A NEW MODEL TO DETERMINE DYNAMIC SURFACE-TENSION AND ELONGATIONAL VISCOSITY USING OSCILLATING JET MEASUREMENTS

We present an integro-differential equation model which, combined with experimental measurements of an oscillating free surface jet, calcula tes dynamic surface tension and elongational viscosity of a fluid. Our model builds upon previous models due to Rayleigh and Bohr in that it self-consistently incorporates the effects of viscosity and gravity. Further, surface tension and viscosity are allowed to be non-constant. The principal result of this paper is a technique for the measurement of surface tension of newly forming surfaces on the millisecond times cale relevant for agricultural spray mixtures. Coincidentally, our mod el independently yields the elongational viscosity of the fluid, altho ugh our present experimental apparatus limits the accuracy of measurem ent of this material property. In this paper we take measurements from physical jet experiments and implement our inverse model to deduce th ese material properties. The model is first benchmarked against standa rd techniques on a well-characterized fluid with constant surface tens ion and Newtonian viscosity. We then apply our method to an agricultur al spray mixture, with non-constant surface tension and non-Newtonian theology. We measure (i) the rapid decay of surface tension from the n ewly formed surface (aged less than a millisecond) to the much lower e quilibrium value, and (ii) the rate dependence of elongational viscosi ty.