10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Numerical Study of Nano-Aerosol Generation through Rayleigh Fission of a Charged Viscous Liquid Drop
NEHA GAWANDE, Mohit Singh, Y.S. Mayya, R.M. Thaokar, Indian Institute of Technology Bombay
Abstract Number: 732 Working Group: Aerosol Physics
Abstract Electro-hydro-dynamic atomization is increasingly used for the production of nanoparticle aerosols. In this technique, a conducting liquid issuing from a needle maintained at a high electric potential with respect to ground, develops a Taylor cone from which highly charged primary droplets are ejected. The charges and the sizes of these droplets, which are generally in the range of several micrometers, depend on the various electro-hydro-dynamic properties of the system. These droplets further disintegrate into smaller droplets by successive Rayleigh fission events. Although the droplets are initially in the sub-Rayleigh limit, they gradually shrink to the radius (a) corresponding to the Rayleigh limit (a3=Qc/(64π2ϵ0γ) where, Qc = charge on the droplet, γ =surface tension, ϵ0=permittivity of free-space) due to evaporation. Although the Rayleigh fission process has been observed experimentally (Duft et al., 2003), the characteristics of the jet formed are still unclear. An understanding of the instability and breakup of such charged droplets from theoretical perspectives is essential for the effective design of the electrohydrodynamic system for the aerosol generation.
Towards this, we model (Gawande et al., 2017) the Rayleigh fission process of a perfectly conducting charged drop of a viscous liquid using a numerical scheme based on boundary element methods. The governing equations involve Laplace equation for the electric potential and Stokes equation for flow field. It is found from the simulations that the drop progressively deforms into a conical shape with pointed ends just prior to break up. As the numerical approach fails to predict charge ejection due to the occurrence of a singularity at this point, the charge loss fraction is estimated by determining how much minimum charge must be removed if the drop is to relax back to a spherical shape. In this manner, the charge loss due to Rayleigh fission (the difference between the original charge and the minimum charge removed) is estimated to be about 39%, which falls within the range of experimental data lying between 20% and 50%. The results on scaling laws, timescales of the process, and the role of various stresses on the dynamics of the drop are also discussed.
[1] D. Duft, T. Achtzehn, R. Müller, B. A. Huber, and T. Leisner, Nature (London),421, 28 (2003). [2] Gawande, Neha, Y. S. Mayya, and Rochish Thaokar, Physical Review Fluids 2.11 (2017).