The surface charge in electrospraying: Its nature and its universal scaling laws

The electrospraying of liquids in steady cone-jet mode follows a well-defin ed EHD mechanism described and quantified in this work using a hybrid exper imental-numerical technique: a collection of emitted microjet shapes corres ponding to several liquids and different flow rates have been digitized and introduced in a quasi-one-dimensional analytical model. A universal Value of the surface charge on the liquid microjet and the resulting charged drop lets, independent of their size and of the liquid permittivity, has been fo und. The surface charge is shown to be always in equilibrium, being the liq uid bulk quasi-neutral. From these findings, we finally present a consisten t general scaling of all EHD variables involved which is experimentally ver ified. In this scaling the electric current I and the characteristic microj et radius R-o, are both proportional to the square root of the emitted flow rare, Q(1/2), and independent of the liquid permittivity epsilon(i). (C) 1 999 Elsevier Science Ltd. All rights reserved.