Light Scattering from Deformed

Droplets and Droplets with

Inclusions:

ARL-TR-2228-I September 2000

David R. Secker, Richard Greenaway, Paul H. Kaye, Edwin Hirst,

David Bartley, and Gorden Videen

Volume I. Experimental Results

Approved for public release; distribution unlimited.
The findings in this report are not to be construed as an

official Department of the Army position unless so

designated by other authorized documents.

Citation of manufacturer’s or trade names does not

constitute an official endorsement or approval of the use

thereof.

Destroy this report when it is no longer needed. Do not

return it to the originator.
ARL-TR-2228-I September 2000

Army Research Laboratory

Adelphi, MD 20783-1197

Light Scattering from

Deformed Droplets and

Droplets with Inclusions:

Volume I. Experimental Results

David R. Secker, Richard Greenaway, Paul H. Kaye, Edwin Hirst 

University of Hertfordshire, Hatfield, UK

David Bartley

National Institute for Occupational Safety and Health, Cincinnati, OH

Gorden Videen

Computational and Information Sciences Directorate, ARL

Approved for public release; distribution unlimited.
This is the first of two volumes dealing with experimental and

theoretical results from the scattering of light by deformed liquid

droplets and droplets with inclusions. Characterizing droplet

deformation could lead to improved measurement of droplet size by

commercial aerodynamic particle sizing instruments. Characterizing

droplets with inclusions can be important in some industrial,

occupational, and military monitoring scenarios. The nozzle

assembly from a TSI aerodynamic particle sizer was used to provide

the accelerating flow conditions in which experimental data were

recorded. A helium-neon (HeNe) laser was employed to generate the

light-scattering data, and an externally triggered, pulsed copper

vapor laser provided illumination for a droplet imaging system,

arranged orthogonal to the HeNe scattering axis. The observed

droplet deformation correlates well over a limited acceleration

range, with theoretical predictions derived from analytical solution

of the Navier-Stokes equation.

Abstract

ii
iii

Contents

1. Introduction 1

1.1 Aerodynamic Particle Sizers .......................................... 1

1.2 Spatial Light Scattering ................................................ 3

2. Apparatus 5

2.1 Light-Scattering Pattern Acquisition ................................. 6

2.2 Droplet Image Acquisition ............................................. 6

2.3 Droplet Generation and Delivery .................................... 7

3. Experimental Data 8

4. Quantifying Droplet Deformation 11

5. Droplets with Inclusions 13

6. Discussion 16

Acknowledgements 17

References 17

Distribution 19

Report Documentation Page 21
iv

Figures

1. Schematic diagram of particle delivery system used in TSI APS

3300 series instruments ......................................................... 2

2. Examples of spatial light scattering patterns from individual

airborne particles ............................................................... 4

3. Schematic diagram of apparatus used to acquire scattering

patterns and real images from individual airborne droplets ......... 5

4. Spatial light-scattering patterns from individual droplets as

function of droplet size and sample flow rate ........................... 8

5. Spatial light-scattering patterns and droplet images for 20-, 25-,

and 30-μm-diameter droplets at various sample flow rates

through APS aerosol delivery nozzle ....................................... 10

6. Feret ratio plotted as function of flow rate for 20-μm oleic acid

droplets ........................................................................... 12

7. Image of 20-μm nominal diameter oleic acid droplet, together with

corresponding computed cross-sectional shape ........................ 12

8. Scattering patterns recorded from droplets generated in 40-%RH

environment ..................................................................... 14

9. Scattering pattern and image of 16-μm-diameter oleic acid droplet

containing inclusion with greater size than that shown in

figure 8 .............................................................................. 15
1

1. Introduction

The authors are currently investigating light-scattering methods that could

be used to correct for the errors that are known to occur in the measurement

of liquid droplets by commercial aerodynamic particle sizing instruments.

In the course of the investigation, it became apparent that the apparatus

used for droplet deformation studies was also capable of reproducibly

generating droplets with inclusions. These droplets with inclusions are important because they can be representative of aerosols found in hospital or

battlefield environments, where biological organisms can survive for prolonged periods when contained within a protective liquid coating. This volume therefore describes the acquisition of experimental light-scattering data

from both deformed droplets and droplets with inclusions. These data may

be used to help develop and test theoretical inversion algorithms, some of

which are described in volume 2, which could ultimately allow rapid characterization and/or identification of these droplet morphologies. This, in

turn, could lead to improvements in commercial aerodynamic particle sizers

and new opportunities for the development of light-scattering aerosol monitoring instruments for bioaerosol detection.

1.1 Aerodynamic Particle Sizers

The size of an airborne droplet may be defined in a number of ways, depending on the method of measurement. However, in studies of therapeutic aerosol sprays, combustion aerosols, paint sprays, and coatings, where

on-line or real-time measurements are required and/or where the airborne

behavior of the droplet is relevant to the application, the aerodynamic size

da is frequently sought. The parameter da is defined as the diameter of a unit

density sphere with the same gravitational settling velocity as the particle

in question. The traditional method of determining the aerodynamic size of

a particle was to measure its gravitational settling velocity with suitable

sedimentation instruments. This process was slow and laborious, and commercial instruments based on the acceleration of particles were developed

as an alternative [1]. The most widely used commercial aerodynamic sizers

are the TSI aerodynamic particle sizer (APS) family of instruments (APS

3300 series, TSI, Inc., St Paul, MN), which measure the particle size by accelerating a sample airstream in which the particles are suspended and measuring the velocity acquired by individual particles. The larger particles

exhibit greater inertia and thus accelerate more slowly, thereby attaining

lower velocities. In the TSI APS instruments, an aerodynamic accelerating

nozzle, in which a sample flow is sheathed in filtered air, produces the necessary accelerating flow, as illustrated in figure 1. The total airflow through

the nozzle is regulated to 5 l/min, 4 l/min of which is filtered to provide the

sheath flow. The time of flight of the particle through two closely spaced
2

laser beams positioned a few hundred micrometers beneath the nozzle exit

is recorded from the pulses of light scattered by the particle to an optical

detector. The particle’s aerodynamic size is assessed by relating the transit

time to an internal calibration function based on measured times of flight of

solid spherical calibration particles. The size range covered by this instrument is from 0.5 to 30 μm aerodynamic diameter, depending on the instrument model used.

However, aerodynamic sizing instruments of this type suffer a significant

limitation in that the measured aerodynamic size can be significantly affected by particle shape. When one attempts to measure nonspherical particles of known density, the shape (and orientation) of each particle subjected to the accelerating air flow governs the drag force it experiences and

hence affects the measured aerodynamic size. The measurement of liquid

aerosol droplets is subject to significant error (25% undersizing reported in

some cases) because the droplets deform to oblate spheroids in the accelerating airflow. As a result of this deformation, their cross-sectional area increases, and they experience a greater acceleration than would be the case

with similar-sized rigid spheres. Despite this phenomenon being well reported in the past by Baron [2] and Griffiths et al [3], there is as yet no

systematic method of measuring the degree of deformation experienced by

individual droplets in the instrument- and material-specific calibration

curves, derived, for example, through gravitational settling techniques.

Twin laser beams

To vacuum pump

Airborne

particles

Acceleration nozzle

Filter for sheath air

Control valve for sheath air

Figure 1. Schematic

diagram of particle

delivery system used

in TSI APS 3300 series

instruments.
3

1.2 Spatial Light Scattering

The spatial distribution of light scattered by a particle, also referred to in

certain texts as the two-dimensional angular optical scattering (TAOS) pattern [4], is a complex function of the size, shape, dielectric structure, and

orientation of the particle, as well as of the properties of the illuminating

radiation (wavelengths, polarization state). Analysis of the scattering pattern can provide a means of characterizing the shape, orientation, and internal structure of the illuminated particle, and many researchers have exploited this property in various ways. Previous work by the authors [5] has

explored the potential of scattering pattern analysis for particle shape and

size classification and has demonstrated [6,7] how such techniques may be

implemented in real-time airborne particle measurement systems designed

for ambient aerosol characterization and asbestos fiber detection.

Scattering patterns can cover different scattering angle ranges, depending

on the light-collection geometry used to acquire them. The examples shown

in figure 2 (see figure caption for description of particle types) were recorded

by the imaging of the pattern of light scattered by individual particles onto

an intensified charge-coupled device (ICCD) camera as the particles were

carried by an airstream through a laser beam. The particle transition time

through the beam was ~2 μs, and in each case light scattered between 5°

and 30° scattering angles and throughout 360° of azimuth was captured as

a 256 × 256 pixel image. The beam direction is perpendicular to the paper

into the center of each image. Each white dot in the patterns corresponds to

a single scattered photon, and the images thus represent photon distribution maps of several thousands to several tens of thousands of scattered

photons. The images illustrate the wide variations of different particle shapes

and orientations that these patterns can assume. It was the potential of spatial light-scattering analysis for real-time particle shape characterization that

initiated the fundamental study of droplet scattering reported here. It also

underpins an ultimate aim of this work—to provide an on-line optical means

of correcting for the errors in measured aerodynamic size caused by droplet deformation in instruments such as those in the APS 3300 series.
4

Figure 2. Examples of

spatial light scattering

patterns from individual airborne particles: (a) hematite

ellipsoid, 2 μm;

(b) copper flake,

~5 μm; (c) sodium

chloride crystal;

(d) sodium chloride

crystal; (e) asbestos

fiber (crocidolite);

(f) irregular background particle;

(g) asbestos fiber

(chrysotile); and

(h) water droplet,

~9 μm diameter.

(a) (b) (c) (d)

(e) (f) (g) (h)
5

2. Apparatus

An apparatus was constructed that allowed the acquisition of both spatial

scattering patterns and real images from individual droplets in the sub30-μm size range as they traversed the measurement space below an APS

3320 sample delivery nozzle (donated by TSI, Inc.). The apparatus is shown

schematically in figure 3. As stated earlier, the actual TSI APS 3300 series of

instruments incorporates two closely spaced cross-polarized beams and

measures particle time-of-flight between the beams. However, for the fundamental results reported here, the spatial light-scattering pattern images

were produced with a single beam from a 633-nm, 15-mW plane polarized

helium-neon (HeNe) laser (model 05-LHP-151, Melles Griot, Inc., Carlsbad,

CA). Furthermore, while the normal operating flow rate of the TSI APS instrument is 5 l/min (1 l/min of sample flow rate and 4 l/min of filtered

sheath flow), we have additionally observed droplet behavior over a range

of flow conditions, both less than and greater than the norm, to gain a greater

understanding of the morphological changes taking place.

Compressor/ VOAG

dryer

CV laser

ICCD

 PC

Filter

Filter

Aerosol

Fiber-optic cable

Trigger PMT

HeNe laser

Particle image Scattering profile

Pump

CCD camera

Vent

Diffusing screen

Figure 3. Schematic

diagram of apparatus

used to acquire

scattering patterns

and real images from

individual airborne

droplets.
6

2.1 Light-Scattering Pattern Acquisition

The beam from the HeNe laser passes through a quarter-wave plate to render the light circularly polarized before it is focused through a cylindrical

lens to produce a beam of elliptical cross section (approximately 1.2 mm by

100 μm) at the point of intersection with the sample air flow. Monodisperse

droplets are generated into the sample flow by a vibrating orifice aerosol

generator (VOAG) (manufactured by TSI, Inc.), as described next. As each

droplet traverses the beam, light scattered in the forward direction between

angles of 5° and 20° is imaged onto an intensified, asynchronously triggered ICCD camera (ICCD225, Photek Ltd., East Sussex, UK). The lower

angular limit is set by a beam stop, while the higher limit is set to avoid

shadowing of the scattered light by the lower surface of the sample delivery nozzle. Images from the camera are digitized, displayed, and stored on

a computer at a rate of several images per second for later analysis. A trigger signal for the acquisition of a scattering pattern by the camera is derived from a separate photomultiplier detector module, which receives light

scattered at a higher scattering angle than the camera, as indicated in

figure 3. The rising and trailing edges of the signal from the photomultiplier detector respectively initiate and terminate the camera integration

period, which is typically ~2 μs duration.

2.2 Droplet Image Acquisition

Real images of the droplets are acquired with a charge-coupled device (CCD)

camera assembly, mounted orthogonal to the HeNe scattering axis. In the

manner first described by Baron [3,8], the droplets are viewed against a

bright diffusing screen. The illumination for this screen is derived from a

copper vapor laser (Oxford Lasers Ltd., UK) that delivers 38-ns pulses at

wavelengths of 510 and 578 nm. Each pulse has an energy of approximately

2 mJ. Again, appropriate triggering of the laser pulses is achieved by use of

the signal from the photomultiplier detector. The imaging system uses a

high numerical aperture microscope objective lens and a standard microscope tube, resulting in an overall image magnification of approximately

10×. A digital, nonintensified CCD camera (model KP-F100, Hitachi Denshi

Ltd., Tokyo, Japan) was used to acquire images, because it offered both high

read-out speed and small pixel size (6.7 μm2). This pixel size produced images of acceptable resolution, the diameter of a 20-μm droplet, for example,

covering approximately 30 pixels within a 1300 × 1030 pixel image. A Foucault knife-edge test [9] was performed to determine the resolution of three

pixels for the image quality attainable with this configuration.

We originally intended to acquire scattering patterns and real images simultaneously from the same droplet. This would have required appropriate optical filtering to prevent the intense illumination from the copper vapor laser from reaching the ICCD camera. However, because the pulsed

radiation was broad-field (as a result of the diffusion screen) and multiwavelength, the task of attenuating this radiation to a degree sufficient to
7

prevent damage to the ICCD camera was extremely difficult without also

significantly attenuating the 633-nm scatter light flux. Fortuitously, the uniformity of droplet size produced by the VOAG for a given set of generating

conditions was such that the scattering patterns and images from successive droplets were remarkably reproducible, the differences being principally a result of the optical quantization noise. We therefore decided to avoid

potential overillumination of the ICCD by recording the scattering patterns

and real droplet images from successive droplets rather than the same droplet, ensuring that the ICCD was disabled during the firing of the pulsed

laser.

2.3 Droplet Generation and Delivery

The VOAG produces droplets by inducing mechanical instability in a fine

liquid jet through excitation with a piezoelectric oscillator. When first formed,

droplets are comparatively large and compose a solute—in our case, either

oleic acid or glycerol—in solution with AR-grade isopropyl alcohol. Dry

(<1 %RH) compressed air is used to transport the droplet aerosol from the

VOAG drying column to the light-scattering instrument, during which time

the alcohol evaporates, to leave pure oleic acid or glycerol droplets. A bleed

valve controls the total flow rate through the nozzle subsystem. For the

experiments described, the total flow rate was set before the scattering data

were acquired, and the sheath air flow rate was measured before and during acquisition with a bubble flowmeter. The measured sample flow rate,

consistently 20 percent of the total flow rate, was found to be stable to within

1 percent during the period of experimental data acquisition.
8

3. Experimental Data

All experimental data presented here were recorded from oleic acid droplets (with the exception of those shown later in fig. 8b, which shows scattering from a glycerin droplet). Figure 4 illustrates the changes that occur in

the spatial light-scattering patterns from individual droplets, as a function

of both droplet size and increasing sample flow rate. Each scattering pattern was recorded over a 2-μs period as the droplet traversed the HeNe

beam beneath the aerosol delivery nozzle. Each scattering pattern therefore

represents the forward scattering from a deformed droplet illuminated at

90° to its axis of symmetry. The images on the top row were recorded from

8-μm-diameter droplets at total flow rates from 1 to 6 l/min. Subsequent

rows show the scattering behavior of 12-, 16-, 20-, 25-, and 30-μm-diameter

droplets for similar values of flow rate. The gain of the image intensifier

was reduced for larger droplet sizes (and lower droplet velocities) to minimize optical saturation effects, although these are still visible as extended

solid white areas in some of the larger droplet/lower velocity patterns.

Ananth and Wilson [10] have carried out a theoretical analysis of the gas

1.0 2.0 3.0 4.0 5.0 6.0

Flow rate (l/min)

8

12

16

20

25

30

Droplet size (

μm)

Figure 4. Spatial lightscattering patterns

from individual

droplets as function of

droplet size and

sample flow rate.
9

and particle velocities present in the APS delivery nozzle. Their results predict that at the normal APS operational flow rate of 5 l/min, the gas velocity

at the measurement point is approximately 160 m/s, and the velocity of a

10-μm spherical particle is approximately 50 m/s.

At the lowest flow rate and smallest droplet size (top left image in fig. 4),

the scattering pattern correlates closely with that predicted by the Mie theory

[11] for a perfect sphere. No droplet deformation is discernible from the

light-scattering data in this image. For increasing flow rate and/or droplet

size, however, the degree of droplet deformation would be evident from an

increasing ellipticity of the scattering maxima and minima evident on the

patterns. This behavior appears well behaved to a point, such as at 16 μm

× 5 l/min, when bright horizontal barring occurs on the image. This is predicted by the theoretical analysis described in volume 2 of this report [12].

This barring may occur either as a predominant central white bar or as a

series of parallel horizontal scattering fringes, such as those visible at 20 μm

× 5 l/min. Increasing the flow rate or droplet size further results in more

complex interference patterns within the images, such as those seen at 30 μm

× 5 l/min. This behavior appears to indicate that the droplets are undergoing a transition from simple oblate spheroidal form to the more complex

morphologies first observed by Baron [2]. It also indicates that the task of

attempting to rapidly assess the deformation of a single droplet by analysis

of its light-scattering pattern (as we desire for on-line correction of deformation effects in aerodynamic sizing) is nontrivial for droplets deformed

beyond a certain degree.

Figure 5 shows light-scattering pattern images for undistorted droplet diameters of 20, 25, and 30 μm, and total flow rates from 1 to 6 l/min. Adjacent to each scattering pattern is the corresponding droplet image. These

latter images indicate the changes in droplet shape that produce the increasingly complex scattering data. The essentially perfect spheres undergo flattening into oblate spheroids, and then become biconcave disks before forming menisci with concave upper surfaces. This morphological transition coincides with the dominant feature of the respective scattering patterns,

changing from a horizontal to a vertical bright band, the former possibly

the result of a morphology-dependent resonance in the droplet, and the

latter a function of the increasingly narrow horizontal cross section that the

droplet presents. We are currently investigating these effects further, with

the hope of elucidating the causes of the dominant scattering features.
10

20 25 30

Spherical droplet size (μm)

1.0

2.0

3.0

4.0

5.0

6.0

Direction of patricle motion

Flow rate (l/min)

Figure 5. Spatial lightscattering patterns

and droplet images

for 20-, 25-, and

30-μm-diameter

droplets at various

sample flow rates

through APS aerosol

delivery nozzle.
11

4. Quantifying Droplet Deformation

Inversion of the light-scattering data to yield droplet deformation descriptors is addressed in volume 2 of this report [12]. As a comparative measure,

a first-order estimate of droplet deformation under given flow conditions

may be obtained from the Feret ratio of the droplet image, defined as the

ratio of Feret diameters, i.e., the maximum distances between particle edges

in the x and y directions of the image (orthogonal and parallel to the particle trajectory, respectively). The edges of the droplets, E(x,y), were determined by a computer-based Gaussian edge-detection algorithm [13]. The

threshold value for this algorithm can be set manually or left to be determined automatically from the background intensity. Since the background

intensity is different for each image, we used the automatic threshold function. In addition, the graininess of the background can make the algorithm

detect erroneous edges there. However, the algorithm generally provides a

continuous line that is evident as an edge in the binary file that the algorithm creates. The Feret ratio can then be determined from this binary file.

Figure 6 shows a plot of Feret ratio versus flow rate for 20-μm oleic acid

droplets. As can be seen from the figure, the Feret ratio is well behaved up

to a 4.5-l/min flow rate. The reduction in the rate of increase of the Feret

ratio immediately after this point coincides with the droplet beginning to

assume a meniscus form (see fig. 5), for which the Feret ratio is less. The

change in the curve at the 4.5-l/min flow rate coincides with the point at

which the horizontal banding appears in the scattering pattern. A significant part of the experimental uncertainty in the Feret ratio measurements is

caused by a blurring of the droplet images in the direction of motion, especially at higher flow rates. Methods to minimize motion blur by reduction

of the laser pulsewidth are currently being explored.

Liquid droplet distortion produced in an accelerating flow field has been

computed through analytical solution of the Navier-Stokes equation by

Bartley et al [14], and this has led to a means of correlating observed droplet

deformation to such parameters as droplet diameter, viscosity, surface tension, and density. A high-Reynolds-number empirical approximation to the

pressure external to the droplet has been used in these calculations. This

procedure, described in detail by Bartley et al, correctly predicts the change

in droplet shape with increasing flow rate from spherical to spheroidal to

biconcave disk. For example, figure 7 shows the image of a 20-μm nominal

diameter oleic acid droplet, together with the corresponding computed cross

section. Beyond this level of distortion, the theoretically computed shape

pattern is not reliable.
12

0 5 10 15 μm 

5 μm

0

Figure 7. Image of

20-μm nominal

diameter oleic acid

droplet, together with

corresponding

computed crosssectional shape.

Computation used

density = 0.90 g/cm3,

viscosity = 0.256

Poise, and surface

tension = 32 mJ/m2.

5

4

3

2

1

0

Flow rate (l/min)

01234567

Feret ratio

Figure 6. Feret ratio

plotted as function of

flow rate for 20-μm

oleic acid droplets.
13

5. Droplets with Inclusions

During the process of collecting experimental light-scattering and particle

image data of the type described earlier, a phenomenon was observed that

has since been attributed to the presence of inclusions in the observed liquid droplets. This is of potential interest, because droplets with internal

bodies may occur in environments such as hospital buildings, where entities such as biological organisms can remain viable over long periods by

encapsulation within liquids. Similar conditions can apply in the dispersal

of biological warfare agents. As a consequence, there is interest in theoretically modeling the light-scattering behavior of such two-component droplets, with the objective of ultimately establishing a means of recognizing

their presence in an environment. Reproducible experimental data from

droplets with inclusions can thus provide a valuable resource for verifying

theoretical inversion methods.

As stated earlier, the VOAG droplets are normally generated into a dry atmosphere, typically <1 %RH. When 20-μm droplets of glycerin, in which

water is soluble, were generated into a moist atmosphere at 40 %RH, no

perturbations of the expected scattering patterns were observed (see

fig. 8a). However, when similar-sized droplets of oleic acid, in which water

is insoluble, were generated into the same atmosphere, light-scattering patterns of the type shown in figure 8b were consistently observed. These preliminary scattering data were recorded at a flow rate of 1 l/min.

The striking feature of the oleic acid pattern is the displaced secondary scattering fringes superimposed on the normal concentric rings from a homogeneous sphere. Figure 8c shows a droplet image from the same oleic acid

aerosol (although, for the reasons given in sect. 2.2, not from the same droplet as shown in fig. 8b); this image shows the presence of an internal dielectric discontinuity. The positions of the secondary scattering fringes relative

to those from the host droplet provide good evidence to support the hypothesis of an inclusion offset vertically upward from the center of the oleic

acid host, and, again, this is supported both by the reproducibility of the

images of the droplets (such as in fig. 8c) and by theoretical analysis [12].

This upward shift of the inclusion within the host may reasonably be assumed to be due to the density difference between the inclusion and host,

as is the case with water and oleic acid (0.998 compared to 0.895 g/cm3).

The higher density water inclusion moves toward the top of the host oleic

acid droplet when the droplet is accelerated downward within the airflow

through the APS delivery nozzle.
14

Figure 8. Scattering

patterns recorded

from droplets

generated in 40-%RH

environment:

(a) 20-μm-diameter

glycerin droplet with

no inclusion and

(b) 20-μm oleic acid

droplet with inclusion. Image

(c) shows type of

droplet giving rise to

scattering in image

(b).

(a)

(c)

(b)
15

These results are preliminary, and the exact mechanism by which the water

inclusion is created is not yet known. One hypothesis is that water vapor

condenses onto the original droplets and dissolves into the alcohol present.

Once the alcohol has evaporated, the water, which has a higher surface tension than oleic acid, forms a single droplet enveloped by the oleic acid.

Thermodynamic modeling of such a system is complex, but has been the

subject of recent work by other authors [15,16]. For smaller host droplet

sizes, the secondary scattering becomes distorted to yield downward bright

arms, as shown in figure 9. Theoretical analysis [12] suggests that this change

in scattering is commensurate with an increase in the size of the inclusion

relative to the host.

Figure 9. Scattering

pattern and image of

16-μm-diameter oleic

acid droplet

containing inclusion

with greater size

(relative to host

droplet) than that

shown in figure 8.
16

6. Discussion

The experimental data presented in this volume illustrate the complex spatial light-scattering behavior of droplets distorted by accelerating flow fields

and droplets containing inclusions. These data are, we believe, the first of

their type to be reported. The reproducibility of the experimental data makes

them a valuable resource in the development and testing of theoretical

inversion models. Such models may ultimately provide a route to the rapid

characterization and/or identification of complex droplet morphologies,

and this, in turn, could lead to advances in instrumentation for droplet aerodynamic sizing and for the detection of potential bioaerosol hazards in hospital and battlefield situations. In volume 2 of this report, numerically

calculated spatial light-scattering data are presented that indicate that theoretical inversion of the light-scattering data is achievable for a variety of

droplet morphologies relevant to the experimental conditions described here.

Acknowledgements

This work was carried out with funding from the UK Engineering and Physical Sciences Research Council (grant GR/L33375) and the National Institute of Occupational Safety and Health, and with generous support from

TSI, Inc., St. Paul, MN.
17

References

1. J. K. Agarwal, and R. J. Remiarz, Development of an Aerodynamic Particle Size

Analyser. National Institute of Occupational Safety and Health, USDHEWNIOSH contract report 210-80-0800, Cincinnati, OH (1981).

2. P. A. Baron, “Calibration and use of the aerodynamic particle sizer (APS

3300),” Aerosol Sci. Technol. 5, 55–67 (1986).

3. W. D. Griffiths, P. J. Iles, and N. P. Vaughan, “The behaviour of liquid droplets in an APS 3300,” J. Aerosol Sci. 17, 427–431 (1986).

4. S. Holler, Y. Pan, R. K. Chang, J. R. Bottiger, S. C. Hill, and D. B. Hillis, “Twodimensional angular optical scattering for the characterization of airborne

microparticles,” Opt. Lett. 23 (18), 1489–1491 (1998).

5. E. Hirst, P. H. Kaye, and J. R. Guppy, “Light scattering from non-spherical

airborne particles: Theoretical and experimental comparisons,” Appl. Opt.

33 (30), 7180–7187 (1994).

6. P. H. Kaye, K. Alexander-Buckley, E. Hirst, and S. Saunders, “A real-time

monitoring system for airborne particle shape and size analysis,” J. Geophys.

Res. (Atmospheres) 101 (D14), 19,215–19,221 (1996).

7. P. H. Kaye, E. Hirst, and Z. Wang-Thomas, “Neural-network based spatial

light-scattering instrument for hazardous airborne fiber detection,” Appl.

Opt. 36, 6149–6156 (1997).

8. K. Willeke and P. A. Baron, Aerosol Measurement: Principles, Techniques, and

Applications. Van Nostrand Reinhold, New York (1993).

9. W. J. Smith, Modern Optical Engineering, McGraw-Hill, New York, pp 439–

443 (1966).

10. G. Ananth and J. C. Wilson, “Theoretical analysis of the performance of the

TSI Aerodynamic Particle Sizer,” Aerosol Sci. Technol. 9, 189–199 (1988).

11. C. F. Bohren and D. R. Huffman, Absorption & Scattering of Light by Small

Particles. Wiley-InterScience, New York (1983).

12. G. Videen, W. Sun, Q. Fu, D. R. Secker, P. H. Kaye, R. S. Greenaway, E. Hirst,

and D. L. Bartley, Light Scattering From Deformed Droplets and Droplets with

Inclusions: Volume 2 Theoretical Results.” U.S. Army Research Laboratory, ARLTR-2228-II (September 2000).

13. MatLab, “The Mathworks, Inc.,” 5.3.0.10183 (R11) ed. (1999).

14. D. L. Bartley, A. B. Martinez, P. A. Baron, D. R. Secker, and E. Hirst, “Droplet

distortion in accelerating flow,” J. Aerosol Sci. (March 2000).
18

15. T. Vesala, M. Kulmala, R. Rudolf, A. Vrtala, and P. E. Wagner, “Models for

condensational growth and evaporation of binary aerosol particles,” J. Aerosol

Sci. 28, 565–598 (1997).

16. T. Mattila, M. Kulmala, and T. Vesala, “On the condensational growth of a

multicomponent droplet,” J. Aerosol Sci. 28, 553–564 (1997).
19

Distribution

Admnstr

Defns Techl Info Ctr

Attn DTIC-OCP

8725 John J Kingman Rd Ste 0944

FT Belvoir VA 22060-6218

DARPA

Attn S Welby

Attn Techl Lib

3701 N Fairfax Dr

Arlington VA 22203-1714

Dir of Defns Rsrch & Engrg

Attn DD TWP

Attn Engrg

The Pentagon

Washington DC 20301

Ofc of the Secy of Defns

Attn ODDRE (R&AT)

The Pentagon

Washington DC 20301-3080

Ofc of the Secy of Defns

Attn OUSD(A&T)/ODDR&E(R) R J Trew

3080 Defense Pentagon

Washington DC 20301-7100

Commanding Officer

Attn NMCB23

6205 Stuart Rd Ste 101

FT Belvoir VA 22060-5275

AMCOM MRDEC

Attn AMSMI-RD W C McCorkle

Redstone Arsenal AL 35898-5240

Dir for MANPRINT

Ofc of the Deputy Chief of Staff for Prsnnl

Attn J Hiller

The Pentagon Rm 2C733

Washington DC 20301-0300

Dir of Chem & Nuc Ops DA DCSOPS

Attn Techl Lib

Washington DC 20301

SMC/CZA

2435 Vela Way Ste 1613

El Segundo CA 90245-5500

TECOM

Attn AMSTE-CL

Aberdeen Proving Ground MD 21005-5057

US Army ARDEC

Attn AMSTA-AR-TD M Fisette

Bldg 1

Picatinny Arsenal NJ 07806-5000

US Army Engrg Div

Attn HNDED FD

PO Box 1500

Huntsville AL 35807

US Army Info Sys Engrg Cmnd

Attn AMSEL-IE-TD F Jenia

FT Huachuca AZ 85613-5300

US Army Natick RDEC Acting Techl Dir

Attn SBCN-T P Brandler

Natick MA 01760-5002

US Army NGIC

Attn Rsrch & Data Branch

220 7th Stret NE

Charlottesville VA 22901-5396

US Army Nuc & Cheml Agency

7150 Heller Loop Ste 101

Springfield VA 22150-3198

US Army Simulation, Train, & Instrmntn

Cmnd

Attn AMSTI-CG M Macedonia

Attn J Stahl

12350 Research Parkway

Orlando FL 32826-3726

US Army Soldier & Biol Chem Cmnd Dir of

Rsrch & Techlgy Dirctrt

Attn SMCCR-RS I G Resnick

Aberdeen Proving Ground MD 21010-5423

US Army Strtgc Defns Cmnd

Attn CSSD H MPL Techl Lib

PO Box 1500

Huntsville AL 35807
Distribution (cont’d)

20

US Army Tank-Automtv Cmnd Rsrch, Dev, &

Engrg Ctr

Attn AMSTA-TR J Chapin

Warren MI 48397-5000

US Army Train & Doctrine Cmnd

Battle Lab Integration & Techl Dirctrt

Attn ATCD-B

FT Monroe VA 23651-5850

US Military Academy

Mathematical Sci Ctr of Excellence

Attn MADN-MATH MAJ M Huber

Thayer Hall

West Point NY 10996-1786

Chief of Nav OPS Dept of the Navy

Attn OP 03EG

Washington DC 20350

Nav Surface Warfare Ctr

Attn Code B07 J Pennella

17320 Dahlgren Rd Bldg 1470 Rm 1101

Dahlgren VA 22448-5100

US Dept of Energy

Attn Techl Lib

Washington DC 20585

Central Intllgnc Agency

Dir DB Standard

Attn OSS/KPG/DHRT

1E61 OHB

Washington DC 20505

Hicks & Associates Inc

Attn G Singley III

1710 Goodrich Dr Ste 1300

McLean VA 22102

US Army Rsrch Ofc

Attn AMSRL-RO-D JCI Chang

Attn AMSRL-RO-EN W D Bach

Attn AMSRL-RO

PO Box 12211

Research Triangle Park NC 27709

US Army Rsrch Lab

Attn AMSRL-CI-AI-R Mail & Records Mgmt

Attn AMSRL-CI-AP Techl Pub (3 copies)

Attn AMSRL-CI-LL Techl Lib (3 copies)

Attn AMSRL-DD J M Miller

Attn AMSRL-IS-EE G Videen (10 copies)

Adelphi MD 20783-1197
1. AGENCY USE ONLY

8. PERFORMING ORGANIZATION

 REPORT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

12a. DISTRIBUTION/AVAILABILITY STATEMENT

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

6. AUTHOR(S)

REPORT DOCUMENTATION PAGE

2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

11. SUPPLEMENTARY NOTES

14. SUBJECT TERMS

13. ABSTRACT (Maximum 200 words)

Form Approved

OMB No. 0704-0188

(Leave blank)

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

12b. DISTRIBUTION CODE

15. NUMBER OF PAGES

16. PRICE CODE

17. SECURITY CLASSIFICATION

 OF REPORT

18. SECURITY CLASSIFICATION

 OF THIS PAGE

19. SECURITY CLASSIFICATION

 OF ABSTRACT 20. LIMITATION OF ABSTRACT

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

Light Scattering from Deformed Droplets and

Droplets with Inclusions:

Volume I. Experimental Results

September 2000 Final, FY99

This is the first of two volumes dealing with experimental and theoretical results from the

scattering of light by deformed liquid droplets and droplets with inclusions. Characterizing

droplet deformation could lead to improved measurement of droplet size by commercial

aerodynamic particle sizing instruments. Characterizing droplets with inclusions can be

important in some industrial, occupational, and military monitoring scenarios. The nozzle

assembly from a TSI aerodynamic particle sizer was used to provide the accelerating flow

conditions in which experimental data were recorded. A helium-neon (HeNe) laser was

employed to generate the light-scattering data, and an externally triggered, pulsed copper vapor

laser provided illumination for a droplet imaging system, arranged orthogonal to the HeNe

scattering axis. The observed droplet deformation correlates well over a limited acceleration

range, with theoretical predictions derived from analytical solution of the Navier-Stokes

equation.

Scattering, aerosols

Unclassified

ARL-TR-2228-I

7FEJ70

61110253A11

N/A

61102A

ARL PR:

AMS code:

DA PR:

PE:

Approved for public release;

distribution unlimited.

27

Unclassified Unclassified UL

2800 Powder Mill Road

Adelphi, MD 20783-1197

U.S. Army Research Laboratory

Attn: AMSRL-CI-EM gvideen@arl.army.mil

U.S. Army Research Laboratory

21

David R. Secker, Richard Greenaway, Paul H. Kaye,

Edwin Hirst (University of Hertfordshire, Hatfield, UK), David

Bartley (National Institute for Occupational Safety and Health,

Cincinnati, OH), Gorden Videen (ARL)

email:

2800 Powder Mill Road

Adelphi, MD 20783-1197