The mass transfer at Taylor cones

Abstract

Electrospraying is a highly attractive method for generation of small particles and also for measurement applications of particles in suspensions. In the cone jet mode, the particle generation rate increases with decreasing flow rate. For applications in the field of aerosol technology flow rates of less than 10µl/h are attractive as particle sizes of less than 200nm can be obtained in this flow rate range. At the same time generation frequency of drops reaches the values required for size measurements by Scanning Mobility Particle Sizers (SMPS). A special challenge in operation of electrosprays at small flow rates is the change of liquid composition due to mass transfer processes between the Taylor cone and the surrounding gas phase. Typically used organic solvents such as alcohols, acetone or acetonitrile evaporate fast (or even evaporate completely) and mixtures containing such volatile species will change their composition significantly due to evaporation from the Taylor cone. Based on key experiments we showa)that the evaporating solvent can be reduced by as much as a factor of 20 by controlling the gas phase saturation. This allows spraying volatile solvents such as e.g. methanol at flow rates down to less than 1µl/h and makes the attractive sub-10µl/h flow rate range accessible to quite volatile solvents as well.b)that changes in the liquid composition by not considering the mass transfer (evaporation) problem lead to significant differences in liquid properties between the feed liquid and the Taylor cone. For example, an increase of surface tension by a factor of 2 is observed for water and t-butanol feed liquid.c)that employing a saturated sheath gas flow allows for very effective manipulation of the solvent composition in the Taylor cone. Based on water as feed liquid we show that a sufficient amount of organic solvent is easily transferred to the Taylor cone liquid from the gas phase to allow spraying the water without electron scavenging gases.Measurement results of the Taylor cone liquid composition are in good agreement with predictions by the presented new model describing the mass transfer between the Taylor cone and the surrounding gas phase. The efficiency of the mass transfer, the predictability of the process and the opportunity of comparably quick changes of the Taylor cone liquid by changing the gas phase saturation state result in a powerful tool for the entire field of electrospraying at low rates.