Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

aries are observed to move if the vibration amplitude is large enough. Here, the lower boundary of the film is in contact with the
bulk suspension, and the mechanical vibration is coupled to the film through capillary waves excited on the bulk surface due to
the Faraday instability. A series of pictures in a figure show how typically grain boundaries evolve under vibration with a frequency
of about 100 Hz. The total length of the grain boundaries decreases, usually in a monotonic fashion, and finally disappears.
The detail process of grain boundary migration is in general rather complex, because the mobility of a grain boundary depends
not only on the relative orientation of the two grains, but also on the direction of the boundary. As a result the mobility is not a
constant but varies along the boundary. However, the evolution of grain boundaries under external forcing does show certain simple
features: (1) Small loops of defect lines are eliminated first, as seen in a figure, and the effect is somewhat similarly to domain
coarsening seen in phase-separating fluids. (2) Before the merging of two domains, the boundary between them becomes fuzzy
and discontinuous. Observations under the high power magnification show a rather continuous transition from the large- to the
small-angle grain boundaries. In the process, domains are seen to rotate coherently. (3) The elimination of small-angle grain
boundaries is much more rapid than the large-angle ones. The application of the external vibration not only offers a mean of studying
defect dynamics but also provides a robust way to grow high-quality 2D single colloidal crystals of size never achieved before.
Because the assembly process relies on the capillary force, particles of different materials and with various surface properties can
be crystallized in this way. Since the freely suspended film provides an environment similar to biological membranes, the technique
may be potentially useful for growing 2D crystals of membrane proteins, of which only a handful have been successfully
crystallized.
Author (revised)
Colloids; Crystallization; Defects; Grain Boundaries; Migration
20010024952 Harvard Univ., USA
Photonic Crystals Based on Self-Assembly of Colloidal Particles
Weitz, D., Harvard Univ., USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December
2000, pp. 1183-1189; In English; See also 20010024890; No Copyright; Avail: CASI; A02, Hardcopy; A10, Microfiche
Photonic crystals are systems in which the refractive index periodically changes in space. These structures show interesting
optical properties which may lead to the suppression of propagation of light within a certain frequency range. During the last years
there have been strong efforts to fabricate such materials with spatial periodicities lying in range of the wavelength of visible light.
One possible approach to fabricate these materials is the use of colloidal particles. We use self-organization of colloidal particles
to produce these periodic structures. by using organic and inorganic materials we control both the elastic and scattering properties
of the particles and the lattice. We determine the structures and optical properties of the colloidal crystal by optical microscopy
and light scattering experiments. by performing light scattering measurements in the optical microscope simultaneously with
imaging, we study the scattering from a region with known defect structure. These new materials have great potential for a variety
of optical communication devices. Possible technical applications are discussed.
Author (revised)
Colloids; Optical Properties; Scattering; Crystals; Photonics
20010024953 NASA Glenn Research Center, Cleveland, OH USA
Gravitational Effects on Flow Instability and Transition in Low Density Jets
Agrawal A. K., Oklahoma Univ., USA; Parthasarathy, K., Oklahoma Univ., USA; Pasumarthi, K., Oklahoma Univ., USA; Griffin,
D. W., NASA Glenn Research Center, USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference;
December 2000, pp. 1190-1192; In English; See also 20010024890; No Copyright; Abstract Only; Available from CASI
only as part of the entire parent document
Recent experiments have shown that low-density gas jets injected into a high-density ambient gas undergo an instability
mode, leading to highly-periodic oscillations in the flow-field for certain conditions. The transition from laminar to turbulent flow
in these jets is abrupt, without the gradual change in scales. Even the fine scale turbulent structure repeats itself with extreme regularity
from cycle to cycle. Similar observations were obtained in buoyancy-dominated and momentum-dominated jets characterized
by the Richardson numbers, Ri = [gD(rho(sub a)-rho(sub j))/rho(sub j)U(sub j)(exp 2) ] where g is the gravitational
acceleration, D is the jet diameter, rho(sub a) and rho(sub a) are, respectively, the free-stream and jet densities, and U(sub j) is
the mean jet exit velocity. At high Richardson numbers, the instability is presumably caused by buoyancy since the flow-oscillation
frequency (f) or the Strouhal number, St = [fD/U(sub j)] scales with Ri. In momentum-dominated jets, however, the Strouhal
number of the oscillating flow is relatively independent of the Ri. In this case, a local absolute instability is predicted in the potential
core of low-density jets with S [= rho(sub j)/rho(sub a)] is less than 0.7, which agrees qualitatively with experiments. Although
the instability in gas jets of high Richardson numbers is attributed to buoyancy, direct physical evidence has not been acquired
in experiments. If the instability is indeed caused by buoyancy, the near-field flow structure of the jet will change significantly
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