Electrospray characteristics of highly viscous liquids

Electrospray characteristics of highly viscous liquids have been investigated by measuring size distributions of droplets emitted from the Taylor cone using glycerol solutions with various conductivities. Since glycerol volatility is very low, droplet sizes can be measured by an aerodynamic size spectrometer (TSI Aerosizer DSP) with negligible evaporation of droplets. For highly conducting and viscous liquids, the sizes of the droplets electrosprayed from the Taylor cone are found to be relatively insensitive to applied voltages and the electrosprays assisted by the corona discharge produce monodisperse droplets as long as the corona intensity is not too high. Near the minimum flow rate where a liquid cone is stable, the spray tends to consist of a one-peak monodisperse distribution of droplets. However, at higher flow rates, the spray bifurcates into a bimodal distribution. This is consistent with the results of Rosell-Llompart and Fernandez de la Mora (J. Aerosol. Sci. 25 (1994) 1093) for their liquids, which are less viscous than the ones we investigated. Experimental results show that the origin of the bimodal distribution may be due to the lateral instability of the electrically charged viscous jet. For liquid flow rates (Q) below 1 nl/s, the droplet diameters measured by the aerosizer are in the range of 0.30–0.44 μm for the most highly conducting solution. The diameters of monodisperse droplets scale approximately with r*=(Qτ)1/3 (J. Fluid Mech. 260 (1994) 155), where τ is the electrical relaxation time of the fluid. However, when compared with several representative scaling laws, the droplet diameters are two to six times greater than those predicted by the scaling laws. This may be closely related to the combined effect of the much higher viscosity and the electrical charge on the jet breakup of glycerol solution.