Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

apply either steady shear flow perpendicular to the stretching direction, or oscillatory squeeze flow parallel to stretching. We
observe that oscillatory, parallel pre-straining accelerates strain hardening in dilute polystyrene-based polymer solutions, while
steady, orthogonal preshearing delays strain hardening. These results are qualitatively consistent with the predictions of the
finitely extensible nonlinear elastic (FENE) dumbbell model. Recent experimental and numerical studies of stretching DNA
chains by Chu & co-workers show that the transient evolution of the polymeric microstructure is very sensitive to the initial configuration
of the chains. Larson recently reported results of Brownian dynamics simulations investigating the effect of preshearing
on the molecular configuration of the chain. The simulations showed that preshearing reduces the occurrence of folds and kinks
in the chain conformations observed during subsequent uniaxial elongation. Simple shear flow perpendicular to the stretching
direction yielded a faster approach of the transient tensile stresses to the steady state plateau. The filament stretching rheometer
presents new possibilities for investigating preshearing effects more quantitatively than has previously been possible. We examine
the effects of (1) steady shear flow perpendicular to the stretching direction, and (2) oscillatory squeeze flow parallel to stretching,
on the transient stress growth in a dilute polymer solution.
Author (revised)
Deformation; Elongation; Microgravity; Polystyrene; Tensile Stress; Hardening (Materials); Stretching
20010025021 National Inst. of Standards and Technology, Process Measurements Div., Gaithersburg, MD USA
Acoustic Study of Critical Phenomena in Microgravity
Moldover, M. R., National Inst. of Standards and Technology, USA; Gillis, K. A., National Inst. of Standards and Technology,
USA; Zimmerli, G. A., National Center for Microgravity Research on Fluids and Combusiton, USA; Proceedings of the Fifth
Microgravity Fluid Physics and Transport Phenomena Conference; December 2000, pp. 1888-1907; In English; See also
20010024890; No Copyright; Avail: CASI; A03, Hardcopy; A10, Microfiche
We plan an acoustic study of xenon near its liquid-vapor critical point in microgravity. We will use a novel, compact acoustic
resonator in microgravity to determine both the speed of sound c and the bulk viscosity zeta. In lowest order, the resonance frequencies
determine c and the ”quality factors” of the resonance modes determine zeta. In microgravity, the data for zeta will be
a factor of 60 closer to the critical temperature T(sub c) than is possible on earth. The microgravity data will provide c the most
rigorous possible test of the fundamental theory for zeta, i.e. the theory for the time-correlations of pairs of critical fluctuations.
by using both Helmholtz and organ-pipe modes of the same compact resonator, the acoustic data will span a 30:1 frequency ratio
at unusually low frequencies: 120 Hz to 4 kHz. Within 6 mK of T(sub c), where the reduced temperature t is identical to (T - T(sub
c))/T(sub c) is less than 2x10(exp -5) , the critical fluctuations are so slow that the equilibrium condition: (acoustic frequency)
x (fluctuation lifetime) is much less than 1 is violated, even at 120 Hz. In this very-near-critical region, c is frequency-dependent
and the attenuation of sound is dominated by zeta. Further from the critical point, where t is greater than 2x10(exp -5) , the low-frequency
speed-of-sound data obtained in microgravity will determine the constant-volume heat capacity C(sub V), perhaps with
higher resolution than direct measurements of C(sub v). If so, the acoustic data will further constrain the theory for the ”crossover”
of C(xub v) from an asymptotic divergence with the critical exponent (alpha) to non-critical behavior. The quality-factor data will
test the theory of the thermal boundary layer in heretofore inaccessible ranges of frequency and fluid properties. The planned
experiment will exploit hardware that two of PIs (Moldover and Zimmerli) helped develop for the Critical Viscosity of Xenon
(CVX) experiment. The CVX flight package consisted of two ”Hitchhiker ” canisters that can be accommodated by many Shuttle
missions. In 1997, the package was flown in the cargo bay of the shuttle Discovery (STS 85). The package successfully measured
the shear viscosity of xenon 100 times closer to T(sub c) than ground-based experiments and it produced the first reliable measurements
of visco-elasticity near any critical point. With minor modifications, the proposed experiment will use the CVX flight thermostat
and hardware for temperature measurement and control. Two of the PIs (Gillis and Moldover) developed the double
Helmholtz resonator (now called the ”Greenspan acoustic viscometer” ) to measure the viscosities of gases accurately. to date,
no acoustic viscometer has been operated with a near-critical fluid. Thus, the first task is to design an acoustic viscometer for use
with a near-critical sample of xenon. This may require modifying electro-acoustic transducers and developing a surface coating
for the viscometer’ s interior. Extensive tests will be required to verify that the viscometer operates in accord with the theory for
the instrument. Then, the viscometer will be used to make the best possible measurements of c and zeta on earth over the full range
of accessible frequencies. These measurements will be used to define the microgravity experiment and they will be used to
compare ground-based and microgravity data.
Author (revised)
Acoustic Properties; Critical Point; Microgravity; Viscosity; Xenon; Liquid-Vapor Interfaces; Acoustics
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