Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

the inorganic synthesis using pressure as an additional process variable, existing synthesis strategies may benefit from enhanced
control of the reaction process, and entirely new strategies may be contemplated.
Author (revised)
Drops (Liquids); Emulsions; Diffusion Coefficient; Pressure Effects; Microstructure
20010024950 Oklahoma State Univ., Dept. of Physics, Stillwater, OK USA
Non-Equilibrium Particle Configuration in Sedimentation
Lei, X.-Y., Oklahoma State Univ., USA; Ackerson, B. J., Oklahoma State Univ., USA; Tong, P., Oklahoma State Univ., USA;
Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December 2000, pp. 1168-1170; In
English; See also 20010024890; No Copyright; Abstract Only; Available from CASI only as part of the entire parent document
In the study of the settling of large non-Brownian particles, one is interested in understanding the effect of the long-range
hydrodynamic interactions on the microstructures formed in the settling suspension and finding the direct link between the microscopic
particle configuration and the macroscopic properties of the suspension, such as the mean sedimentation velocity and the
velocity fluctuations (the rms velocity) as a function of the particle concentration. We have recently proposed that statistical or
spontaneous number fluctuations of the particles can produce regions having more (or less) particles than the average. These
heavier (or lighter) regions are convectively unstable, when the size becomes larger than the diffusive dissipation length delta(sub
d). Using the Rayleigh criterion, the value of delta(sub d) is calculated and the result agrees with experiment. Above the length
delta(sub d), a set of coarse-grained equations of motion is used to describe the concentration and velocity fluctuations in sedimentation.
The model provides a physical interpretation for the emergence of a velocity cut-off length, which prevents hydrodynamic
dispersion coefficients from being divergent. to test these theoretical ideas, we conduct measurements of the particle configuration
in a settling suspension of large non-Brownian particles. The particles used in the experiment are 40 microns diameter polymer
latex spheres suspended in polyalkylene glycol at the volume fraction 0.4%. A sheet of laser light is used to illuminate a vertical
section of the settling suspension and images of the particles are obtained using a CCD camera situated normal to the illuminating
light sheet. A figure shows a typical particle configuration in sedimentation. Superimposing a series of such pictures taken at
15-second intervals shows a turbulent looking velocity field with rather large velocity fluctuations. The particle configurations
in sedimentation look random but the number fluctuations of the particles are found to be suppressed when compared with Poisson
statistics for pure random configurations. A figure compares typical number fluctuations (number of particles in a circle of fixed
area) measured in sedimentation (upper curve) with those in pure random configurations (lower curve). Systematic statistical analysis
shows that the number fluctuations of the settling particles become more and more suppressed with time. At the later stage
of the settling process, the suppression is most striking at large length scales. These findings are consistent with the notion that
the convective response of the suspension to ordinary number fluctuations reduces and eliminates them, bringing the suspension
to a marginally stable state, in which the statistics are such that unstable convection is minimized.
Author (revised)
Sediments; Settling; Microparticles; Dispersions; Suspending (Mixing)
20010024951 Pittsburgh Univ., Dept. of Physics and Astronomy, Pittsburgh, PA USA
Dynamics of Grain Boundaries in 2D Colloids
Wei, Q.-H., Pittsburgh Univ., USA; Wu, X.-L., Pittsburgh Univ., USA; Proceedings of the Fifth Microgravity Fluid Physics and
Transport Phenomena Conference; December 2000, pp. 1171-1182; In English; See also 20010024890; No Copyright; Avail:
CASI; A03, Hardcopy; A10, Microfiche
Grain boundaries are the longest known crystal defects and the development of grain structures plays a critical role in the
properties of many kinds of materials such as ceramics, metals, and semiconductors. While the equilibrium structures of grain
boundaries have been known very well to atomic resolution through electron microscopy and soap bubble model studies, the
dynamics process of grain boundary migration driven by external forces or thermal fluctuation is not well established. In this presentation,
we report the non-equilibrium behavior of grain boundaries in a model system, consisting of a monolayer of colloidal
particles confined in a soap film. A great advantage of this model system is that structure and dynamics of grain boundaries can
be studied instantaneously. The 2D film is formed by drawing a thin plastic frame with a hole in the center from a bulk monodispersed
colloidal suspension. Due to the gravity and water evaporation, the thickness of the soap film becomes comparable to the
particle diameter, leading to the curving of the water-air interface. This curved meniscus between particles poses an attractive
interaction between colloidal particles, which are otherwise hard-sphere like. This attractive capillary force causes the colloidal
particles to self-assemble forming crystals which are usually polycrystalline with an average grain size up to hundreds of lattice
constants, as shown. Using a larger magnification, one can easily distinguish large- and small-angle grain boundaries, which, as
indicated respectively by L and S as seen in an accompanying figure, are different from those commonly seen in textbooks. An
interesting observation is that when an external periodic vertical vibration is applied to the particle suspension, the grain bound-
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