(192i) Ac Electrospraying: Quasi-Steady Cones and Microjets

Electrospraying under a direct current (DC) electric field has been studied in detail due to its manifold applications. The deformation of the liquid meniscus into a conical shape and the ejection of a thin, steady jet from the cone tip have been explained by charge separation and accumulation in the liquid phase under the influence of the applied field. The variation of the conical angle as well as the meniscus shape and the number of jets with the applied voltage and liquid properties, particularly conductivity and flow-rate has been analyzed and explained in detail, both experimentally and theoretically. Recently, our group has scrutinized the electro-hydrodynamics of a drop under an alternating current (AC) electric field, beginning with a micro-jetting phenomenon at high voltages and high frequencies (> 10 kHz) (Yeo et al. Phys. Rev. Lett. 2004). This talk summarizes the new results at high frequencies and moderate voltages. At low frequencies below 10 kHz the natural vibration frequencies for a free drop and its overtones coincide with the spraying frequency, giving rise to a combined oscillation and spraying behavior, which can enhance the spraying rate. At high frequencies, the meniscus shape changes and cylindrical microjets start emerging periodically to form large individual droplets. Another complication is introduced by local temperature rise of the metal spraying needle under the high frequency conditions. We describe the variation in the spray ejection dynamics with variation in liquid properties, flow-rate and applied field for the moderate voltage but high-frequency ranges. Under a constant pressure-head the spraying behavior is relatively uniform, but the imposition of a constant flow-rate allows the observation of multiple spraying modes. At specific voltages, a quasi steady conical shape is observed, in spite of the imposition of very high frequencies that dictates the formation of transient cones in tune with the applied frequency, as is observed at low frequencies. Moreover the observed cone half-angle of ~10º is much less than that seen under the application of a DC voltage, all other conditions being the same. This suggests the entrainment of some charge at the meniscus, which does not get dispersed every other half-cycle and builds up every period resulting in an enhanced polarization at the liquid tip which elongates the meniscus with time, stretching it farther than the DC cone. Different mobilities for the cations and anions leading to incomplete dispersal of one of them during the comparatively short half cycles because of the high frequencies seems to be causing the charge build-up. Ultimately these cones become unstable due to the large polarization at the tip, as well as instabilities due to its high aspect ratio, and hence break down and subsequently form again. These conical meniscus shapes are very sensitively dependent upon the liquid properties, such as conductivity, viscosity and surface tension, as well as the imposed flow-rate and voltage, and get very easily replaced with periodic ejection of large drops from an unsteady meniscus. We find this ejection frequency and consequently drop dimension to also be strongly dependent on the applied frequency and liquid conductivity and viscosity. Conductivity affects the charging and relaxation times of free charges in the meniscus, as is consistent with the above theory, and might explain why AC sprays are so frequency-dependent. In this talk we will discuss all the spraying modes observed for AC sprays and their relation to the experimental parameters quantitatively, with emphasis on these quasi-steady cones, and our theoretical attempts to elucidate the charging mechanism behind these variations.