Instability and Jetting of a Charged Milimeter-Scale Water Drop Evaporating in a Strong Electric Field

The stability of a charged liquid drop has been an interesting topic for more than a century due to its wide implications such as sprays, aerosols, ink-jet printing, and lightning in a cloud. In 1882, Rayleigh developed a mathematical model to predict the instability of a charged liquid drop. Decades later, Taylor studied the breakup of an uncharged water drop in an electric field. Recently, Shrimpton combined both Rayleigh and Taylor to consider both the drop charge and the external electric field simultaneously. Almost all experimental studies so far have been conducted with micron-scale liquid drops, which makes it extremely difficult to capture the dynamics of a drop. This presentation summarizes our efforts to visualize the instability of a charged millimeter-scale water drop in a strong electric field and identify the mechanism of the observed prematured jetting. A series of experiments were conducted using the Solution Electrostatic Levitator (SEL) developed at Iowa State University. SEL levitates a charged (~400 pC) liquid drop of 2.5-3.5 mm in diameter within a strong (~10⁵ V·m^-1) electric field. As the levitated drop evaporates, its diameter decreases down to ~0.5 mm and the surface charge density increases. As the coulomb force between surface charges approaches to the surface tension force, the drop becomes unstable and deforms into a prolate spheroid. Upon further evaporation, when the surface charge density reaches the jetting limit, some of surface charge is ejected by jetting and the drop regains its near-spherical shape. The changes in drop size and drop charge were measured as a function of time and relative humidity. In the presentation the mechanism of drop charge, the cause of charge loss, and the influence of a strong electric field on drop jetting will be discussed.