Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

Foams could be used very extensively in space (liquid foams in everything from shaving cream, to fire extinguishers, and
solid foams in thermal insulation, anti-explosion fuel tanks, and structural support elements). One interesting question is whether
liquid foams under gravity differ in their fundamental physics from foams in microgravity conditions. Foam experiments during
parabolic flights have shown new phenomena. An initially dry foam sucks liquid from a reservoir located below the foam and
an upwards moving wetting front develops. The liquid now redistributes inside the foam, while preserving the total wetness. As
a result, the membrane thickness distribution will differ from that on Earth, which will change the stability and viscosity of the
foam. A new scenario for foam aging could develop in microgravity. Possibly, foam could reach a new metastable configuration,
containing several large bubbles with thick membranes. For the foam to reach the global, no bubble, equilibrium (which is the
only final state for foam evolution on the Earth) will require almost infinite time, suggesting that defoaming could be a real problem
in microgravity. Once created, foam could persist almost forever.
Author (revised)
Foams; Microgravity; Life (Durability); Experimentation
20010024944 NASA Glenn Research Center, Cleveland, OH USA
Phase-Shifting Liquid Crystal Point-Diffraction Interferometry
Griffin, DeVon W., NASA Glenn Research Center, USA; Marshall, Kenneth L., Rochester Univ., USA; Mercer, Carolyn R.,
NASA Glenn Research Center, USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference;
December 2000, pp. 1137-1139; In English; See also 20010024890; No Copyright; Abstract Only; Available from CASI only as
part of the entire parent document
Microgravity fluid physics experiments frequently measure concentration and temperature. Interferometers such as the Twyman
Green illustrated have performed full-field measurement of these quantities. As with most such devices, this interferometer
uses a reference path that is not common with the path through the test section. Recombination of the test and reference wavefronts
produces interference fringes. Unfortunately, in order to obtain stable fringes, the alignment of both the test and reference paths
must be maintained to within a fraction of the wavelength of the light being used for the measurement. Otherwise, the fringes will
shift and may disappear. Because these interferometers are extremely sensitive to bumping, jarring and transmitted vibration, they
are typically mounted on optical isolation tables. Schlieren deflectometers or the more recent Shack-Hartmann wavefront sensors
also measure concentration and temperature in laboratory fluid flows. Ray optics describe the operation of both devices. In a
schlieren system, an expanded, collimated beam passes through a test section where refractive index gradients deflect rays. A lens
focuses the beam to a filter placed in the rear focal plane of the decollimating lens. In a quantitative color schlieren system, gradients
in the index of refraction appear as colors in the field of view due to the action of the color filter. Since sensitivity is a function
of the focal length of the decollimating lens, these systems are rather long and filter fabrication and calibration is rather difficult.
A Shack-Hartmann wavefront sensor is an array of small lenslets. Typical diameters are on the order of a few hundred microns.
Since these lenslets divide the test section into resolution elements, the spatial resolution can be no smaller than an individual
lenslet. Such a device was recently used to perform high-speed tomography of heated air exiting a 1.27 cm diameter nozzle. While
these wavefront sensors are very compact, the limited spatial resolution and the methods required for data reduction suggest that
a more useful instrument needs to be developed. The category of interferometers known as common path interferometers can eliminate
much of the vibration sensitivity associated with traditional interferometry as described above. In these devices, division
of the amplitude of the wavefront following the test section produces the reference beam. Examples of these instruments include
shearing and point diffraction interferometers. In the latter case, shown schematically, a lens focuses light passing through the test
section onto a small diffracting object. Such objects are typically either a circle of material on a high quality glass plate or a small
sphere in a glass cell. The size of the focused spot is several times larger than the object so that the light not intercepted by the
diffracting object forms the test beam while the diffracted light generates a spherical reference beam. While this configuration
is mechanically stable, phase shifting one beam with respect to the other is difficult due to the common path. Phase shifting enables
extremely accurate measurements of the phase of the interferogram using only gray scale intensity measurements and is the de
facto standard of industry. Mercer and Creath 2 demonstrated phase shifting in a point diffraction interferometer using a spherical
spacer in a liquid crystal cell as the diffracting object. by changing the voltage across the cell, they were able to shift the phase
of the undiffracted beam relative to the reference beam generated by diffraction from the sphere. While they applied this technology
to fluid measurements, the device shifted phase so slowly that it was not useful for studying transient phenomena. We have
identified several technical problems that precluded operation of the device at video frame rates and intend to solve them to produce
a phase-shifting liquid crystal point-diffraction interferometer operating at video frame rates. The first task is to produce high
contrast fringes. Since the diffracted beam is much weaker than the transmitted beam, interferograms have poor contrast unless
a dye is added to the liquid crystal to reduce the intensity of the undiffracted light. Dyes previously used were not rigorously characterized
and suffered from hysteresis in both the initial alignment state of the device and the electro-optic switching characteristics.
Hence, our initial effort will identify and characterize dyes that do not suffer from these difficulties and are readily soluble in the
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