Ice Particle Experiments
Recent Publications
Laboratory Measurements of Light Scattering by Single
Levitated Ice Crystals
Neil J. Bacon and Brian D. Swanson
J. Atmos. Sci. 57 2094-2104 (2000)
Abstract
The authors have measured the differential
light-scattering cross sections and phase functions of single vapor-grown
hexagonal ice particles levitated in an
electrodynamic balance. The ice particles, grown at
temperatures $-5^\circ > T > -10^\circ$C, were typically
$\approx$ 50 $\mu$m in diameter and tended to orient with the $c$ axis
either nearly vertical (parallel to the scattering plane normal) or
horizontal (in the scattering plane). He-Ne laser light scattered by a
levitated crystal was collected in the angular ranges
20$^{\circ}$-65$^{\circ}$ and 115$^{\circ}$-160$^{\circ}$ with a
1024-element linear photodiode array with an angular resolution of about
0.05$^{\circ}$. The particle size and orientation were measured a few
seconds before
and after the scattering measurements
with top- and side-view video telemicroscopes. Three basic features are
found in the scattering from vertically aligned
crystals: (i) a strong ``halo" peak between about 21$^{\circ}$-35$^{\circ}$,
(ii) a secondary peak with ripple structure between
about 30$^\circ$ and $70^\circ$, and (iii) a weaker peak in the
backscatter between 115$^\circ - 160^\circ$. The ripple
structure is interpreted in terms of two-beam
interference and show that it provides a sensitive measure of crystal
dimensions. The experimentally measured peak positions are compared with a simple model, and the authors discuss the effects of surface roughness, crystal imperfections, and tilted ori
entations on the measured scattering cross sections.
Laboratory Measurements of Light Scattering by Single Ice
Particles
Neil J. Bacon, Brian
D. Swanson, Marcia B. Baker and E. James Davis (1998)
J. Aerosol Sci. Vol. 29, S1317--S1318
Electrodynamic Trapping and Manipulation of Ice Crystals
Brian
D. Swanson, Neil Bacon, E. James Davis and Marcia B. Baker, (1999)
Q. J. Roy. Meteor. Soc. 125, 1039-58
Abstract
We have developed a double-ring double-disk electrodynamic
balance (EDB) with humidity control provided by an internal thermal
diffusion chamber to study atmospherically important processes involving
10 -- 200 $\mu$m ice particles. The balance constants needed to
quantify the particle size and mass were
determined by a three-dimensional relaxation method calculation of the
non-axisymmetric electric fields, and by an experimental
measurement of the onset of particle stability using polystyrene latex (PSL)
microspheres of known size and mass. We have levitated frost particles
of various shapes and sizes under a range of atmospherically relevant
temperature and humidity conditions. The measured
growth and sublimation (g/s) rates are consistent with
predictions based on mass and thermal diffusion to equivalent spheres.
The EDB is shown to be a useful tool for
exploring single particle g/s rates and for measuring their light-scattering
characteristics.
The Breakup of Levitated Frost Particles
Neil J. Bacon,
Brian D. Swanson, Marcia B. Baker, and E. James Davis, (1998) J.
Geophys. Res. Vol. 103, 13,763-13,775
Abstract
We have studied the sublimation and breakup of single
$100-200$ $\mu$m frost particles
levitated electrically at temperatures in the range $-2^{\circ}$C to
$-30^{\circ}$C. Breakup rates were largely independent of temperature
and humidity in this range, but strongly dependent on particle shape.
Irregular particles of high aspect ratio were most likely to break up and
sublimation was characterized by an increasing aspect ratio. Heightened
sublimation rates of thin neck regions were not observed, in line with a
linearized sublimation model. Estimates of the forces involved suggest
that the breaking strength of these frost particles is considerably less
than that of bulk ice. We discuss possible implications of our results
for ice particle multiplication in clouds.
Characterization of the electric field and particle trapping in a double-ring
electrodynamic balance
C.L. Aardahl, R. Vehring, E.J. Davis, G.
Schweiger and B.D. Swanson, (1997) J. Aerosol Sci. Vol. 28,
1491-1505
Abstract
A new technique employing electrodynamic trapping of a pair of microparticles
has been explored for characterizing the electric field and particle stability
in a double-ring electrodynamic balance. The electric field in the
neighborhood of the nullpoint
was examined by comparing the oscillatory motion of the two-particle system
with a theoretical analysis. In addition, the relevant balance constants were
studied by five methods: (i) determination of the stabilization strength
constant, C1, using measurements on two-particle arrays, (ii) determination
of the levitation strength constant, C0, using measurements on single
particles of known mass and charge, (iii) computation of C1 and C0 by
solving the three-dimensional Laplace equation for the non-axisymmetric
electrode system, (iv) computation of C0 using a ring charge simulation
technique, and (v) determination of the ratio C1/C0 by measurements of the
marginal stability limit. The results of the different methods are compared
and shown to be consistent.
Experimental Measurements of Morphological Changes During Sublimation
and Growth of Levitated Ice Particles
B. D. Swanson, E. J. Davis and
M. B. Baker, Bull. Am. Phys. Soc. 41 (1996).
Laboratory Measurements of Ice Crystal Evaporation Rates
B. D.
Swanson and M. Baker, Proc. of Am. Met. Soc. Conf. on Cloud Physics,
Dallas, TX (1995).
This page is maintained by: Brian
Swanson
Last modified Oct. 14, 1999