Effects of bulk charge and momentum relaxation time scales on ac electrospraying

Abstract

The behavior at the liquid meniscus under the application of a high voltage (>4×103Vp-p), high frequency (>10kHz) alternating current (ac) signal is shown to depend very sensitively on the liquid conductivity and the applied frequency. For pure ethanol, after resonance effects become unimportant, the meniscus oscillates and then changes to a growing conical mode with increasing frequency. For ionic-liquid doped samples with higher conductivities, however, this transition to an oscillating mode and a conical mode is delayed and additional tip-streaming and elongated-fast dripping modes are seen. A linear proportionality between the transition frequency and the sample conductivity indicates that the critical period is approximately 20 times the bulk charge relaxation time scale; the mode transition takes place only when a specific and universal fraction of charge in the liquid bulk is unrelaxed. This observation for ac cones is in contrast with direct current cones with complete charge relaxation in the bulk. It also suggests that bulk Coulombic forces due to a cumulative net charge in the liquid bulk are responsible for growth of the much more slender ac cones. The vibration frequency of the oscillating mode and the ejection frequency of the elongated-fast dripping mode are found to be related to the momentum diffusion time scale across the drop and not the applied frequency.