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Kovalchuk Capillary Pressure Studies Under Low Gravity Conditions 2010

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Capillary pressure studies under low gravity conditions V.I. Kovalchuk a, ⁎, F. Ravera b , L. Liggieri b , G. Loglio c , P. Pandolfini c , A.V. Makievski d , S. Vincent-Bonnieu e , J. Krägel f , A. Javadi f , R. Miller f a Institute of Biocolloid Chemistry, Vernadsky str. 42, 03142 Kiev, Ukraine b CNR - Istituto per la Energetica e le Interfasi, 16149 Genoa, Italy c University of Florence, 50019 Sesto Fiorentino (Firenze), Italy d SINTERFACE Technologies, 12489 Berlin, Germany e European Space A
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  Capillary pressure studies under low gravity conditions V.I. Kovalchuk a, ⁎ , F. Ravera b , L. Liggieri b , G. Loglio c , P. Pandol fi ni c , A.V. Makievski d , S. Vincent-Bonnieu e , J. Krägel f  , A. Javadi f  , R. Miller f  a Institute of Biocolloid Chemistry, Vernadsky str. 42, 03142 Kiev, Ukraine b CNR - Istituto per la Energetica e le Interfasi, 16149 Genoa, Italy c University of Florence, 50019 Sesto Fiorentino (Firenze), Italy d SINTERFACE Technologies, 12489 Berlin, Germany e European Space Agency - HSF-US, 2200 AG Noordwijk, The Netherlands f  MPI of Colloids and Interfaces, 14424 Potsdam/Golm, Germany a b s t r a c ta r t i c l e i n f o Available online xxxx Keywords: Microgravity conditionsInterfacial dynamicsCapillary pressure tensiometryOscillating drops and bubblesSurfactant adsorption layers For the understanding of short-time adsorption phenomena and high-frequency relaxations at liquidinterfaces particular experimental techniques are needed. The most suitable method for respective studies isthe capillary pressure tensiometry. However, under gravity conditions there are rather strong limitations, inparticular due to convections and interfacial deformations. This manuscript provides an overview of the stateof the art of experimental tools developed for short-time and high-frequency investigations of liquid dropsand bubbles under microgravity. Besides the brief description of instruments, the underlying theoreticalbasis will be presented and limits of the applied methods under ground and microgravity conditions will bediscussed. The results on the role of surfactants under highly dynamic conditions will be demonstrated bysome selected examples studied in two space shuttle missions on Discovery in 1998 and Columbia in 2003.© 2010 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02. Capillary pressure instruments for space experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02.1. Geometry and mechanical architecture of the fl uid cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02.2. The instrument's functionality and performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02.3. Optical observation of drop oscillation in the high-frequency range by the spectrum translation/compression technique. . . . . . . . . 02.4. In- fl ight calibration functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02.5. Illustrative example of FASTER experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03. Theoretical basis for drop/bubble oscillations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03.1. The pressure variation in the cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03.2. Complex resistance (impedance) of the capillary with the attached meniscus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03.3. Calculation of  ε  from oscillating drop/bubble experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 04. Experimental results on surfactant adsorption layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 1. Introduction Emulsionsandfoamsaresystemscomposedoftwoimmiscible fl uidsinwhichdropletsofoneliquidorgasbubblesare fi nelydispersedintoasecond liquid. Such liquid disperse systems are found everywhere,nature-made or man-made. For example food products, such as milkand mayonnaise are emulsions, and whipped cream and ice cream areeven foamed emulsions. There are many more products like cosmeticsand pharmaceutical formulations that belong to this broad class of materials.Already in the 19th century the fi rst fundamental interest incapillarity without gravity was reported when Plateau used neutral Advances in Colloid and Interface Science xxx (2010) xxx – xxx ⁎ Corresponding author. E-mail address: Vladim@koval.kiev.ua(V.I. Kovalchuk). CIS-01078; No of Pages 13 0001-8686/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.cis.2010.02.012 Contents lists available atScienceDirect Advances in Colloid and Interface Science  journal homepage: www.elsevier.com/locate/cis ARTICLE IN PRESS Please cite this article as: Kovalchuk VI, et al, Capillary pressure studies under low gravity conditions, Adv Colloid Interface Sci (2010),doi:10.1016/j.cis.2010.02.012  buoyancy techniques for studying capillary phenomena[1]. Morerecently,studies inmicrogravity have progressedthanksto thedevel-opment of drop-tower facilities, aircraft fl ying parabolic trajectories,sounding rockets and space missions. Since 1977, the European SpaceAgency performed more than 380 experiments involving systemswith liquid interfaces[2].Most of these experiments exploited the virtual absence of hydrostatic pressure, sedimentation and buoyancy to study static anddynamiccapillaryeffects,Marangoniconvection,interfacialinstabilitieslike Rayleigh – Taylor or Kelvin – Helmholtz, wetting and adsorptionphenomena,liquidsnearthecriticalpoint,andde-mixingofimmisciblealloys.Among the pioneers' experiments, in 1981 on sounding rocketTexus3and4,Brücknershowedthatthermocapillaryconvectiondoesnotneed interactionwithbuoyancyforces to becometimedependent[3]. In 1990, on board the Maser 4 rocket Passerone et al. performedthe fi rstinterfacialtensionmeasurementwiththePressureDerivativeMethod, later re-used in the FAST experiments[4]. In 1993, Vignes-Adler et al. investigated on Maser 6 solutal Marangoni effects occur-ring when a surfactant transfers from a drop immersed in an aqueoussolution without/with an interfacial exchange reaction, and deter-mined the srcin of interfacial turbulences at the drop surface[5]. Inrelation to this, numerous experiments aiming at understanding themicrostructure formed in immiscible alloys upon de-mixing followedby solidi fi cation were performed on Texus 5[6]. After several basicstudies, the microgravity experiments on capillarity have movedtowards increasingly complex phenomena, mostly involving interfa-cial dynamics as in the two FAST missions in 1998 and 2003. We canalso refer to recent experiments by Colinet et al. on Maser 9 and 10,who investigated the convection pattern[7], boiling experimentsstudying the temperature distribution, and the bubble shape duringthe growth and two-phase fl ows experiments[8].Foams and emulsions, even those made by nature, are stabilisedagainst breakdown by surface active molecules, which adsorb at thesurface of the liquid drops or bubbles, preventing their aggregationand coalescence. The challenge for science and technology consists inthe controlled stabilisation or destabilisation of these dispersesystems, which in turn is directly linked to the stability of the liquid fi lmsastheirmainbuildingblocks[9].A topdownapproachbrings usto the basis of these systems, the corresponding adsorption layersaround the bubbles and drops in foams and emulsions. The philos-ophy of this approach is to learn the most important properties of these adsorption layers and fi nd correlations to the behaviour of liquid fi lmsandthe fi nalfoamsandemulsions[10 – 13].Itseemsrathereasy to comprehend that the formation of a foam (foamability) or anemulsion depends directly on the rate of adsorption of surfactants atthe interface[14,15]. To analyse the properties for the stabilisation of the resulting disperse liquid systems, however, it is not trivial andnotatallcleartodaywhatthekeyparametersoftherespectiveadsoprtionlayers are, with respect to the liquid fi lm and also not with respect tothe fi nal foams and emulsions. This is obviously due to the fact thatthe stability of these complex systems can be controlled by differentmechanisms, such as drainage, Ostwald ripening, and coalescence[9,16 – 18].Thus,knowingallmainpropertiesoftherelevantadsorptionlayers is only a necessary basis but does not automatically answers thegeneralquestionsfromparticetothefundamentalscienceofinterfaces.Moreover, foams and emulsions are not necessarily stabilised bysurfactants, but also by proteins[19 – 21]and particles[22 – 25]or inmany cases by a mixture of them with surfactants[26 – 29].Stabilisation mechanisms are, however, not the target of this com-munication but rather the possibilities for the quantitative analysis of adsorption layers at the liquid/gas and liquid/liquid interfaces. Thisincludes essentially single drop and bubble investigations under dy-namic conditions. For the application of these experimental tools,microgravityisbene fi cial,asitavoidsgravity-drivenconvectioncausedby temperature or concentration gradients. Microgravity also providesinterfaces of constant-curvature, i.e. spherical geometry of drops andbubblesindependentoftheirabsolutesize,whileonground,thesurfaceshape varies strongly.There are quite a few activities dedicated to the improvement of knowledge about mechanisms controlling the formation and stabilityof liquid disperse systems[12].During the recent years, the ESA project FASES, which stands for “ Fundamental and Applied Studies of Emulsion Stability ” , using the module FAST (Facility for Adsorptionand Surface Tension) and its re fi ned version FASTER (Facility forAdsorption and Surface TEnsion Research)[30].The objective of the FASES project was to establish a link between emulsion stability andthephysico-chemicalcharacteristicsofdropletinterfaces,ofemulsion fi lms, and the modelling of emulsion dynamics. Additional projectswere supported by national space agencies and brought about quite anumber of fundamental achievements in the present fi eld, to a largeextend under ground but also under the favourable low microgravityconditions in space. The target of this manuscript is a summary of these results. Mainly on the basis of experimental data from the STS-95 and STS-107 shuttle missions, it will be demonstrated howmicrogravity signi fi cantly expands the effectiveness of establishedmethods, for example for the investigation of interfacial dilationalrheology to much higher frequencies. 2. Capillary pressure instruments for space experiments Capillary Pressure Tensiometry (CPT), effectively developed duringthe last two decades, is overall based on the application of the Laplaceequation to obtain dynamic surface/interfacial tension from themeasurement of the capillary pressure across a droplet interface andof its curvature radius. This technique allows monitoring dynamicinterfacialtensionchangesathighsamplingrates.Itisthereforesuitabletoinvestigatedifferentdynamicphysico-chemicalaspectsofadsorptionlayers, related to surfactant transport and dilational rheology[31],depending on the kind of interfacial area stimuli. The Expanded Drop[32,33]andtheFastFormedDrop[34]methodswereproposedtostudy the adsorption kinetics after creating a nearly freshly formed interface,allowing adsorption processes with characteristic times from a fewseconds to several minutes to be investigated. The Pressure Derivativemethod[4]allows the measurement of the interfacial tension of pureliquids, while the Growing Drop/Bubble methodology[35,36]wasdesigned for investigations of the dynamic aspects of adsorption atcontinuously expanding interfaces. Finally, the dilational rheologicalpropertiesofadsorptionlayerscanbestudiedbytheOscillatingDroporBubble[37 – 40]at small expansion/compression amplitudes.In a typical CPT experiment a submillimetric droplet (or bubble) isformedatthetipofacapillary.Apressuretransducerisusedtomeasurethepressuredifferenceacrosstheinterface,whilethedropradius, R ,canbeeither measured by imaging techniques, or calculated from theinjected liquid volume. The interfacial tension is then obtained via theLaplace equation. γ t  ð Þ = P  c  t  ð Þ R t  ð Þ 2 ð 1 Þ where P  c  is the measured capillary pressure. The interfacial areastimuli are usually provided by a piezoelectric linear actuator, while asyringe pump is utilised for coarser adjustments.CPT instruments have been suitably utilised in previous experi-mental investigations of adsorbed layers based on the measurementsof the dynamic interfacial tension[41,42]and of the dilational visco-elasticity[39,43,44]both at water/air and water/oil interfaces.The “ FAST ” and “ FASTER  ” instruments are essentially capillarypressure tensiometers, designed for the measurement of interfacialdynamic properties of  fl uid/ fl uid interfaces under microgravityconditions with controlled temperature and the option of changingsurfactant concentration in a certain range. 2 V.I. Kovalchuk et al. / Advances in Colloid and Interface Science xxx (2010) xxx –  xxx ARTICLE IN PRESS Please cite this article as: Kovalchuk VI, et al, Capillary pressure studies under low gravity conditions, Adv Colloid Interface Sci (2010),doi:10.1016/j.cis.2010.02.012  TheperformedexperimentsconductedduringSTS-95andSTS-107space shuttle missions, demonstrated quite good capabilities andexcellentperformancesfortheimplementationandsatisfactionofthescienti fi c requirements, allowing to validate theoretical models[30,45]. The present accommodation of the FASTER facility on theColumbus Laboratory of the International Space Station involved anew detailed design, manufacturing and assembly, capable of dosingincreasing amounts of two different surfactants and of observingsurface tension phenomena as responses to different kind of stimuli,like oscillations or trapezoidal pulses at fl uid/ fl uid or fl uid/gasinterfaces.TheextensionofFASTERtocoverthin fi lmrheology,asencounteredin foams and emulsions, will offer to the experimentalists the oppor-tunity to increase their knowledge in advanced foam and emulsiontechnology.  2.1. Geometry and mechanical architecture of the fl uid cell The FASTER facility in its fl ight con fi guration is composed by twomain subsystems, the “ Experiment Unit ” and the “ Facility Controller ” .The experiment unit is the experimental segment of the facility andhouses two fl uid cells, together with all their stimuli and detectionelements, and contained in a so called “ Secondary ExperimentContainment ” (SEC),whichprovidestheexternalleakprooftightness.The fl uid cells consist of  fl uid containers, which are made up byfused silica, and of an external metallic structure (made of Al), whichkeeps fi xed the fl uid cell and is used as main mechanical structureamong the sensors and actuators. In addition it represents a thermalbridge among the cell segments. The fl uid cell may have the lowercontainer fi lledwithpurewaterforexperimentcell1(EC1)orwithn-hexane for experiment cell 2 (EC2), and the upper container fi lledwith matrix fl uid, which is Paraf  fi n oil (EC1) or water (EC2).As shown in the cell functional scheme of Fig. 1, the lower cellchamber holds a pressure transducer and a temperature sensor (inadditiontoapassivecompensationunit).Theupperchambercontainsvarious devices, sensors and actuators, namely: ã two injection units allowing the use of two types of surfactantsolutions; ã the stirrer unit to homogenise the solution after each surfactantinjection; ã a passive compensation unit (which is excluded during the runningof the experiments); ã the compensation piston unit for the positioning and control of theliquid meniscus inside the capillary; ã the piezoelectric actuator for the fi ne meniscus movement, dropletcreation and droplet excitation; ã apressure transducer; ã two temperature sensors.The two pressure transducers are relative to the same referencepressure, being connected on the rear side by means of a referencepipe. In this way the differential pressure between the two fl uids isdirectly measured. Moreover, the upper chamber has optical fusedsilica windows for the illumination of the droplet and for the imageacquisition by a CCD camera.A capillary connects the two chambers through a main valve. Thecapillary tip, protrudinginto the upperchamber,holds a drop centredin the focal plane of the CCD-camera objective.  2.2. The instrument's functionality and performances The experimental module FASTER is controlled by a dedicatedhardware, that is a built-in computer and the pertinent electronics,and by a proper software. The instrument automatically performs aprogrammed sequence of measurement procedures, according to anestablished time-line. The main tasks comprise a) the generation of drops,oroptionallyofairbubbles,atthecapillarytipwithacontrolleddimension; b) the real time acquisition and processing of the drop/bubble image in order to measure and control the drop dimension(height, radius, volume, interfacial area) with high precision; c) realtimemeasurementofthedifferentialpressurebetweentheinnerdrop fl uid and the outer matrix fl uid with high accuracy; d) excitation of drop/bubble sinusoidal oscillations (actuated by software in the 0.01 – 0.5 Hzrangeandbyhardwareintherange0.5 – 1000Hz);e)modulationofdropinterfacialareaaccordingtoarbitrarytimefunctions(increasingor decreasing ramps, trapezoidal pulses, fast step expansions, etc.);f)recordingof the strain-gaugesignalwhichmonitorstheposition andgoverns in a closed-loop mode the displacement (extension/contrac-tion) of the piezoelectric actuator; g) fl uid temperature measurementand control.Infutureplanned missions,different kinds of experiments,including thin fi lms experiments, can be implemented additionally bymeans of software recon fi guration (uploading from ground).The measurement and control accuracy of the drop dimension canreach, when an optical magni fi cation of 2.5× is used, values betterthan 4 μ  m on height and on all the points of the droplet pro fi le whichareusedtocalculatethescienti fi cradius(thatmeansbetterthan2 μ  monradiusitself).Thefrequencyofthedropareavariationiscontrolledwithin a bandwidth of the order of 1 Hz.Fordynamicmeasurements,whenthedropletisexcitedinvariousways, the pressure measurement accuracy is ±1 Pa, after calibration.The temperature accuracy after calibration is ±0.1 °C in the range0÷50 °C.At regulartime-intervals,all sensor signalsand images are acquiredin synchronism (the time uncertainty is less than 0.1 ms), stored in acomputer memory and subsequently downloaded to ground mission-centre.As far as the facility control is concerned, its architectural designtakesintoaccounttheneedforahighly fl exibleelectronics.Moreover,itis possible to design new experimental sequences with the maximumpossible fl exibility during the whole duration of the mission.Actually the FASTER instrument, albeit capable of automaticallyexecuting all measurement procedures and operations, howeverleavesaccessforuploadinga largenumberofexperimentparameters,look-up-tables de fi ning arbitrary time functions, measurementsequence modi fi cations and time-line variations or, in the limit, thewhole controlling software.  2.3. Optical observation of drop oscillation in the high-frequency rangeby the spectrum translation/compression technique Adistinguishingfeatureof theFASTERfacility,inrespecttotheSTS-95andSTS-107 fl ights,istheavailabilityofdropimagesacquiredwithasub-millisecond exposure window atparticularde fi nite time-intervals,achieved by a digital video camera. Considering the characteristics of atypical digital camera, in particular the possibility of setting theelectronic shutter and the frame rate by externally uploaded para-meters, the technique of the spectrum translation/compression (SCT)[46]can be optionally implemented during in- fl ight high-frequencysteady-state drop oscillations.The important new issue introduced by the SCT execution is thatanevaluationofthecellintrinsicelasticityasafunctionoffrequencyisallowed, by reconstructing the oscillation amplitude in an extendedtime-interval and comparing the visualised changes of drop volumewith the displaced volume of the piezoelectric actuator. Actually, thecell intrinsic elasticity is a crucial parameter for the data analysis andinterpretation of high-frequency drop oscillations and, hence, for thereliabledeterminationoftheinterfacialdilationalvisco-elasticity,asitwill be demonstrated further below in the theoretical section.Essentially, the SCT measurement technique is based on sub-sampling an image at a properly-adjusted lower rate than the requiredrate for representing the cycle characteristics, as schematicallyrepresented in the plot of inFig. 2for a pure sinusoidal oscillation. The 3 V.I. Kovalchuk et al. / Advances in Colloid and Interface Science xxx (2010) xxx –  xxx ARTICLE IN PRESS Please cite this article as: Kovalchuk VI, et al, Capillary pressure studies under low gravity conditions, Adv Colloid Interface Sci (2010),doi:10.1016/j.cis.2010.02.012  example of the fi gure shows that the shape of a single high-frequencycycle at f  1 =52Hz can be reconstructed by sampling just one point at20ms intervals (i.e., at frequency f  2 =50Hz) in a sequence of cyclesobserved in an expanded time domain of 0.5 ms. Obviously, in order todown-scale a periodic multi-frequency oscillation, the recording timemust be conveniently expanded.In mathematical terms, the sampling operation is expressed by thesifting propertyof a fi nite series of equally spaced Dirac deltafunctions ∫ + ∞−∞ ∑ n = Q n =0 δ t  − nt  s ð Þ I t  ð Þ dt  = I nt  s ð Þ ; n = 1 ; 2 ; … ; i ; … Q  ð 2 Þ where I  ( t  )= I  0 sin(2 π   ft  ) and t  s is the sampling acquisition period. Inorder to optimize the effectiveness of this technique for thereconstruction of the periodic real signal it is important to assume asuitable relationship between t  s and the frequency of the real signal.The example of Fig. 2illustrates the simplest case of a sinusoidaloscillation. For a multi-harmonic periodic signal, constituted by afundamental frequency f  0 and N  harmonics, we have: I t  ð Þ = ∑ N n = − N  a n exp − 2 π inf  0 t  ð Þ ð 3 Þ where the highest harmonic component is, obviously, f  max = N f  0 . Fig. 1. FASTER instrument: functional scheme of one of the two experiment cells, contained in the “ Secondary Experiment Containment ” (SEC).4 V.I. Kovalchuk et al. / Advances in Colloid and Interface Science xxx (2010) xxx –  xxx ARTICLE IN PRESS Please cite this article as: Kovalchuk VI, et al, Capillary pressure studies under low gravity conditions, Adv Colloid Interface Sci (2010),doi:10.1016/j.cis.2010.02.012
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