Ultrasonic droplet generator investigation: Correlation between droplet size distribution and solution properties

Kai Zhong (1), Alex Langrock (1), George Peabody (1), Howard Glicksman (2) and SHERYL EHRMAN (1),

(1) University of Maryland, College Park, MD (2) DuPont Electronic Technologies, Research Triangle Park, NC

     Abstract Number: 463
     Last modified: May 12, 2010

Abstract
Ultrasonic atomization, developed for liquid atomization, plays an important role in applications including drug delivery, precursor transport and environmental improvement, with the advantages of tunable and relatively narrow droplet size distributions. Different groups have reported measurements of ultrasonic generated droplets with diameters from microns to hundreds of microns, depending on the geometric structures of the generator and the physical and chemical properties of the solution such as density, viscosity and surface tension. This paper describes our study comparing the ultrasonic droplet size distribution of droplets formed from aqueous copper nitrate solutions and solutions of copper nitrate dissolved in a mixture of water and an organic solvent. . In our experiment, an ultrasonic droplet generator was designed with cooling water and precursor solution continuously flowing through the generator. Nitrogen gas flow was used to carry the droplets out of the generator and into a tube furnace based aerosol reactor. Experimental conditions were adjusted by careful control of copper nitrate concentration from 0.5M to 2M, co-solvent volume ratio from 0% to 40% and the carrier flow rate from 1L/min to 4L/min. Viscosity and surface tension of the solutions ranged from 1mm2/s-7mm2/ and 50mN/m- 58mN/m respectively. The size distributions of droplets were measured by a spray particle analyzer system (Spraytec, Malvern Instruments Ltd) and volume average diameter ranged from 0.9 micron to 3.2 microns. Statistical analysis showed that either increasing viscosity or increasing surface tension increased droplet average diameter. With increasing viscosity of the solution, the size distribution changed from monodisperse to a bimodal distribution, however the mechanism of the formation of the second peak located at about 50 microns is under further investigation. The atomizer efficiency was characterized by measuring solution mass change during the atomization process, and a high efficiency was obtained by either decreasing the viscosity or increasing the carrier gas flow rate.