Experimental Investigation of Scaling Laws for Electrospraying: Dielectric Constant Effect

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Abstract
Experiments were performed to investigate the effect of liquid dielectric constant on existing scaling laws for the electrospraying process. The variations of the droplet size and the emitted current were measured as a function of the dielectric constant for the electrospray operating in the cone-jet mode. Eight different solvents with dielectric constants, κ, ranging from 12.5 to 182 were tested. The residue particle size distributions were measured using a TSI scanning mobility particle sizer, (SMPS). The produced liquid droplet sizes were then calculated from the known solution concentrations. The results show that: (1) For the produced droplet size, Dd , experimental data are in agreement with the scaling law (G(κ) = 1.66κ −1/6) proposed by Gañán-Calvo et al. (1994) for solvents with high dielectric constants. The derivation in low dielectric constant cases may be that the assumption of Gañán-Calvo (1994) on the characteristic length may not hold for the present system; (2) for the emitted current, I, experimental data deviate from those given by Fernández de la Mora and Loscertales (1994) except for the case of benzyl alcohol. The difference may be partially explained by the use of different electrolytes. A larger deviation is found in comparing with the equation, f(κ) = 6.46κ1/4, provided by Gañán-Calvo et al. (1994). The reasons may be due to the inapplicable characteristic length assumption and the role of ion drifting current in the total emitted current. Based on the experimental data collected, some problems are pointed out regarding the criteria proposed in the previous studies for predicting the minimum liquid feed rate. A formula is given based on these data to estimate the maximum feed rate and to explain the observations reported in Chen et al. (1995).