Journal of Electrostatics 66 (2008) 184–189 Optimization of liquid electrostatic coating J. Abu-Ali, S.A. Barringer à Department of Food Science and Technology, The Ohio State University, 2015 Fyffe Road, Columbus, OH 43210-1007, USA Received 12 July 2007; accepted 13 December 2007 Available online 17 January 2008 Abstract Soybean oil with emulsifiers was atomized by electrohydrodynamic spraying. The relationships between voltage (20–40 kV), flow rate (28–88 g/s), emulsifier content (10–15%), apparent viscosity (45–53 mPa-s), conductivity (0.1–0.2 mS/m) and surface tension (43.1–46.0 mN/m) and both coating reproducibility and efficiency were mapped utilizing Response Surface Methodology. Voltage had the most significant effect on reproducibility, followed closely by conductivity, and then flow rate. Viscosity had the least significant effect on coating reproducibility and was only significant through interaction with other factors. Surface tension was not a significant effect. Reproducibility was increased by increasing charge concentration, which decreased droplet size. This was achieved at intermediate voltage, low conductivity, high viscosity and low flow rate. Flow rate had the most significant effect on efficiency followed by voltage and emulsifier content. Efficiency increased at low flow rate and voltage. Optimum conditions produce an optimum charge concentration, resulting in the most reproducible and efficient coating. r 2007 Elsevier B.V. All rights reserved. Keywords: Liquid coating; Electrostatics; Efficiency; Conductivity; Viscosity 1. Introduction The electrostatic atomization of liquids is used in many diverse fields, including pollution research, meteorological experiments, crop spraying, paint spraying, electrostatic printing and the propulsion of space vehicles [1]. Electro- static spraying systems have been used successfully in many industries for applying liquid sprays with improved spray depositions [2]. One of the emerging technologies in food manufacturing is the utilization of liquid electrostatic coating systems for coating foods with oils, emulsifiers and flavors. Electro- static spraying has been used for the application of smoke flavors during smoke curing of fish and pork products, for impregnation of bread with vegetable oil, and for applica- tion of polishing and other liquid coating agents to sugar, confectionary and chocolate products [3]. The reason for this interest is the high degree of controllability and efficiency associated with the electro- static atomization of liquids. The use of electrostatics has enabled certain industries to significantly improve their processes. In the agricultural application of pesticides, the use of electrostatics increased the deposition by a 2.3:1 ratio over traditional non-charged systems [4], increased evenness of the pesticide on the leaves [2], and decreased the amount of pesticide needed to control insects and weeds [5]. The areas that are most important in coating operations, including those utilizing electrostatics, are control, quality and cost reduction. Current coating systems have problems in these areas. Theoretically, electrostatics offers great potential for improvement. However, a lack of under- standing of the underlying relationships between operating variables and spray quality in food systems has delayed the integration of these systems into the food industry. In general, reproducibility and efficiency in electrostatic oiling have not been studied but droplet size and charge to mass have been. The factors having a statistically significant effect on droplet size and charge-to-mass are voltage, conductivity, mass flow rate, viscosity and surface tension, with conductivity having the greatest effect and ARTICLE IN PRESS www.elsevier.com/locate/elstat 0304-3886/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2007.12.003 à Corresponding author. Tel.: +1 614 688 3642; fax: +1 614 292 0218. E-mail address: Barringer.11@osu.edu (S.A. Barringer).