Electrospray of a very viscous liquid in a dielectric liquid bath
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1272-KK07-02
Electrospray of a very viscous liquid in a dielectric liquid bath
F. J. Higuera E. T. S. Ingenieros Aeron´auticos, UPM, Pza. Cardenal Cisneros 3, 28040 Madrid, Spain
ABSTRACT Numerical computations and order-of-magnitude estimates are used to analyze a jet of a very viscous liquid of finite electrical conductivity that is injected at a constant flow rate in an immiscible dielectric liquid under the action of an electric field. The conditions under which the injected liquid can form an elongated meniscus with a thin jet issuing from its apex (a cone-jet) are investigated by computing the flow, the electric field, and the transport of electric charge in the meniscus and a leading region of the jet. The boundaries of the domain of operation of the cone-jet mode are discussed. The current transfer region determining the electric current carried by the jet is analyzed taking into account the viscous drag of the dielectric liquid surrounding the jet. Conditions under which the electric current/flow rate characteristic follows a square root law or departs from it are discussed.
INTRODUCTION Electrostatic dispersion is a technique used to generate emulsions of electrically conducting liquids in baths of immiscible dielectric liquids. It leads to drops with narrow distributions of sizes which can be controlled in the range from a few tens of nanometers to hundreds of micrometers, and therefore has potential applications to the synthesis of nanoparticles, the encapsulation of antibacterial or antifungal agents, and the manufacturing of drug-laden particles for targeted delivery, among others. The technique originated in the work of Barrero and coworkers [1, 2], who showed that the cone-jet mode of an electrospray, which is often used to atomize conducting liquids in air or vacuum [3-8], can also be used in a bath of a dielectric liquid. These authors investigated fundamental properties of the cone-jet mode in a liquid bath, such as the current/flow rate and current/voltage characteristics, the scaling laws for the droplet size, and the enhanced whipping instability of the jets of these electrosprays compared with their counterparts in air. Gundabala and Fern´andez-Nieves [9] have carried out work along these lines for electrosprays embedded in microfluidic channels, and Alexander [10] investigated the pulsating modes of an electrospray in a liquid bath. In this paper, numerical computations and order-of-magnitude estimates are used to fur-
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Figure 1: Sketch of the meniscus and the jet near the end of the capillary tube. ther analyze the problem and propose scaling laws. Attention is focused on the stationary current transfer region of the electrospray’s jet where convection of the electric charge accumulated at the interface between the two liquids begins to account for a fraction of the electric current of the spray. A square root current/fl
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