~AerosolSci. Vol. 31, Suppl. 1, pp. $971-$972, 2000 Pergamon www.elsevier.com/locate/j aerosci Session 9B - Particle eharaeterisation and monitoring I MEASUREMENTS OF DEFORMED DROPLETS AND DROPLETS WITH INCLUSIONS IN AN AERODYNAMIC PARTICLE SIZER D.R. SECKER, E. HIRST, and P.H. KAYE Science and Technology Research Centre (STRC), University of Hertfordshire, Hatfield, Herts, ALl0 9A13, United Kingdom. Keywords: Aerodynamic Particle Sizer, Droplet Deformation, Light Scattering, Particle Characterisation. INTRODUCTION This paper describes the progress on a project aimed at quantifying the degree of deformation experienced by liquid droplets in the TSI Aerodynamic Particle Sizer with a view to providing automatic correction for errors in measured aerodynamic size caused by the deformation. The approach is based on recording the spatial light scattering from individual droplets as they pass through the instrument's measurement space. Theoretical inversions of the scattering from deformed droplets are presented. The paper also contains observations of droplets with inclusions. METHODS Commercial aerodynamic particle sizers generally achieve the measurement of aerodynamic size by accelerating the sample air stream in which the particle is suspended and recording the velocity of individual particles. The velocity is derived from recording the transit time of a particle as it traverses two closely spaced laser beams. The particle aerodynamic size can then be determined from an internal calibration function, which is generally achieved by using incompressible spherical particles such as polystyrene latex spheres of known size and density. Despite their widespread use aerodynamic particle sizers suffer a potential limitation in that the measured aerodynamic size can be significantly affected by particle shape. For example, two identical non- spherical particles of known size and shape can be reported as having different aerodynamic sizes if their orientation is different during measurement. Liquid droplets are also subject to this type of error since the droplets deform into oblate spheroids in the accelerating air-flow. This deformation is perpendicular to the flow and as a result of their greater cross-sectional area to the flow field, the droplets experience a greater acceleration than if they were spherical. This leads to the under-sizing of liquid droplets as reported by Baron (1986), and Griffiths el al. (1986). The error increases with increasing droplet size. Figure 1 shows a plot of measured liquid droplet aerodynamic size as a function of undistorted droplet diameter. 25 m~15 "0 -, o10 ~g / I ' oleicacid ] ........ z'" 10 1'5 Calculated diameter (micrometers) 2'0 Figure 1 25 In previous research the authors have shown how size, shape and orientation of individual particles can be characterised by the analysis of the particles' spatial light scatter intensity, see Kaye et al. (1997). In order to achieve fundamental measurements of the deformation of liquid droplets within the TSI APS the authors have built a laboratory apparatus at the centre of which is an aerosol delivery system from an APS3320 instrument (TSI Inc.). An intensified charge coupled device (CCD) camera is used to record the spatial light scattering as the droplets pass through a HeNe laser beam passing beneath the nozzle exit, and an additional CCD camera, placed orthogonal to the intensified camera, $971