Experiments with the wetting behavior of immiscible fluids against the container below the critical temperature are being performed in the MSFC Drop Tower Facility. Microgravity conditions extending up to three seconds (of the 4.5 second drop) are generated for the experiment. Specimens consist of glass cylindrical ampoules partially filled with fluid phases. How the fluids develop the meniscus geometry as well as hot the fluid interfaces respond to the microgravity induced oscillations is recorded during the experiment with on-board cameras. Drops are made at various temperatures to determine the interfacial energy variation as a function of temperature

Kaulker, William F.

English

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CRITICAL POINT WETTING DROP TOWER EXPERIMENT
Contract No. NAG8-511
Dated 1988
Semi-Annual Report
JNASA-CR-183062) CRITICAL POINT WETTING N88-26603
DROP TOWER EXPERIMENT Semiannual Report
(Alabama Univ.) 10-p CSCL 20D
Unclas
I G3/34 . 0149310 --
By
Dr. William F. Kaulker, Ph.D
Principal Investigator
School of Science
Chemistry Department
The University of Alabama in Huntsville
Huntsville, Alabama 35899
Prepared for
National Aeronautics and Space Administration
George C. Marshall Space Flight Center
Attn: Mr. Richard E. Black/JA6k
Marshall Space Flight Center, Alabama 35812
July, 1988
https://ntrs.nasa.gov/search.jsp?R=19880017219 2020-03-20T05:31:55+00:00Z
CRITICAL POINT WETTING DROP TOWER EXPERIMENT
SEMIANNUAL REPORT 1988
•
William F. Kaukler
Department of Chemistry, The University of Alabama in
Huntsville, Huntsville, Alabama 35899
Introduction
Experiments with the wetting behavior of immiscible fluids
against the container below the critical: temperature are being
performed in the MSFC Drop Tower facility. Microgravity condi-
tions extending up to three seconds (of the 4.5 second drop)
are generated for the experiment. Specimens consist of glass
cylindrical ampoules partially filled with fluid phases. How
the fluids develop the meniscus geometry as well as how the
fluid interfaces respond to the microgravity induced oscilla-
tions is recorded during the experiment with on-board cameras.
Drops are made at various temperatures to determine the inter-
facial energy variation as a function of temperature.
This work originally began several years ago, but with
endless delays in getting experiments performed the grant has
been no-cost extended until July 1989. There are results of
some value. Recent facility improvements should increase the
quality of the data during the next year. During these years,
the experiment has been modified, improved and refined with
some initial objectives canceled due to funding and time.
MSFC Drop Tower Facility
The Experiments described here rely on the character of
the gravity environment produced by the Drop Tower. Initially,
the experiment package (270 kg and Ixlxl meter dimensions) sits
within the drag shield at the top of the tower (100 meters
nominal). Upon release, a pressurized gas rocket thruster ac-
celerates the (over 1000 kg) drag shield such that the experi-
ment package floats up from the drag shield floor (about 5 cm).
Microgravity conditions within the experiment begin at this
point and have levels reaching 10~3 to 10~5 g. The thruster
ensures the air drag doesn't influence the experiment and keeps
the drag shield falling with the rate of 1 g.
The drag shield is decelerated by a catch tube which per-
mits a controlled release of compressed air as the close fit-
ting drag shield enters the tube. The package settles on the
floor and finally all comes to rest with up to a maximum of 25
g deceleration.
Experiment Summary
During the drop, accelerometer data and photographic
recording of the fluid phenomena is taken for later analysis.
Most of this work has been performed with the one well
documented immiscible system having a TM/ the perfluoromethyl-
cyclohexane and isopropanol (P-I) system found by Schmidt and
Moldover. The critical temperature, T0, is 90.5 *0.5°C, and
the TM was found to be 38.0 *0.1°C. At temperatures above Tw,
the more dense perfluoromethylcyclohexane-rich phase (with
density of 1.768 g/cm at 22°C) will form a wetting layer be-
tween the vapor phase and the upper, less dense isopropanol-
rich phase (of density 0.826 g/cm at 22°C). Experiments were
performed in the temperature range 35 to 55 °C, encompassing
the Tw.
Results
At first, the contact angle between the interfluid phases
and the container wall as well as the overall meniscus shape
were the most important sources of proof that a wetting transi-
tion had occurred.. Due to the presence of small lateral g-.
forces during a drop, the meniscus shape determinations are not
a sufficiently sensitive test for the critical wetting condi-
tion (in the P-I system).
An observation was made of a subtle rate of oscillation
period change when the temperature was raised from below TM to
above Tw. Displacement of the mid-section or apex of the fluid
interface results in a symmetrical, opposing movement of the
contact line against.the ampoule wall since there is no volume
change. See Figure 1. A dashed line marks the meniscus
profile at one-g. The sides of the graph represent the ampoule
walls. The inter-fluid interfaces 'slide' up and down along
the ampoule wall in response to the pulse change in g-level.
The period of oscillation was found not to remain constant with
each cycle. As the oscillations damped out, the velocity of
the contact line on the ampoule wall also diminishes and so the
dissipation from the contact line 'friction' will be considered
as a function of its velocity. As it is the wetting conditions
at the wall which is being tested, altering the wetting condi-
tions should affect the oscillation rate and damping of the os-
cillations.
Contact Line Phenomena
The consideration now is directed to dynamic contact
angles, those established during the movement of the fluid in-
terface. Experiments have shown there is a contact angle de-
pendence on contact line velocity. The second Figure was drawn
using data from>a paper by Johnson (R. E. Johnson Jr., R. E.
Dettre and D. A". Brandreth, " Dynamic Contact Angles and Con-
tact Angle Hysteresis", J. Coll. and Int. Sci. 62(2) (1977)
205-212. ) The static contact angle hysteresis (CAH) limits are
the two values of 8 where U = zero. More significantly, there
is a variation in G with velocity. Young and Davis (G. W.
Young and S. H. Davis, paper submitted to J. of Fluid
Mechanics, "A plate oscillating across a liquid interface: ef-
fects of contact angle hysteresis") found that both the CAH and
steepening of the contact angle with increasing contact line
velocity are dissipative effects. For example, wave amplitude
against a vertical wall (gravity waves) is dampened not only by
viscosity but also by the contact line 'friction' against the
wall.
If a wetting layer of lower phase is formed between the
upper phase and the container and/or vapor (at temperatures
above Tw), then the contact line 'friction1 should be different
from that for partially wetting conditions found at tempera-
tures below Tw. This layer of the lower phase separates the
upper phase from the glass and vapor. The contact line which
sweeps up and down is no longer the tri-junction between the
upper fluid, lower fluid and the glass phases.
It is known that fully wetting fluids do not have contact
angle hysteresis (as the contact angle is zero). We cannot ex-
pect the magnitude of the dissipative effects at the contact
line to be the same during partial wetting conditions. Young
and Davis state that the dissipative effect of contact angle
steepening with increased contact line speed is suppressed for
cases of fixed contact angle and fixed contact line when the
contact line is independent of contact line speed. Full wet-
ting means fixed contact angle. Increased dissipation of the
meniscus oscillation is therefore expected for T < Tw.
Differentiating the dissipative effect of viscosity from
that of contact line 'friction' is possible when experiments
are performed with conditions such that the rate of relaxation
of the fluid at the solid-liquid-gas junction is much greater
than the velocity. Johnson et al could ignore viscosity ef-
fects for their experiments because their conditions satisfied
the above assumption.
This Experiment
Interfacial free energy, viscosity and density difference
all diminish as temperature increases. There is no singularity
in these functions. CPW is a first-order wetting transition
which should mr.nifest a singularity in Young's equation or in
fluid layer thickness. By using the meniscus oscillations and
comparing them in the temperature regime about Tw, the wetting
transition should lead to a singularity in the damping behavior
of these oscillations. There has been qualitative evidence to
indicate there is an effect. To date, there is too little data
of sufficient quality to confirm the theory.
There is clear evidence that the damping of the oscilla-
tion wave amplitude is greater for T < Tw in the P-I system.
This might be attributed to the increased viscosity. Measure-
ments or calculations will need to be made to verify that vis-
cosity is not the sole dissipation mechanism. Alternately,
more data can be collected to permit an empirical curve to be
drawn showing the damping factor as a function of temperature.
A difference in period of oscillation has been observed
between temperatures above and below the Tw. Below TM the
frequency of oscillation is slightly higher. This effect may
be due to the increase in interfacial tension as temperature is
decreased. There is not enough data to fit curves which could
show any more than this. :
The observation that initiated the contact line 'friction'
concept is a very subtle difference in the rate the period
changes as the meniscus oscillates during a drop between the
temperatures above and below Tw. At T < Tw, the period of os-
cillation decreases constantly while the amplitude is damped.
At T > Tw, the period remains constant. Note that this is a
different effect than damping with constant fundamental
frequency. Measurements will be needed to ensure viscous
relaxation of the fluid is more rapid than the contact line
velocity. Contact line velocity has been measured to have a
maximum of 2.6 mm/sec. This value places it roughly in the
non-viscous controlled domain. The observation is not con-
firmed because to clearly show it, the oscillation data needs
to be improved.
Experiment Status to Date
The major delays have been with the tower itself. After a
year of struggling to get the video telemetry system working,
it now seems to function as it should have. Clearly, the sys-
tem was defective as received from the manufacturer.
A drop was made just a few days ago where the whole video
system was functionally operative. Adjustments are neccesary
to permit data collection, but these little problems will be
ironed out over the next month. Video from the experiment was
received and recorded from prior to the release of the package
to the impact point. A picture was received for nearly the
whole duration of the drop except when the drag shield encoun-
tered a rough spot on the guide rails. This must be repaired
soon. As a result, the picture was distorted or briefly inter-
rupted near the end of the drop. From the video recording it
was possible to confirm that although the drag shield was
knocked about by hitting the rail, the experiment inside was
not affected since it was adequately levitated within. When
the drag shield hit the rail very hard, the laser telemetry was
knocked out off alignment briefly since it is directly mounted
to the top of the drag shield.
From all the drops made this last year, only one (DT 7-87)
provided the accelerometer data for both recording methods.
The radio telemetered data and the on board Geo-tek system were
both working properly such that a comparison between them could
finally be made. Since the sampling rates were not equal and
not synchronized, a one-for-one correspondence cannot be made.
The qualitative match between the waveforms was very good.
Quantitatively, only the steady lo-g portion of the drop can be
compared. The NASA Lo G Accelerometers cannot provide the
unity-g response during the release of the drag shield. Geo-
tek data cannot be taken with as high a resolution as the NASA
accelerometers can (8 bits versus 24). Both accelerometer data
sets showed good agreement of g-level (3 X 10~3 g for both)
given the resolution. In order to record the quality of the
release acceleration, the Geo-tek was set for a less sensitive
scale. As a result, the maximum resolution of lo-g could not
be obtained or compared.
During March of 1988, several drops were made to determine
the quality of the forces the drag shield sustains during a
drop. The results of dropping a high-g and low-g accelerometer
have shown that a fault exists on the guide rail and .that the
experiment package could benefit from a higher levitation
point. The dent in the drag shield seems not to be getting
worse at present. Results from from these tests and other ex-
aminations of Tower data indicated the following courses of ac-
tion, all of which affect the CPW experiment...
1. The rail is deformed and needs replacement in one
location near the bottom of the drop distance. A first attempt
at improving it will be to grind the high spots off. If nec-
cesary, the rail section will be replaced. The quality of the
low-g and of the laser telemetry is compromised until some
repair is made.
2. The thrust pressure needs to be increased. This will
produce a higher boost at the start which will cause the
package to be raised higher within the drag shield. One con-
sequence of this is the higher anticipated impact forces both
on the drag shield and the experiment.
3. For better cushioning at higher thrust, the cushioning
material in the catch tube will need upgrading or replacement.
4. Better package balancing will reduce the need for in-
creasing the thrust.
All these facility improvements will be required for the
CPW experiment. From the onset (1985), the quality and length
of the low-g period of the drop has deteriorated (drop from 3.2
seconds to 2.5 seconds). Longer, bump-free low-g times will
give better results when the analysis of interface oscillations
is done.
Processing of the telemetered data is performed under a
contract by NTI at MSFC. For the last set of conversions, a
delay was encountered since funding for the effort was not
readily available, although it was supposed to have been.
There is no problem in getting the data converted so long that
there is money in the contract for this work and so long as
there are no higher priority jobs on their manifests. Further
analysis may not be needed so long as the Geo-tek data can be
relied upon. The need for radio telemetry will also be
eliminated. Only a pressure warning device needs to be added
to the drag shield to satisfy safety requirements and
eliminated telemetered monitoring of the thruster sphere pres-
sure.
Up to date results and analysis from this experiment was
presented at the Microgravity Materials Processing Session at
the TMS-AIME Conference in Phoenix Az. in January 1988. A
paper was written that describes the experiment results in
detail. This paper has been submitted and is expected to be in
the Proceedings of the conference and will be a publication.
The cost for the trip was covered from this grant. All remain-
ing monies in the grant are to be applied to salary, overhead
and benefits for completion of the grant. There is too little
money available to extend the grant any further than it is.
This experiment is no longer the only one for the
facility. A metals levitation experiment is being developed at
Vanderbilt University by Bayuzak et al. A Japanese
Astronaut/Scientist, named Mouri, is also performing experi-
ments on the tower in metals casting. Scheduling of drops will
now be neccesary among us. Mouri expects 40 drops in the next
year, and the CPW experiment will need 5 good ones, where prob-
ably 10 will need to be made in order to get the 5 good ones.
Using the NASA Lewis Drop Tower
Due to the never-ending delays, it was suggested at a
recent review that the Lewis Drop Tower be used for this ex-
periment. While the experiment could have been initially
designed to be performed in that facility, in its present con-
struction, at this late date, it is far to large to be accom-
modated at the Lewis Tower.
Experimenters at the Lewis facility do have high grade
technical support people. The facility is properly maintained.
Much of this comes from the high visibility this multi-million
dollar facility has. The advantage of the MSFC tower is the
large experiment size and weight that can be accommodated. The
experiment need not operate in a vacuum environment. Arranging
to drop is not a major bureaucratic undertaking. However, sup-
port from NASA for the MSFC Drop Tower is at best weak. •*
The shape of the drop capsules for the Lewis Tower is also
difficult to work within, approximately 2 foot diameter by 10
foot long cylinders are dropped and caught in a bed of foam
chips. The g-level at impact is dangerously high (greater than
30 g's) and no on-board video is available during the drop.
For these and other reasons, it is not recommended that the
Lewis Tower be used for this experiment.
Conclusion
More drops will be made with the new video telemetry sys-
tem and analysis of this data will follow as the video results
arrive. The year extension provides the time to obtain this
data and prepare the final science report. The quality of the
data will determine how well the wetting transition can be
delineated by this experiment technique. So far, only marginal
results have been obtained. There will be confirmation of the
theory by these experiments if the data quality is as good as
is expected.
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Critical point wetting drop tower experiment

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