The characteristics of electrospraying of highly viscous liquids have been investigated in vacuum as well as at atmospheric pressure. At the same time, a freezing method has been developed to measure submicron droplets in electrosprays. With the freezing method, submicron droplets varying from 0.1 to 0.4㎛ can be sampled with their original sizes preserved and analyzed by a transmission electron microscope (TEM). This method is found to be more accurate in measuring droplets with the sizes ranging from 0.1 to 0.4㎛ than an aerodynamic size spectrometer (TSI Aerosizer DSP).
Under atmospheric pressure, the size distributions of electrospray droplets from the Taylor cone in cone-jet mode are measured by sampling their electrosprays into an aerodynamic size spectrometer. At fixed flow rate (Q), the size distributions consist of one or two fairly monodisperse classes of droplets with diameters. Near the minimum flow rate where the cone is stable, the spray tends to consist of primary and satellite droplets only. However, at larger flow rates, bimodal size distributions appear. The main peak bifurcates into two branches at a critical flow rate, which may be due to the onset of lateral kink instability of the jet. For liquid flow rates below 1 nl/sec, the measured droplet diameters by the aerosizer are in the range of 0.30 to 0.44 ㎛ for a solution of 1.42 M NaI glycerol. The diameters of monodisperse droplets scale approximately with $r^*=(Qτ)^\frac{1}{3}$(Fernandez de la Mora and Loscertales, 1994), 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 factors 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. The breakup mechanism and the diameter depend on the viscous parameter $Π={γ^2 ρτ^\frac{1}{3}$/μ(μ=fluid viscosity). These trends of our results can be extended to more viscous regime, compared with the previous data. Comparing with liquid of small viscosity, highly viscous liquid has more narrow applied voltages and liquid flow rates in the ranges where a liquid cone is stable. In addition, the spray current of highly viscous fluid in the cone-jet mode depends on the applied voltages while that of the lower viscous fluid is almost constant. For highly conducting liquid, the sizes of the droplets electrosprayed from the Taylor cone are found to be relatively insensitive to applied voltages and electrosprays assisted by the corona discharge produce monodisperse droplets as long as the corona intensity is not too high.
In vacuum, the cone is stable in a wide range of applied voltages and flow rates than at atmospheric pressure. In the cone jet mode, the spray current increases with increasing applied voltages because ions in the solution of glycerol doped with NaI are extracted at higher voltages. The size distributions of droplets are measured by the freezing method and a TEM image processing technique. Compared with the case of atmospheric pressure, the droplets have more monodisperse and a little bit smaller sizes, which may be due to the sharpness of a liquid cone tip. Visualization of cone shapes shows that liquid cones have cusp shapes in vacuum while those have literally conical shapes at atmospheric pressure.
