Spreading of spray mixtures on leaf surfaces: I. relative effectiveness of various physico-chemical predictors

Surface tension, contact angles and critical micelle concentration were not effective as practical predictors of spreading. Contact angles could not quantify spreading past the point where the angles became zero, although differences could be discerned by eye. Measurement of diameter of spread allowed quantification of these differences. Hansen parameters, used instead of surface tension, underlined the importance of hydrogen-bonding and charge effects, but could not explain all the observed phenomena. The results emphasise the complexity of spreading on leaf surfaces, and problems experienced with several approaches encountered in the literature are discussed.