10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
High Speed Imaging of Rayleigh Breakup of Charged Droplets Levitated in an Electrodynamic Balance
Mohit Singh, Neha Gawande, Y.S. MAYYA, R.M. Thaokar, Indian Institute of Technology Bombay
Abstract Number: 712 Working Group: Aerosol Physics
Abstract Electrospray is a well-known method for production of aerosols of nanometer size for various applications. These involve drug nanoparticle generation, coating, agricultural and automotive sprays, ink-jet printers, spray-cooling of hot surfaces, plasma coating, ion mass spectroscopy etc. An important question pertains to the Rayleigh break-up mechanisms which is responsible for the transformation of the micro-meter sized droplets ejected from the Taylor cone into nano-sized particles, as observed with instruments such as scanning mobility particle sizers. Specific question pertains to the estimate of charge and mass losses per fission. However, being a fast process, it is nearly impossible to carry out controlled observations of break-up process in electrospray itself and hence alternative tools facilitating careful observations are required. In order to accomplish this task, an electrodynamic balance in which charged droplets are levitated stably for long periods of time has been developed (Singh et al. 2017). It is made up of copper based hemispherical end-cap electrodes and circular ring electrode, connected to a AC power supply. Being large in size, it leaves sufficient gaps for inserting various observational tools such as laser beam, droplet injection needle, fibre-optics arrangement connected to PMT and low/ high speed camera. The PMT arrangement records the laser light scattered by the droplet and provide continuous signal indicating the positional and the surface stability state of the droplet. When the surface becomes unstable just prior to the Rayleigh break-up, it triggers the high speed camera for recording the emitted jet/satellite droplets. The charge is established by changing the AC frequency until the droplet becomes positionally unstable (Mathieu instability). The quantitative observations have been made on the droplets of about 70 to 200 μm, carrying charges about 15% of their respective Rayleigh charges. As the droplet undergoes evaporation, the droplet size decreases increasing the fissility to that corresponding to the Rayleigh limit, and it is observed that finally the droplet breaks up in a succession of multiple ejections. All the successive events of droplet oscillations, deformation, breakup and relaxation of drop after ejection have been captured by high speed camera connected with stereo-zoom at 130-220k fps. The experiments yield loss of about 29-40% of its original charge and 2 to 3 % of its original mass, having jet diameter of 2 to 6 μm. It has also been possible to study the dynamics of many drop levitation, their interaction and pattern formation, using the Electrodynamic balance. The presentation will discuss, the quantification of these data along with the results of numerical simulations.
Reference: Mohit Singh, Y.S Mayya, Jitendra Gaware and Rochish M. Thaokar: J. Appl. Phys. 121, Art.No. 054503, (2017).