Non-coalescence of two bodies of the same liquid and the suppression of contact between liquid drops and solid surfaces is being studied through a pair of parallel investigations being conducted at the Georgia Institute of Technology and the Microgravity Research and Support (MARS) Center in Naples, Italy. Both non-coalescence and contact suppression are achieved by exploiting the mechanism of thermocapillary convection to drive a lubricating film of surrounding gas (air) into the space between the two liquid free surfaces (non-coalescence) or between the drop free surface and the solid (contact suppression). Experiments performed to date include flow visualization experiments in both axisymmetric and (nearly) two-dimensional geometries and quantitative measurements of film thickness in the contact-suppression case in both geometries

Neitzel, G. Paul

NASA Technical Reports Server

Non-Coalescence Effects in Microgravity

Non-coalescence of two bodies of the same liquid and the suppression of contact between liquid drops and solid surfaces is being studied through a pair of parallel investigations being conducted at the Georgia Institute of Technology and the Microgravity Research and Support (MARS) Center in Naples, Italy. Both non-coalescence and contact suppression are achieved by exploiting the mechanism of thermocapillary convection to drive a lubricating film of surrounding gas (air) into the space between the two liquid free surfaces (non-coalescence) or between the drop free surface and the solid (contact suppression). Earlier experiments performed included flow-visualization experiments in both axisymmetric and (nearly) two-dimensional geometries and quantitative measurements of film thickness in the contact-suppression case in both geometries. Work done in the second year has focused on obtaining quantitative results relating to the effects of variable air pressure, development of analytical and numerical models of non-coalescing droplets and to pursuing potential applications of these self-lubricated systems

J NASA/CR---/_,_ 208199
Non-Coalescence Effects
/ x., _ Y-c",C.
/3 7-_/_
in Microgravity
(NAG 3-1894)
Performance Report for the period
17 June 1997- 16 June 1998
Submitted to:
Dr. Ray Skarda
Microgravity Fluids Branch, Mail Stop 500/102
NASA Lewis Research Center
21000 Brookpark Road
Cleveland, OH 44135
by
Prof. G. Paul Neitzel, Principal Investigator
The George W. Woodruff School of Mechanical Engineering
Georgia Institute of Technology
Atlanta, GA 30332-0405
May, 1998
https://ntrs.nasa.gov/search.jsp?R=19980111133 2020-06-15T23:38:38+00:00Z
I. Summary
Non-coalescence of two bodies of the same liquid and the suppression of contact
between liquid drops and solid surfaces is being studied through a pair of parallel
investigations being conducted at the Georgia Institute of Technology and the Microgravity
Research and Support (MARS) Center in Naples, Italy. Both non-coalescence and contact
suppression axe achieved by exploiting the mechanism of thermocapillary convection to
drive a lubricating film of surrounding gas (air) into the space between the two liquid free
surfaces (non-coalescence) or between the drop free surface and the solid (contact
suppression). Earlier experiments performed included flow-visualization experiments in
both axisymmetric and (nearly) two-dimensional geometries and quantitative measurements
of f'dm thickness in the contact-suppression case in both geometries. Work done in the
second year has focused on obtaining quantitative results relating to the effects of variable
air pressure, development of analytical and numerical models of non-coalescing droplets
and to pursuing potential applications of these self-lubricated systems.
II. Discussion of Research
Non-coalescence, as defined for the purposes of the present research grant, is the
ability of two liquid masses to refrain from coalescing (joining) when brought into apparent
contact under the proper conditions. Non-wetting, refers to the inhibition of contact
between a liquid mass and a solid surface normally wetted by this liquid. Both of these
phenomena are due to the separation of the liquid masses and/or the liquid and solid by the
presence of a thin, lubricating film of the gas in which the systems are immersed. In the
case of non-coalescence, this film may be provided by driving relative motion between the
two liquid surfaces, either through the action of thermocapilladty or through the forced
motion of one of the surfaces (see Dell'Aversana, Banavar & Koplik, 1996); for non-
wetting, thermocapillarity is employed to drive the film by cooling the solid surface
sufficiently below the temperature of the liquid droplet.
The research done in the second year of this project has been divided between
experimental, numerical and theoretical work. The theoretical portion of the research has
been carried on at Georgia Tech in collaboration with a new graduate student, Ms. Shelley
Chen. Ms. Chen is engaged in the theoretical development of a lubrication model to
describe the droplet motion and fihn flow. In addition, she will be continuing with the
experiments of Mr. John Nalevanko, a former M.S. student, who designed and built an
apparatusfor studyingnon-coalescencebetweentwo threadsof the same liquid and non-
wetting between a thread and a solid surface..
Numerical computations have been underway at the MARS Center with the aim of
simulating the flows in both the liquid and gas phases associated with two-dimensional,
non-coalescing droplets. This problem is a difficult one since droplets are of order
millimeters in size while the film thickness is known to be (see Dell'Aversana, Tontodonato
& Carotenuto, 1997) of the order of microns. Hence, there are three orders of magnitude
difference in the relevant length scales. Work has focused on the use of grid-
transformation schemes to account for the length-scale disparity. Early results appear
promising, but the problem of allowing the free surface to move, being determined as part
of the solution, still needs to be tackled.
Experimental efforts have been aimed at gathering further quantitative results of the
effect of surrounding gas pressure and at exploring possible uses for "self-lubricated'"
systems. We are now able to enclose a pair of non-coalescing droplets in a sealed vessel
which can be evacuated to precisely controlled pressures. For a given liquid, droplet size
and relative displacement, we then measure the pressure at which coalescence occurs.
Results to date have been surprising with respect to the magnitudes of these threshold
pressures, which are so low as to bring into question the validity of the continuum
hypothesis for the gas in the film. This subject will receive further attention in the
upcoming project year.
With respect to possible applications of non-coalescing systems, one type of
experiment has been performed in which we were able to levitate a small device containing
a magnetized needle above a bath of silicone oil. The levitation was accomplished by
attaching a number of droplets to the bottom of the device and heating these radiantly. The
magnetized needle permitted to device to rotate in response to nearby ferromagnetic
materials and also, in the absence of these substances to align with the earth's magnetic
field. This experiment confirms our earlier hypothesis that it may be possible to use such
non-coalescing systems as beatings in low-load situations such as might be experienced in
a microgravity environment. We have also conducted very preliminary experiments using
acoustic excitation to suppress coalescence of a droplet with a bath of the same liquid. A
potential use for this mechanism might be to use acoustic forcing to inhibit coalescence
within clouds of droplets, which could be beneficial in a combustion application. This is
likewise being explored further in the coming year.
3
We notewith pride the appearanceof an articlein Physics Today, the monthly
publication of the American Physical Society, which resulted from an invited talk given at
the APS March Meeting last year. In addition to the article itself; a photograph from our
research graced the cover of the January, 1998 issue in which it appeared, giving welcome
visibility to our work.
III. References
DelrAversana, P., Banavar, J. R. & Koplik, J. 1996 Suppression of coalescence by
shear and temperature gradients. Physics of Fluids 8, 15.
Dell'Aversana, P.; Tontodonato, V.; Carotenuto, L., 1997 Suppression of coalescence
and of wetting: the shape of the interstitial film. Physics of Fluids 9, 2475.
IV. Project Personnel
at Georgia Tech:
G. Paul Neitzel, Professor, Principal Investigator
Shelley Chen, Graduate Research Assistant
at the MARS Center:
Luigi Carotenuto, Researcher
Dario Castagnolo, Researcher
Pasquale Dell'Aversana, Researcher
V. Publications and Presentations
Dell'Aversana, P., Tontodonato, V. & Carotenuto, L. 1997 Suppression of coalescence
and wetting: the shape of the interstitial film, Physics of Fluid 9, 2475-85.
Neitzel, G. P. & Dell'Aversana, P. 1998 When liquids stay dry, Physics Today 51, 38-
41.
4
• i-
Neitzel, G.P. 1997 Thermocapillary convection in microgravity," invited talk presented
at JASMAC-13 (proceedings, pp. 17-20), Meeting of the Japan Society of Microgravity
Application, Fukuoka, Japan, October 30-31.
DeU'Aversana, P., Neitzel, G. P., Castagnolo, D. & Tontodonato, V. 1997 Non-
coalescence and wetting suppression via thermocapillarity," presented at the Fiftieth
Meeting of the Division of Fluid Dynamics of the American Physical Society, San
Francisco, CA, November 23-25.
Appendix
MARSCenterPerformanceReport
(Exhibitsnot included)
6
lM icrogravity Advanced Research and Support C enter
Non-Coalescence Effects in Microgravity
Performance Report
for the period
June 17, 1997 through April 16, 1998
(Year 2)
Subgrantee's part
Subgrant number:
Subgrantee:
Subgrantee address:
Principal Investigator:
E-25-L43-G I
Microgravity Advanced Research and Support
Center (MARS)
Via Comunale Tavernola,
80144, Napoli, Italy
Phone: +39-81-234-4580
Fax: +39-81-234-7100
Dr. Luigi Carotenuto
Prime grant number:
Grantee:
Principal Investigator:
NAG3-1894
Georgia Institute of Technology
Prof. G. P. Neitzel
Date: 30 April 1998
Luigi Carotenuto Francesco Laccetti
(General Manager)
Non-Coalescence Effects in Microgravity
Performance Report
Year 2
The present document reports on the Subgrantee's activities being performed for the
second year of a study concerning the phenomenon of stable non-coalescence between
liquids 1. The work is being executed jointly with the Georgia Institute of Technology
and will be carried out over a four-year period. The activities of the second year are
expected to be completed by June 16, 1998, as scheduled. They are now being
performed according to what announced in the statement of work [Exhibit A] and
completed to about 70%.
The mechanism at the basis of the permanent inhibition of coalescence and of
wetting has been basically understood and illustrated in the course of the first year of the
project. Nevertheless, part of the present activities continue to pursue the investigation of
the scientific aspects of non-coalescence. In fact, the experiments are expected to yield
new elements to complete the analytical description of the phenomenon. However, with
respect to the previous year, a larger effort is being spent today to investigate the
mechanical features of non-coalescing and non-wetting systems in view of possible
applications. In the following, we shall occasionally refer to such systems as "self-
lubricated" in line with the usage of this phrase in a previous publication 2.
During the first year of the project, a sealed cell was built in which it was possible to
decrease the air pressure to determine the minimum pressure capable of sustaining a non-
coalescing state between two droplets. Such a critical pressure was found to depend on
the drop reciprocal squeezing, for any fixed temperature difference AT imposed between
the droplets. In this second year, these measurements have been repeated to enhance the
statistics and additional measurements have been performed at three different AT's to
examine the temperature dependence of the pressure threshold for the coalescence
occurrence. These measurements are intrinsically interesting (because they promise to
yield new data to better describe the phenomenon analytically) and difficult to perform
(because even a small dust particle or a vibration may affect the results). Therefore, in
order to perform new and more accurate measurements of coalescence between droplets
under conditions of controlled temperature and pressure, a new sealed cell has been
obtained. With respect to the old one, the new sealed cell has several improved features
including a quick access door to speed up operations, a more precise micrometer to
displace the sample drop, sensors and electrical feed-throughs located on removable
blank windows to permit a more flexible design. It also has the capability to accept a
replacement bottom of transparent glass. This will permit us to perform observations of
wettinginhibition aswell, at adefinedair pressure.Suchobservationscanbeperformed
by meansof an interferometerwhich hasbeenimprovedwith respectto the one used
previouslyto reduceits overall dimensions.In addition,to achieveabettercontrol of all
the important parameters,the pressure-and temperature-controlsoftware has been
perfected.Today,with thenewhardwareandsoftwaresystemit is possibleto displacea
sampledrop with 1 lainprecision,to control thetemperaturewithin 10.2K andto control
thepressureinsidethecell within 0.2mbaror one-thousandthof thesetpoint.
In thematterof usingself-lubricatedsystemsasbearings,somemeasurementshavebeen
executedto test the load capacityof a drop pressedeither againsta bath of the same
liquid or againsta flat glasssurface. An apparatushasbeenset up where the drop is
suspendedfrom a precisionbalanceandheatedby radiationso asto not affect the force
measurement.The bath(or theglasssurface),which is at a lower temperature,hasbeen
raisedbeneaththedropby aDC motorto contactthedrop andpushit up. Forcesaslarge
as20 dynesor morehavebeenmeasuredbothwith thebath andthe glasswhena 5 lal
dropletof 3 mm diameterwasused.
Suchmeasurementsled usto the conclusionthat a small, yet useful payloadcould be
carded by a self-lubricated systemeven in a terrestrial environmentand that it was
worthwhile to constructa working to illustratethis fact. Sucha devicehasbeenbuilt: it
is a small compasshovering upon a silicone oil bath,sustainedby 19non-coalescing
dropletsof thesameliquid workingasbearings.Thetotalmass,dropletsincluded,is not
muchmore than0.2 g and,whenit is madeto deviatefrom its equilibrium position, it
realignsitself with the terrestrial magneticfield, showing a considerablesensitivity to
anymagneticdisturbances.
In order to expand the set of the explored self-lubricatedsystems,several new non-
coalescingconfigurationshave beenobservedwheremore than two liquid bodiesare
involved. Someof thesecasesare illustrated in the photographsof the article here
attachedasExhibit B andotherconfigurationsexist wherea non-coalescingdrop is not
attachedto arod andis radiantlyheatedfrom above. In someof thesecasesa largenon-
coalescingdrop is in "contact" with a bathandasmallersatellitedrop is formedcloseto
it. From the video from which fig. 3 in the enclosedarticle wastaken, it is possibleto
observethat the motion existing within the satellite droplet is rotating in a single
direction, rather than being of a multicellular nature.Owing to the smallnessof the
involved friction, a gyroscopiceffect dueto sucha convectioncould be observableand
exploitedfor someapplication,providedthatall theotherenvironmentaldisturbancesare
removed.
As far asthe theoretical and numerical part of the researchis concerned,new ideas
regardingthe relevanceof theKnudsen-numbervariation in the low-pressurerangehave
beensuggestedto explain the occurrenceof coalescenceat a certain critical pressure.
Sucha variation,to beconfirmedby futureexperiments,couldaffect thepersistenceof a
sufficientno-slipat the liquid/gasinterfacesto ensuretheformationof theair lubricating
film. A new grid generationalgorithm hasbeen implementedto perform numerical
calculationsthat,in thefuture,couldhelpto understandalsothispoint.
The budgetallotted for travelhasbeenusedto allow onepersonfrom MARS to attend
the50thAmericanPhysicalSocietyDivision of Fluid Dynamicsconference,held in San
Franciscoon 23 - 25 November1997.At the meetingDr. Dell'Aversanapresentedthe
talk "Non-Coalescenceand Wetting Suppression via Thermocapillarity " prepared in
collaboration with Prof. Neitzel et al.
In conclusion, we wish to point out that, besides of the publication of the article, "When
Liquids Stay Dry", attached to this document as Exhibit B, the editors of Physics Today
decided to dedicate the cover of the January 1998 issue of the magazine to the topic of
this research.
Following the publication of this article, the authors have received many comments and
requests for reprints from the readers, and invitations to write on about non-coalescence
from the editors of Philosophical Magazine B, and the World Scientific Publishing Co.
References
1p. Dell'Aversana, J. R. Banavar and J. Koplik: "Suppression of coalescence by shear and
temperature gradients," Phys. Fluids 8, 15-28, (1996).
2p. DelrAversana, V. Tontodonato and L. Carotenuto: "Suppression of Coalescence and
of Wetting: the Shape of the Interstitial Film "Phys. Fluids, 9, 2475-2485 (1997).


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