Press Releases

An Innovative Method to Control the Incipient Flow Boiling through Grafted Surfaces with Chemical Patterns

An Innovative Method to Control the Incipient Flow Boiling through Grafted Surfaces with Chemical Patterns
of 5
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Transcript An Innovative Method to Control the Incipient Flow Boiling throughGrafted Surfaces with Chemical Patterns R. Rioboo,* ,† M. Marengo, ‡ S. Dall’Olio, ‡ M. Voue, † and J. De Coninck † † Laboratory of Physics of Surfaces and Interfaces, University of Mons, Parc Initialis, Av. Copernic,1, B-7000 Mons, Belgium, and   ‡ Faculty of Engineering, University of Bergamo, Viale Marconi 5,24044 Dalmine, ItalyReceived February 6, 2009. Revised Manuscript Received April 17, 2009 Theonsetofflowboilingofaliquidislinkedtothesuperheatconditionthatisnecessarytoactivatethenucleationsitesoncontacting surfaces. The nucleation sites are usually represented by cavities in the rough surface of the heat exchanger. Onsmooth surfaces, the region where bubble detachment does not occur due to the lack of superheating may constitute aserious limitation for microfluidic devices. This paper shows the first experimental evidence that the position of the activenucleation sites can be controlled through chemical patterning of smooth surfaces: in this study, the heated surfaces arechemicallygraftedwithalkylsilaneself-assembledmonolayersbymicrocontactprinting.Theanalysisofthepropagationof thebubblezoneareaquantitativelyshowsthatthebubblesremainlocalizedontopofthegraftedzoneandthat,intheinitialphase of the experiment, the center of mass of the bubble zone only moves along the vertical axis, without lateral drift. The increase in functionality and power consumption of electronic components and microdevices creates a problem:heating. This heat energy has to be removed in order to ensurethe reliable working of these devices, without negatively affectingtheir performance. One of the techniques used in electronics andmicrofluidic devices utilizes so-called “phase-change coolingsystems” (PCCS). Among possible PCCS, the heat pipe, forexample, has the advantage of being “passive”, yielding a veryhigh degree of long-term reliability. 1,2 In each PCCS, there is azonewhere liquid, solid,and gasare allpresent together,allowingsurface features, such as wettability and roughness, to play animportant role. 3,4 Much of the thermal effectiveness of PCCS isrelated to the characteristics of their evaporation and condensa-tionzones,their length,the thicknessofthethinevaporating film,and the dynamic contact angle. 5 Fixing the liquid/solid/vaporcontactlinetopredefinedpositionsmay,then,helptoenhancethethermal efficiency of PCCS. This can be performed by means of heterogeneities of the surface. 6 The work presented here studies the novel possibility of controlling the onset position of incipient flow boiling of a liquidduringitspassagethroughaheatedsmoothchannelbychemicallygrafting specific wettability patterns. Such a specific, highlyresistant surface modification results in surfaces without cavitiesat micro- and submicrometer scales.The onset of flow boiling of a liquid is linked to the so-called“superheat condition” which is necessary to activate the nuclea-tion sites on a surface. It is known that any topographicalheterogeneity of the solid surface may act as a nucleation site. 7 Since the smaller the cavities, the higher the superheat, phasechanges upon smooth surfaces, such as the ones encountered inthermal microdevices, demand a higher superheating to occur.Thus, thermal efficiency is decreased. Control of the location of the incipient boiling is therefore crucial to bypass this drawback.Whenaliquidflowinginachannelisheatedviaadiabaticsolidsurface, it will dissipate the heat by convection. With high heatfluxes, the liquid will boil by forming bubbles starting fromnucleation sites. The formation of the vapor bubbles is notimmediately linked to the attainment of the saturation tempera-tureoftheliquidatthegivenpressure,butitisnecessarytohaveasuperheatconditiontoinitiatethebubbles. 2 Thismeansthatthereis a separation between the position where the flowing liquidreaches the equilibrium saturation temperature and the positionwhere the firstbubbles appear. Thisdistance, named theincipientboiling distance or the incipient boiling length (Figure 1b), mayvary according to changes in various experimental parameters.Yet in 1962, Hsu 8 founded that the activation of the nucleationsites on smooth surfaces was only possible for large superheats.Similarly, in 1988, Bar-Cohen 9 confirmed that highly wettingliquids flooded all but the smallest cavities, hence depleting thevapor embryos needed for boiling inception. In other words, ahigher incipient boiling superheat is required to activate smallercavities and to initiate boiling (Figure 1). Once very small vaporbubbles obtain enough energy to nucleate and to subsequentlyseparate from the cavities, boiling takes place. Therefore, theincipient boiling length is a measure ofvapor bubble deactivationon the heated surface. As a consequence, an increase in incipientboiling length is accompanied by lower heat transfer rates. Thismeans that in microdevices, for example, difficulty in activatingsmoothsurfaceswillleadtopoorheattransferefficiencies,since alarge part of the channels are in the region of the nonboilingcondition.In the case of pool boiling, i.e., when the liquid is at rest andheated, the wettability effects are well-known, even if not fully *Corresponding author. (1) Kakac - , S.; Bergles, A. E.; Oliveira Fernandes, E.  Two-Phase Flow HeatExchangers: Thermal-Hydraulic Fundamentals and Design (NATO Science SeriesE) ; Springer: Berlin, 1988.(2) Kakac - , S.  Boilers, Evaporators, and Condensers ; Wiley: New York, 1991.(3) Wang, T. A.; Reid, R. L.  ASHRAE Transactions: Research  1996 ,  102 , 427– 433.(4) Kim, S. J.; Bang, I. C.; Buongiorno, J.; Hu, L. W.  Appl. Phys. Lett.  2006 ,  89 ,153107.(5) Wen, D. S.; Wang, B. X.  Int. J. Heat Mass Transfer  2002 ,  45 , 1739–1747.(6) Basu, N.; Warrier, G. R.; Dhir, V. K.  J. Heat Transfer-Trans. ASME   2002 , 124 , 717–728.(7) Bergles, A. E.; Rohsenow, W. M.  J. Heat Transfer  1964 ,  86 , 365–370.(8) Hsu, Y. Y.  J. Heat Transfer  1962 ,  34 , 207–214.(9) Bar-Cohen, T.W. S.  Heat Transfer Eng.  1988 ,  9 , 19–31. Published on Web 4/30/2009 © 2009 American Chemical Society DOI: 10.1021/la900463b  Langmuir   2009,  25(11), 6005–6009  6005  quantitatively described. In any case, the wettability issues and inparticularthe effect of the local staticcontact angle on boiling arealways linked to the presence of cavities. 10,11 The fact thatchemical heterogeneities on smooth surfaces may influence theboiling onset in a liquid flow was up to now, to the best of ourknowledge, not considered in the literature.Thecommonuseofhighsurfaceenergysolidmaterials(metals)and mostly low surface tension liquids that completely wet thesolids has discouraged research linking two-phase heat exchangeto the control of surface hydrophobic properties, such as theirpattern.Thomas et al. 12 used different self-assembled monolayers(SAMs) and showed that the boiling behavior of a liquid duringfast transient heating events in a pool was a function of thewettability of the solid surface. The nucleation temperatures werelower with hydrophobic SAMs on solid surfaces. 13 In anotherpaper,Takataetal. 14 showedthatsuperhydrophilicTiO 2 surfacescharacterized by zero static contact angle presented a highercritical heat flux than a surface with a 20   static contact angleunder conditions of pool boiling 15 as predicted by Kandlikar. 16 In the case of flow boiling, the influence of wettability, throughtheuseofsurfactantsolution,hasbeenaddressedbyJeongetal. 17 They experimentally demonstrated that wettability had indeedan important role, but its influence was also a function of themass flux. In 2003, Hibiki and Ishii 18 showed that the numberof nucleation sites was a function of the static contact angle of the liquid on the solid surface. Finally, Agrawal et al. 19 investi-gated the presence of nanobubbles on nanopatterned surfacesby atomic force microscopy. They showed that, under iso-thermal condition, their number is larger on hydrophobicsurfaces than on hydrophilic ones. The extent to which thesenanobubbles play an active role in the onset of the boilingphenomenon remains an open question. The present work isthe first successful experimental demonstration that not only canchemical patterning of the exchanger surface lead to bubbleformation, but it may result in controlling the position of thenucleation sites and may therefore be considered an appropriateway to change the incipient flow boiling length. Such techniquesassurfacepatterningwithSAMs 20 maybeofbenefitforsmallandsmooth surfaces by increasing the density of vapor embryos.Aflowboilingexperimentwithlowmassfluxwasconsideredasa first experiment. Figure 2 presents the experimental setup.Deionized Milli-Q water was used after degassing using heliumbubble flow in a closed reservoir. The water was heated to 90   Cand then brought to the heated channel using a magnetic gearpump (ColeParmer). The tubes providing the water from thereservoir to the gear pump and from the pump to the heatedchannelwereinsulatedwithpolystyrenetoreducethethermallossandtokeepthetemperatureofthefluidenteringthetestsectionasclose as possible to that of the reservoir. The water temperature Figure 1.  The physical meaning of the incipient boiling length issketchedinthefigure,sincethereisadistancebetweenthepositionin the channel where the liquid has reached the saturation tem-perature and the position of the actual formation of bubbles. Insuch incipient boiling length, the heat transfer is low and nobubbles are activated from the cavities. Figure 2.  (a) Scheme of the experimental setup. The channel isclosedonitsbothsides,belowandtop.(b)Schematictopviewofaheated channel with an example of “U” shape for the patternedzone. (10) Wang, C. H.; Dhir, V. K.  J. Heat Transfer-Trans. ASME   1993 ,  115 , 659– 669.(11) Yang, S. R.; Kim, R. H.  Int. J. Heat Mass Transfer  1988 ,  31 , 1127–1135.(12) Thomas, O. W.; Cavicchi, R. E.; Tarlov, M. J.  Langmuir  2003 ,  19 , 6168– 6177.(13) Balss, K. M.; Avedisian, C. T.; Cavicchi, R. E.; Tarlov, M. J.  Langmuir 2005 ,  21 , 10459–10467.(14) Takata, Y.; Hidaka, S.; Cao, J. M.; Nakamura, T.; Yamamoto, H.;Masuda, M.; Ito, T.  Energy  2005 ,  30 , 209–220.(15) Sefiane, K.; Benielli, D.; Steinchen, A.  ColloidsSurf.,A 1998 , 142 , 361–373.(16) Kandlikar, S. G.  J. Heat Transfer-Trans. ASME   2001 ,  123 , 1071–1079.(17) Jeong, Y. H.; Sarwar, M. S.; Chang, S. H.  Int. J. Heat Mass Transfer  2008 , 51 , 1913–1919.(18) Hibiki, T.; Ishii, M.  Int. J. Heat Mass Transfer  2003 ,  46 , 2587–2601.(19) Agrawal, A.; Park, J.; Ryu, D. Y.; Hammond, P. T.; Russell, T. P.;McKinley, G. H.  Nano Lett.  2005 ,  5 , 1751–1756.(20) Rioboo, R.; Marengo, M.; Dall’Olio, S.; Vou  e, M.; De Coninck, J. Devicesand method for enhanced heat transfer. European Patent Application EP07113887.9 - 1266, August 6, 2007. DOI: 10.1021/la900463b  Langmuir   2009,  25(11), 6005–6009 6006  Letter   was measured at the exit of the channel using a simple K-typemicrothermocouple.The channel itself was heated from below with a ChromaloxWS-605 heatsource providing 500 W heatat its surface (approxi-mately 120 mm  60 mm). The heater was regulated by a powercontroller (Watlow DIN-A-MITE), a temperature controller(Watlow EC), and a J-type thermocouple. The temperaturecontroller was connected to the heater and to the thermocouple.The thermocouple was used to measure the temperature of thesurface of the heater and to compare it with the preset value.The test section comprised a 2-mm-thick aluminum plateprovided with a cavity to fit the patterned surfaces. The size of the cavity was adjusted to the size of optical microscope glassslidesused for the patterning, (i.e.,1 mm thick,75 mm  25 mm).Theplatefurthercomprisedaslittoallowthefillingofthechannelwith the incoming water. The channel walls were created using0.6-mm-thickTeflonspacers,andaglassplateprovidedthetopof the channel, allowing visualization of the flow. The test sectionwas fixed on the heater with high-conductivity paste in order toensuregoodcontactanduniform heatfluxbetweenthetwoparts.The patterned surface which comprised hydrophilic and hydro-phobicareaswasfixedinthecavityusinghigh-conductivitypaste.The overall channel dimensions were height 0.6 mm, width18 mm, and length  ∼ 100 mm. The water leaving the channelwas not recirculated.Two types of patterned surfaces were used: a glass plate and asilicon plate cut to the same dimensions on which an octadecyl-trichlorosilane (OTS) monolayer was grafted by microcontactprinting 21,22 in defined zones. The surfaces were activated asfollows: each surface was rinsed twice in an ultrasonic bath of chloroform for 5 min; exposed toUV/O 3  for30min toremoveallpossible organic contaminants; immersed in a piranha solution(H 2 O 2 /H 2 SO 4  30:70 v:v), and finally rinsed with Milli-Q waterand dried under nitrogen. Microcontact printing was performedimmediately after activation of the surface in a low humiditychamber (RH: 6%). A polydimethylsiloxane (PDMS) stamp wasused, dipped in an OTS solution (10 mM in hexane). The PDMSstamp was brought to contact with the activated solid surface for30 s. 22 In these experiments, the OTS stamped areas were either aband (width of approximately 7 mm) oriented transversally withrespect to the flow direction or a U-shaped area.The wettability of the surface was measured using the sessiledrop method 23 after several heating and cooling runs. The partsthatwillbereferredtoas“hydrophobic”partshadadvancingandreceding static contact angles of   θ adv  = 107.3  ( 9.2   and  θ rec  =80.3  ( 8.7  . The rest of the plate, the one that will be referred toas“hydrophilic” parts,had advancingand recedingstaticcontactangles of   θ adv  = 94.8  ( 1.8   and  θ rec  = 55.5  ( 8.5  . The highvalue of these angles can be explained by the fact that the high-energysurfacereceived,duringseveralheating - coolingprocessesand hours of experiments, all the possible contaminants of thecircuit. Nevertheless, it was still possible to measure a wettabilitycontrast of approximately 23   between the hydrophobic zonesand the more hydrophilic ones.Thecharacteristicsofeachexperimentwerecompletelydefinedby fixing three variables: the volumetric flow rate of water, thewater inlet temperature, and the heater surface temperature.During the experiments, these three parameters were adjustedin order to localize the position of boiling onset to within a fewcentimetersoftheentranceofthechannelforthetwovolumeflowrates tested (2.2 cm for the 20 mL/min mass flow rate and 2.8 cmforthe25mL/min).Inbothcases,thepositionoftheboilingonsetwas almost the same and coincided with the position of the hydrophobic area. The temperature of the heater was fixedat 110  (  1   C, and we confirmed a uniform temperaturedistribution along the test section.Experiments were performed at two volume flow rates (result-ing in liquid mean velocities of 0.0309 and 0.0386 m/s) and withthe two types of surface (glassslides and silicon wafers) to testthepossible influence of these parameters. The experimental para-metersare summarized inTable 1.A firsttestwasperformed onaglass surface without any hydrophobic area. No particularpattern on the appearance of the bubbles and their localizationwas observed. This was considered the negative control experi-ment. Figure 3 represents the time evolution of the appearing of boiling over the patterned surfaces. Each image is a snapshot of  Table 1. Experimental Parameters for the Incipient Flow BoilingExperiments case A B Csurface glass glass siliconvolume flux (mL/min) 20 25 25 Figure 3.  Bubble evolution over time from an arbitrary zero time(top) until 75.9 s (bottom). In each image the flow is going fromright to left. Three cases (A, B, and C) are represented. Thehydrophobic zone is presented in the first image of each sequence:it isthe zone between the two white,dashed lines. The volume fluxand surface is varied in the following way. Volume fluxes are for(A),20mL/min;(B),25mL/min;(C),25mL/min.Graftedsurfacesarefor(A,B),glass;andfor(C),silicon.Allimagesaretakenatthesame magnification; on the (C) sequence vertical lines spaced byone millimeterare present ontheimages. Imagesrepresentanareaof 24.9 mm  12.4 mm. The scale bar is 5 mm long. (21) Kumar, A.; Whitesides, G. M.  Appl. Phys. Lett.  1993 ,  63 , 2002–2004.(22) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G.  Langmuir  1997 ,  13 ,3382–3391.(23) JohnsonR.E.;Dettre,R.H.In  Wettability Berg,J.C.Ed.; MarcelDekker:New York, 1993; p 1. DOI: 10.1021/la900463b  Langmuir   2009,  25(11), 6005–6009  6007  Letter   the channel,taken from above.On each image, the liquid isgoingfrom right to left. For each case, the zone where bubbles appear,the “bubble zone”, is clearly expanding over the hydrophobicparts independently of the direction of the flow. The three casespresented on this figure clearly show the effect of the wettabilitycontrastontheevolutionofboiling.Thecycleswereperformedatleast 5 times to ensure the reproducibility of the localized boilingeffect and the thermal stability of the chemical patterns.It should be stressed that the liquid present behind theexpanding bubble zone (left side of the images) received moreheating thanthe liquidinthe grafted zone. Despitethis, the shapeof the bubble zone still coincides with that of the grafted zone. Itcan be seen that at 40.2 s after the first visual appearance of bubbles,forthestraightzones (A and B inFigure3), thebubbleisconfined to the grafted zone. The silicon surface gave similarresults in the straight zone. In order to check that indeed thebubble appearing is following the hydrophobic zone we decidedto use another shape (a U-shape), which is unsymmetrical to theflow direction. For the U-shaped zone (C in Figure 3), theexpanding bubble follows at least half of the grafted circuit until75.9 s after the first bubble formation.Let us now consider some quantitative aspects of the propaga-tion of the bubbles. The series of images corresponding to theexperimental case A (Table 1) was analyzed using the CompixSimplePCI image analysis software (v 5.1, Hamamatsu Corp.),and the area of the bubble zone was recorded as a function of thetime.TheseresultsarepresentedinFigure4a.Theareacoveredbythe bubbles is made dimensionless by dividing its value by thevalue ofthe SAM-grafted area. Two analyses were considered. Inthe first one, the whole images were analyzed, showing that thebubble zone grows beyond the SAM surface at least for  t  > 55 s(continuousredcurve).Atpoint3,thearearemainsconstantover15 s and rapidly vanishes as soon as the liquid flow is rapidlyincreasedtoflushouttheexperimentalcell(point4).Inthesecondcase, the seriesofimageshas beenreanalyzedinthe samewaybutconsidering only the part of the bubble zone in contact with theSAM (dashed green curve). From point 1 to point 2, both curves(almost) coincide, confirmingthe visualresultaccording towhichthe bubbles remain confined above the SAM-grafted surface. Atpoint 2, 90% of the SAM-grafted zone is covered by the bubbles,and the bubble zone starts to grow outside the SAM zone. Twoother parameters have also been analyzed: the position along the X  and Y  axesofthe centerofmassofthe bubblezone(Figure 4b).The trajectory has been represented in two parts: the first one(from point 1 to point 2) corresponds to the displacement of thecenter of mass inside the SAM area (continuous blue curve). Thispart of the trajectory confirms that the bubbles remain confinedabove theSAM zoneand thatthe X  positionofthecenterofmassonly deviates by less than 4%. In the meanwhile, the  Y   positionincreases from the bottom of the SAM zone to approximatelyreach the mid-position of it. During that part of the experiment,theedgesofthebubblezonearetrappedbytheSAMborders.Thesecond part of the trajectory (dashed blue curve) between points2 and 4 ischaracterized bya bubblezonewhichspreads to the leftofthe SAM zone. Afterpoint 4,the wholechannel wasflushedbyan increase of volume flow rate in order to pass to another test.Subsequently, the bubble is removed from the grafted zone.Clearly, the SAM can affect the distribution of the heat on thesurface by enhancing liquid - vapor phase change on the graftedzone. Such behavior is unexpected, since on the surfaces we haveconsidered there were no microcavities and the roughness waskept very low (typically, the mean roughness amplitude of siliconwafers and microscope glass slides is below 1 nm 24 ). A possibleexplanation for such behavior is the presence of nanobubbles,already proven in isothermal conditions by different authors. 19,25 It has been shown that the density of nanobubbles on hydro-phobic patterns is higher than on hydrophilic zones. 19 It canthereforebesupposedthatthetriggeringofboilingisenhancedbythe higher nanobubble density. The U-shaped experiment indeedshowsthatthephasechangeisstronglyaffectedbythewettabilitycontrast, which reinforces the hypothesis that the phase change isdue to nanobubble density differences.We believe this study should be the first of many experimentsdesignedtoquantifythewettabilityeffectintermsofthelocalheatflux and heat distribution on the solid surface as a function of thewettabilitycontrast.Anotherimportantbenefitisthesimplicityof the method compared with the usual control of the surfaceroughness and topographical elements.In conclusion, the incipient boiling length, which results fromthe superheating necessary for the activation of the nucleationsites, is an important issue for the overall efficiency of smallthermal devices,such asmicroheating exchangersand microheat-ing pipes. In such cases and generally for smooth surfaces, theincipient boiling length may have the same length scale as themicrochannels, resulting in poor heat transfer rates comparedwith devices where the heat transfer channels have much higherlength scales. The present work provides a first qualitative andquantitative confirmation that the use of chemical grafting toproduce patterns through microcontact printing on solid smoothsurfaces can control the position of the incipient boiling. Thesilicon and glass surfaces used have no cavities bigger than a fewnanometers and the coating is of molecular thickness (at most, a Figure 4.  (a) Time evolution of bubble zone area (expressed inpercentoftheSAM-graftedarea).Continuousredcurve:totalareaofthebubblezone.Dashedgreencurve:areaoftheboilingzoneincontact with the SAM-grafted surface. (b) Trajectory of the centerof mass of the bubble zone. Dashed black lines: limit of the SAM-grafted zone. Points 1 to 4 are described in the text. (24) Rioboo, R.; Marengo, M.; Tropea, C.  Atomization Sprays  2001 ,  11 , 155– 165.(25) Cavicchi, R. E.; Avedisian, C. T.  Phys. Rev. Lett.  2007 ,  98 , 124501. DOI: 10.1021/la900463b  Langmuir   2009,  25(11), 6005–6009 6008  Letter   few nm): this means that our results are not due to the standardnucleation sites usually linked to roughness and/or micrometricsurface defects. We suggest that the phase change is due to thelocal density of nanobubbles, which is dependent on the hydro-phobicity of the surface. The global heat flux on the solid surfacecan then be strongly affected by the chemical heterogeneities andwettability contrast. Similar results coming from experiments onmaterials with different heat conductivity and using differentvolume flow rates invite the possibility to generalize this effectto other cases. The proposed methodology may be applied toan enormous number of applications, from boiling to cooling,where multiphase thermo-fluid-mechanical phenomena are thecontrolling processes. Acknowledgment.  Marco Marengo and Stefano Dall’Oliowere supported through Italian National funding projects,PRIN2005 and PRIN2007, “Two-phase flows in micro- andmini-channels” led by Prof. Marco Spiga, University of Parma.This work is partially supported by the Minist  ere de la R  egionWallonneand the BelgianFundsfor Scientific Research(FNRS). DOI: 10.1021/la900463b  Langmuir   2009,  25(11), 6005–6009  6009  Letter 
Similar documents
View more...
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks