SYMPOSIA PAPER Published: 01 January 1990

Morphology Dependent Resonance Measurements of Berglund-Liu Generator Droplet Dispersion


This research exploits the well-documented morphology dependent resonances (MDRs), observed from lasing spherical droplets in the supermicron range, for highly accurate measurements of the size of droplets produced by the Berglund-Liu vibrating orifice mono-disperse aerosol generator. The measurement is several orders of magnitude more accurate than established methods.

Single dye-doped ethanol droplets, in the size range a = 15 to 45 μm, of known refractive index, from a nominally monodisperse distribution, were illuminated with the green frequency doubled beam of a single-longitudinal-mode neodymium: yttrium aluminum garnet (Nd: YAG) laser, and scattered light was collected at 90° to the incident beam. The collected light was dispersed and detected on an optical multichannel analyzer. The recorded spectrum is the broadband fluorescence spectrum, modulated by the regularly spaced MDRs, which act as wavelength-selective high-Q, cavities. At high values of the input intensity, the cavity provides positive feedback and amplification, characteristic of laser radiation. The smooth temporal profile associated with a single-longitudinal-mode laser improves signal-to-noise ratios above those achieved in previous work using a multimode pulsed laser. A good signal-to-noise ratio is vital if accurate information about droplet size is to be gained.

The study of the wavelength locations of the observed MDRs can provide highly accurate measurements of the droplet size and size variations. The accuracy of these measurements is limited only by imperfect knowledge of the refractive index, by asphericity of the illuminated droplet, and by experimental resolution and noise levels. Advantages over established methods include the high-speed, nonintrusive nature of the measurement and greatly improved measurement accuracy.

Author Information

Bayvel, LP
Driver, HST
University of Cape Town, Cape Town, South Africa
Knight, JC
University of Cape Town, Cape Town, South Africa
Robertson, GN
University of Cape Town, Cape Town, South Africa
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Developed by Committee: E29
Pages: 19–29
DOI: 10.1520/STP25409S
ISBN-EB: 978-0-8031-5141-3
ISBN-13: 978-0-8031-1459-3