Med. Fac. Landbouww. Univ. Gent, 67/2, 2002 37 THE EFFECT OF STATIC AND DYNAMIC SURFACE TENSION ON ATOMISATION WITH AGRICULTURAL NOZZLES Pieter SPANOGHE 1 , Hilde VAN EECKHOUT 2 , Walter STEURBAUT 1 & Paul VAN DER MEEREN 2 1 Department of Crop Protection 2 Department of Applied Analytical and Physical Chemistry Ghent University, Coupure Links 653, 9000 Gent, Belgium INTRODUCTION Droplet generation by agricultural nozzles is a complex process. Therefore, to un- derstand why different nozzles generate diverse droplet types of spray it is nec- essary to comprehend the fundamental processes and principles involved. The atomization process in general can be set up in 3 phases. Initially the liquid pours out of the nozzle and a liquid jet is formed. The intact liquid jet then disin- tegrates and forms large ligaments and thin membranes (Shavit and Chigier, 1995). The thin membranes become thinner as their area increases. Some small areas of this liquid film attenuate more rapidly than the remainder and lead to holes in the film. When the liquid film has reached its critical thickness it will break up spontaneously and will be scattered into small droplets. Three mecha- nism of sheet break up are proposed: sheet disintegration by oscillation caused by external disturbances, sheet disintegration by perforation and rim disintegra- tion (Butler Ellis et al., 1997). When these small liquid drops move on, finally evaporation as well as coalescence may occur. This can cause a different particle size from the initial one. Next to the use of a specific operator pressure and a different shape of nozzle, a key factor in these applications is the inclusion in the water solution of surface- active agents, which are also referred to as surfactants. It is well known that sur- factants lower the surface tension at the air-water interface and at the water- plant interface. The fundamental reason for the fact that these molecules are ac- tive at the surface is that they contain both a hydrophilic and a hydrophobic part. The surface tension reducing power of a surfactant depends on the hydrophilic- hydrophobic relationships as well as on the ability to arrange themselves at the different interfaces. Under these fast dynamic circumstances, the whole process of diffusing and adsorbing of surfactants at the surface is changing in function of time. The “dynamic” surface tension, where the surface tension is measured at short times (the value of surface tension changes in time), refers to this effect. A “static or equilibrium” surface tension is measured at an equilibrium liquid sur- face (the value of surface tension at time endless). This constant value is time independent. For pure liquids, the composition of the surface will not change over time; hence, the surface tension of a pure liquid does not have a dynamic character.