Pin-Plate Electrode System for Emulsification of a Higher Conductivity Leaky Dielectric Liquid into a Low Conductivity Medium Sameer Mhatre and Rochish Thaokar* Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai, Maharashtra 400076, India ABSTRACT: In this experimental study we propose the use of highly nonuniform electric field to emulsify a leaky dielectric oil into another oil. Specifically, a pin-plate electrode system is used to emulsify castor oil into silicone oil. The method is suitable for the emulsification of a higher conductivity leaky dielectric oil dispersed in a lower conductivity medium and is suitable for an already existing emulsion, unlike electrospray methods. The process is stabilized by charging of the dispersed phase and the associated electrohydrodynamic flow. A balance of electrocoalescence and breakup leads to a stationary drop size distribution. A short process time indicates its suitability for continuous operation. 1. INTRODUCTION Emulsions exist in a large number of daily life products such as medicines, detergents, paints, lubricants, foods, etc., 1-3 as well as in industrial applications such as phase separation, heat and mass transfer, etc. Different methods used for making emulsions employ mechanical devices and methods such as impellers, rotor-stator mixers, colloid milling, flow-injection, electrostatic, high pressure, ultrasonic, etc. Most of these emulsification methods involve high shear which disintegrates the discontinuous phase into small droplets. However, in the electrostatic method, a high electrostatic stress on account of the applied field can overcome the surface tension force resulting in the breakup of a drop. In recent times such electrostatic methods have evolved as faster and more efficient emulsification techniques, 4,5 especially when the continuous medium is poorly conducting. Furthermore, the emulsion produced by the electrostatic method is fairly monodispersed. 6 The credit of the discovery of electrospray technology goes to Jean-Antoine Nollet, who first observed that the water flowing through a vessel aerosolizes when the vessel is kept under the influence of an electric field. He also noted that a wound in an electrified person does not bleed normally, instead it gives a blood spray. 7 In a first scientific study, Zeleny 8 observed that a liquid drop when raised to a high electric potential can break, thereby giving rise to a string of small droplets. The size of the resulting progeny droplets is a function of the applied potential. The electrostatic method, generally referred to as electrospraying, employs the phenomenon of disintegration of a liquid using electric fields. The application of electric potential for the preparation of emulsions was first introduced by Nawab and Mason. 4 Industrial applications such as atomization, inkjet printing, electro-spinning, drug delivery, aerosol formation, etc. employ electrostatic energy to generate droplets of desired size. A conventional electrospraying device contains a metal capillary, which is connected to a high potential source, and a counter electrode connected to the ground. Fluid to be disintegrated is pumped through the high potential capillary. At the opening of the capillary, electrostatic forces at the meniscus draw the fluid in the form of a jet. Depending upon the applied electric field, two jetting modes, i.e., single jet or multijet modes, can be observed. 9 Electric field-induced instability further disintegrates the jet into a string of small droplets. Also, the size of the resulting droplets is governed by many factors such as magnitude of applied potential, flow rate, capillary size, electrical properties (conductivity and polarizability), physical properties (viscosity and surface tension), etc. A drop in an electric field can further break in to smaller progeny droplets depending on the capillary number and the charge acquired by it. 10 The mechanism of jet formation at the tip of the capillary does not change on changing the polarity of the applied potential although it may disturb at high field because of corona discharge when the surrounding medium is air. 9 In the present work, we report a novel method of emulsifying a leaky dielectric (LD) fluid into another leaky dielectric fluid, using high electric field generated by a pin-plate electrode system. As demonstrated earlier by Mhatre and Thaokar, 11 a conducting drop dispersed in a leaky dielectric medium when subjected to a nonuniform electric field, such as that found in a pin-plate electrode system, is attracted to the pin electrode. On contact with the electrode, a drop gets charged and is repelled from the electrode, and sustained oscillations are observed. Similar periodic motions of a conducting drop in the pin-plate electrode system was reported by Ahn et al. 12 However, in the case of a LD-LD system (leaky dielectric phase dispersed in leaky dielectric medium), when the dispersed phase is relatively more conducting than the medium phase, a drop moves to the pin electrode by positive dielectrophoresis and is straddled at the pin. The drop then exhibits a shape similar to that of a fluid emerging out of a capillary in electrospraying. As shown in Figure 1, the cone formation and its subsequent breakup gives rise to a string of small droplets. Emulsions with a LD fluid dispersed in another LD phase exist in many applications. Prominent examples include (1) a phenomenon called phase inversion in liquid-liquid dis- persions which involves swapping between dispersed and medium phases, i.e., the dispersed phase becomes continuous Received: April 30, 2014 Revised: July 17, 2014 Accepted: July 17, 2014 Published: July 17, 2014 Article pubs.acs.org/IECR © 2014 American Chemical Society 13488 dx.doi.org/10.1021/ie5017893 | Ind. Eng. Chem. Res. 2014, 53, 13488-13496