Experimental analysis of thermo-physical processes in acoustically levitated heated droplets Abhishek Saha a , Saptarshi Basu b , C. Suryanarayana a , Ranganathan Kumar a,⇑ a Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, United States b Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India article info Article history: Received 2 June 2010 Received in revised form 31 July 2010 Accepted 31 July 2010 Available online 16 September 2010 Keywords: Levitated droplet Droplet vaporization Acoustic streaming Nanoceria abstract An experimental setup using radiative heating has been used to understand the thermo-physical phe- nomena and chemical transformations inside acoustically levitated cerium nitrate precursor droplets. In this transformation process, through infrared thermography and high speed imaging, events such as vaporization, precipitation and chemical reaction have been recorded at high temporal resolution, lead- ing to nanoceria formation with a porous morphology. The cerium nitrate droplet undergoes phase and shape changes throughout the vaporization process. Four distinct stages were delineated during the entire vaporization process namely pure evaporation, evaporation with precipitate formation, chemical reaction with phase change and formation of final porous precipitate. The composition was examined using scanning and transmission electron microscopy that revealed nanostructures and confirmed highly porous morphology with trapped gas pockets. Transmission electron microscopy (TEM) and high speed imaging of the final precipitate revealed the presence of trapped gases in the form of bubbles. TEM also showed the presence of nanoceria crystalline structures at 70 °C. The current study also looked into the effect of different heating powers on the process. At higher power, each phase is sustained for smaller duration and higher maximum temperature. In addition, the porosity of the final precipitate increased with power. A non-dimensional time scale is proposed to correlate the effect of laser intensity and vaporization rate of the solvent (water). The effect of acoustic levitation was also studied. Due to acoustic streaming, the solute selectively gets transported to the bottom portion of the droplet due to strong circulation, providing it rigidity and allows it become bowl shaped. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Droplet evaporation has several interesting and important appli- cations in cloud physics, combustion of fuel, ink-jet printing and sur- face-patterning. Majority of these studies involve the interaction of drops with surfaces. Very few studies have analyzed transport phe- nomena in binary droplets undergoing vaporization. Solute migra- tion problem in a binary drop has been studied [1,2] in which the particles in the drop move toward the edge of the drop in a well known ‘coffee ring’ phenomenon. Here, the competing effects be- tween convection and Marangoni flows allow various deposit pat- terns [3]. In contrast, this paper explores solute transport and segregation in an evaporating acoustically levitated 500 lm cerium nitrate droplet at given concentration irradiated by a CO 2 laser. In or- der to delineate the wall–droplet and droplet–droplet effects, a wall- less containerless environment has been chosen to study droplet vaporization. As seen in the droplet evaporation studies on a solid surface, in a levitated droplet, the evaporation flux due to laser heat- ing generates a non-uniform temperature distribution. Irradiation increases the surface temperature which sets up a surface tension gradient, which induces Marangoni convection. This paper will study the strength of Marangoni flow compared to that of the signif- icant flow perturbation known as acoustic streaming induced by the levitator acoustic field and propose a time scale for convective flow within the droplet. The time scale due to acoustic streaming is much shorter and it will be shown that this effect is dominant in a levitated droplet which allows the formation of a bowl-shaped morphology. The containerless processing technique using levitation of mag- netic, non-magnetic materials and live animals have been shown to be effective in counteracting gravity [4,5]. The drop shape de- pends on the balance between the curvature-induced stress caused by the surface tension and the acoustic radiation pressure on the drop surface [5]. The acoustic pressure on the droplet surface is such that the two polar regions contract inwards and the equato- rial region expands outwards. However, a small droplet of 500 lm used in this study does not deform into an oblate spheroid due to curvature effect. Ceria is widely used in solid oxide fuel cells, catalytic convert- ers, and for oxidation-resistive coatings [6–9]. Recently, use of nan- 0017-9310/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2010.08.016 ⇑ Corresponding author. E-mail address: Rnkumar@mail.ucf.edu (R. Kumar). International Journal of Heat and Mass Transfer 53 (2010) 5663–5674 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt