Study pulsating electrospray of non-Newtonian and thixotropic sodium alginate solution A. Tabeei,A. Samimi * , M. Khorram,H. Moghadam Department of Chemical Engineering, University of Sistan and Baluchestan, Daneshgah Blv., Zahedan, Iran a r t i c l e i n f o Article history: Received 16 June 2011 Received in revised form 22 October 2011 Accepted 26 October 2011 Available online 10 November 2011 Keywords: Electrospray Pulsating electric field Sodium alginate High viscous fluid a b s t r a c t The main objective of investigation was to study the effect of pulsating electrospray of high viscous and non-Newtonian sodium alginate on mono-dispersity of droplets. In the jet mode, employing the pulsating DC electric field at a critical frequency led to narrow size distribution of beads, produced from the droplets.Above the criticalfrequency,the size of beads was distributed over a wider range. The relationship between the dimensionless group representing diameter and the groups including: Weber, Reynolds,Electric Field and Frequency numbers was modeled according to genetic algorithm. The experimental results were found in good agreement with the model. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Spray of a liquid is of importance in a number offields.It is a physical process in which a liquid stream of droplets are dispersed in a gaseous environment. The spray of droplets is usually employed to achieve two primary functions:increasing surface area of dispersed liquid droplets to enhance evaporation and distributing a liquid over an area. Lord Rayleigh [1] presented a theory in which the instability of a laminar jet and its wavelength was related to the jet diameter. Using a linearised stability analysis, Rayleigh showed that for an inviscid liquid a jet broke up when the developed wavelength was greater than the circumference of the jet. Weber [2] extended the theory of Rayleigh to viscous liquids. He showed that the optimum wavelength, for the jet breakup depen- ded also directly on viscosity, and inversely on density and surface tension of the liquid. The laminar jet breakup may cause production of large drops depending on viscosity and surface tension of the liquid. Furthermore, for non-Newtonian and highly viscous liquids and suspensions, it may lead to an irregular jet breakup causing a wide size distribution of the droplets. The breakup of a liquid jet occurs by the propagation and growth of the waves along the jet axis inducing the formation and separation of droplets from the jet tip. The wave leaving the nozzle moves down the stream along the jet, whilst its amplitude grows and as a result of which the liquid jet breaks up. Droplet formation under electrical potential difference is known as either electroehydrodynamicatomization or electrospray, depending on the actual jet breakup wavelength under flow instabilities [3]. In electroehydrodynamic spraying, the possibility of manipulating the liquid with electric fields has led to several technological applications including polymeric particle fabrications for drug encapsulation [4], pharmaceutical productions [5,6], thin films [7], coating and film deposition technology [8], fuel injection [9],ink-jet printing [10]. The idea of using an electric field between a capillary tip such as a nozzle and a flat counter electrode was initially employed to reduce the diameter of droplets by applying an additional force (i.e. electric force) in the direction ofgravitationalforce in order to overcome the upward capillary force of liquid [11]. In this regard, the electric field would be either constant or pulsating. In varying the electric potential one of the latest works belongs to Sato [12], who used four different types of wave pattern: an AC sine wave, a rectified negative AC, a positive sine wave and a positive square pulse wave.Balachandran et al. [13] and Huneiti et al. [14] super- imposed an AC potential on a DC potential to control the liquid jet breakup in a uniform state. However, almost all of the above works have been concentrated on low viscosity liquids. Speranza [15] reported the effect of pulsating DC frequency on the jet breakup of viscous (40e140 m Pa s) and conductive liquids made of different concentration of PVA. He showed that under DC frequencies * Corresponding author. E-mail address: a.samimi@eng.usb.ac.ir (A. Samimi). Contents lists available at SciVerse ScienceDirect Journal of Electrostatics j o u r n a l homepage: w w w . e l s e v i e r . c o m / l o c a t e / e l s t a t 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.10.006 Journal of Electrostatics 70 (2012) 77e82