfactants (four organosilicones, two 'conventional' hydrocarbon-based surfactants and an organosilicone + hydrocarbon-based blend; 0.5 to 5.0 g litre-1) ha ve been combined with data on the adhesion of their spray droplets to pea leaf. The latter were determined by a method enabling the surface age, and thus surface tension of the droplets at impact, to be calcula ted. Adhesion was directly related to the dynamic reduction in surface tension (2 to 50 mN m-1) prior to impact. The organosilicone surfacta nts and the blend reduced surface tension more rapidly and to lower va lues than did the organic surfactants. Adhesion was also inversely rel ated to droplet size (I 10 to 880 mum diameter), droplet velocity (0.3 to 2.8 m s-1) and angle of incidence on the leaf (0 to 67.5 degrees). These physical factors were of less importance than surface tension i n controlling adhesion. Relative adhesion varied among the surfactants both with their concentration and with surface tension. These finding s are discussed with respect to their implications for the possible in volvement of surface viscosity in the spray impaction process.