Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

along the heater surface and departed with smaller diameter at the same gravity level in the direction normal to the surface. For
PF5060, even a very small horizontal gravitational component caused the sliding of bubble along the surface. The numerical simulation
has been carried out by solving under the condition of axisymmetry, the mass, momentum, and energy equations for the
vapor and the liquid phases. In the model the contribution of micro-layer has been included and instantaneous shape of the evolving
vapor-liquid interface is determined from the analysis. Consistent with the experimental results, it is found that effect of reduced
gravity is to stretch the growth period and bubble diameter It is found that effect of reduced gravity is to stretch the growth period
and bubble diameter at departure. The numerical simulations are in good agreement with the experimental data for both the departure
diameters and the growth periods. In the study on dynamics of multiple bubbles, horizontal merger of 2,3 4,and 5 bubbles
was observed. It is found that after merger of 2 and 3 bubbles the equivalent diameter of the detached bubble is smaller than that
of a single bubble departing at the same gravity level. During and after bubble merger, liquid still fills the space between the vapor
stems so as to form mushroom type bubbles. The experimental and numerical studies conducted so far have brought us a step closer
to prediction of nucleate boiling heat fluxes under low gravity conditions. Preparations for a space flight are continuing.
Author (revised)
Bubbles; Gravitational Effects; Heat Transfer; Microgravity; Nucleate Boiling
20010024898 Princeton Univ., Depts. of Physics and Chemical Engin. and Princeton Materials Inst., NJ USA
Physics of Hard Spheres Experiment: Microscopy of Colloidal Particles
Ruiz, J. C., Princeton Univ., USA; Megens, M., Princeton Univ., USA; Hollingsworth, A. D., Princeton Univ., USA; Harrison,
C., Princeton Univ., USA; Russell, W. B., Princeton Univ., USA; Chaikin, P., Princeton Univ., USA; Cheng, Z.-D., Princeton
Univ., USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December 2000, pp.
215-251; In English; See also 20010024890; No Copyright; Avail: CASI; A03, Hardcopy; A10, Microfiche
In preparation for the next phase of the Physics of Hard sphere experiment, an investigation of hard sphere nucleation and
growth using a specially designed microscope on the space station, we have developed new colloidal particles, as well as some
new techniques. The new colloidal systems are flourescently dyed, index and density matched spheres with screened and
unscreened electrostatic interactions, as well as microlithographically prepared disks. Confocal imaging of the nucleation process
shows dominant surface nucleation with an amorphous first layer and then well defined crystallite propagation into the bulk. We
have studied the nucleation and growth of colloidal crystals in the confined geometry of a 150 micron thick sample between slide
and cover slip in a confocal microscope. The samples were PMMA-PHSA stabilized .956 micron spheres fluorescently labeled
with fluorescene using a modification of the techniques developed in ref.1. The initially dyed and washed particles were found
to be charged when suspended in the index and density matching solvent decalin-tetralin-cycloheptylbromide. Even in this highly
non-polar solvent the coulomb interactions could be effectively screened using the organic salt Tin(II) 2-ethylhexanoate. The confocal
images shown correspond to ”slices” 1, 4 and 48 microns from the cover slip, for a sample with volume fraction 0.52, 60
minutes after homogenization. What is striking is the low density and amorphous character of the first layer, the crystallinity of
the forth layer, and the mixed crystal-liquid character of the 50th layer. The crystallites have clearly nucleated on the surface but
the layer closest to the surface is not crystalline. This indicates the two, not necessarily constructive, effects of the surface. One
is the wettability, the other is the constraint on particle motion. The excluded volume interaction is repulsive depleting the layer
closest to the wall and leaving it below the freezing transition. The next layer has a higher volume fraction but still has restricted
motion in the direction perpendicular to the wall. This is where the crystal first nucleates. With confocal microscopy we can scan
the volume of the cell with an area of 200x200 microns squared and a depth of 60 microns in a 5 minutes. We can then follow
the growth of the crystal liquid front as it propagates into the bulk. The observed front growth goes from 1 micron/10minutes to
1 micron/min as the volume fraction changes from .50 to .60. In the coexistence region the nucleation proceeds incompletely leaving
a metastable boundary between the crystal and liquid phases. We show the three dimensional structure of the crystallites in
this situation. The crystal structure of each slice of the sample has been analyzed by an algorithm which locates the particles, finds
the local crystal structure and the direction of the crystal axes and assigns the next layer to the same crystal if its orientation is
within 5 degrees of the underlying layer. (There is no distinction made here for FCC vs HCP, the structures all conforming to
RHCP). The crystals grow epitaxially from the substrate layer with the initial (2 nd layer) domain structure unannealed in the
growth process. The stalagmite shape of the crystallites indicates the dendritic growth instability. Since the design of the flight
microscope incorporates sample cells with submicroliter capacity, our initial experiments showed that extensive investigations
of colloidal systems could be done with miniscule (by conventional standards) quantities of material. It then became practical to
design our own non-spherical, colloidal particles using variations of optical lithography. On a three inch wafer it is possible to
make approx. 2 microliters of colloidal particles at volume fraction 0.5. Depending on the size and shape this amounts to approx.
300,000,000 particles, well beyond numerical simulation capabilities. The example presented in the figure is a suspension of 4
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