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ESJphysics2017 Research Article Open Access Olivier EMILE* and Janine EMILE Towards an optofluidic pump? iAdbicstsryasctte:mMsoastreofbathseedviobnraltoicnaglmheeachtianngisomfsmoefmopbtroaflnues- 532 nm beam scphloitpeprerframe soap film that induces liquid flow. We report here a new type of D 7 r diaphragm pump in a liquid film based on the optical 543 nm a) 1 0 radiationpressureforce.Wemodulatealowpowerlaser 633 nm 2 thatgenerates,atresonance,asymmetricvibrationofa interferences D n freestandingsoapfilm.Thefilmlifetimestronglyvaries g a from 56 s at low power (2 mW) to 2 s at higher power λ=543 nm J (70 mW). Since the laser beam only acts mechanically u.) 0.98 λ=633 nm 6 n. on the interfaces, such a pump could be easily imple- n ( b) ] mented ondelicatemicro-equipment on chipsor inbio- o 0.96 s si c logical systems. mis ti s 0.94 p Keywords:Soapfilm,laserinducedvibration,resonance n a o tr . s *Corresponding Author:Olivier EMILE:UniversitédeRennes1, 4 8 12 time (s) c i 35042 Rennescedex,France,E-mail:[email protected] s JanineEMILE:IPR,UMRCNRS6251, UniversitéRennes1, Fig.1.Experimentalsetup:a)a532nmmodulatedlaserlight y 35042 Rennescedex,France,E-mail:[email protected] deformsaverticalfreestandingthinsoapfilm.Thethicknessis h p measured fromthetransmissioninterferencesoftwolowpower [ lasers. BS:beamsplitter,DgandDr:photodiodes.b)exampleof interferencesignals. 1 1 Introduction v 9 2 Experimental procedures 5 At the photon level, the value of the linear momentum 6 depends on the refractive index of the medium. At an 1 interface,withtwomedia withdifferentindexes, for ex- The experimental set-up, already described in [11], is 0 . ample at an air/water interface, the linear momentum shown in Fig. 1a. A chopped green laser (CristalLaser, 1 0 isdiscontinuous.Thisleadstoaradiationpressureforce P = 100 mW, that can be attenuated to P = 2 mW, 7 [1–3] that may then induce an interfacial deformation. waist w = 300 µm, λ = 532 nm) illuminates a vertical 1 However,inordertobendit,onehastocompensatefor free standing draining film supported by a glass frame. : v the surface tension. Apart from experiments that use Fromtherelativepositionoftheintensityextremaofthe i X evanescent wavestoactontheinterface[4–6],thereare interferencesintransmissionfromtwocontinuous(CW) two main ways to compensate for it. On the one hand, He-Ne lasers (Melles Griot, P = 1 mW, w = 400 µm, r a one can apply high power focalized laser beams [1, 7– λ=633nmandλ=543nm),onemeasurestheabsolute 9] or, on the other hand, one can use two liquids with film thickness [12] with a precision better than 2 nm. similarsurfacetensions[2,10].Anyway,fromamechan- The chopped laser and the two probing beams are su- icalstandpoint,itiseasiertodeformaliquidfilmunder perposed on the centre of the soap film. The vertical illumination than an interface. Besides the liquid film free draining film (laser off) has a lifetime of 27±1 s. may be considered as a model case. It is therefore es- We have checked that when the solid state laser is not sential to determine the parameters of the parametric modulated nor applied, and when the probe lasers are amplificationexcitationof the system that may lead to off,thefilmlifetimeisunchanged.ThetwoHe-Nelasers resonant vibrations. The aim of this letter is to investi- have thus no visible effect on the film. gatethelifetimeofasoapfilmirradiatedbyalowpower The glass frame sustaining the film is a d = 3 cm- chopped laser and look for potential applications. diameter toroidal ring made from a 5 mm-diameter glass rod. The soap solution is made of SLES surfac- 2 OlivierEMILEandJanineEMILE u.) laser on 56 s 23.5 Hz a. al ( CW 50 P = 2 mW gn 632 nm film ) si 543 nm deflation e (s40 m i t e f 0.9 film m li30 P = 9 mW l /4 inflation 2a fil 20 P = 70 mW 0.25 S 17 s 0.88 10 10 11 time (s) 2 s frequency (Hz) Fig.2.Dynamicalresponseoftheinterferencesoftheprobe 0 0 10 20 30 40 50 laserswhenthelaserisswitchedon.Thedottedlinescorrespond totheinterferenceswithoutthelaser.Forthesakeofclarity,the Fig.3.Filmlifetimeversusmodulationfrequencyforthreediffer- interferencefringeshavenotbeensuperimposed. entpowers, showingaresonanceat23.5Hz. tant (Sodium Lauryl Ether Sulfate, Cognis), 0.1% v/v when the laser is suddenly switched off with a dip on diluted in pure water, below the critical micellar con- both interfaces. centration (CMC). The temperature is controlled to T=20.0±0.2○C,aswellastheairhumidityto50%±5%. The intensities of the two transmitted probe lasers are 3.2 Resonance recorded on two photodiodes and registered on a com- puter (see Fig. 1b) with an acquisition rate of 1 ms or Since the system is strongly damped, one wouldexpect 100 ms respectively. a resonance frequency higher than the 4 Hz frequency, i.e.inthetenthofhertzrange.Wehavethenmodulated the laser (2 mW) with different frequencies. The film 3 Results lifetimeversusthemodulationfrequency isdisplayedin Fig.3. Every measurement is done at least three times, 3.1 Dynamical response of the film. withaverygoodreproducibility.Atlowpower(2mW), the curve exhibits a sharp resonance at a frequency of We have first registered the dynamical response of the 23.5±0.1 Hz. One also notes that when the light is not film when suddenly illuminated by a CW laser (see modulated (CW laser), the lifetime is unchanged. At Fig. 2). One notes that the soap film inflates when the high frequency, the lifetime is also unchanged (27 s). laser is switch on, as expected. This inflation 2a is the As the power is further increased, the resonance is variation of the signal with the laser off (dotted line) continuously turned into an antiresonance. At a power and with the laser on (plain line). It can be evaluated of 70 mW, the film lifetime is only two seconds. When by computing the transmissionsignalversus film thick- illuminated with a chopped light at 23.5 Hz, the film ness that leads to interferences. This establishes a cor- nearly immediately breaks. This is not due to heat- respondence between the detected signal and the film ing and thermal effects since, with CW light, even at thickness [12]. This then enables the evaluation of the high power, the film lifetime is unchanged. It is only 2avariationthatequals14nminourcase,bothwiththe because theamplitudeof theoscillationsincreaseswith greenandtheredlights.Itissmallcomparedwiththe1 the power that the film becomes brittle. Actually, we µm film thickness. Thus each interface is bent (bump), havenotmeasuredtheamplitudeoftheoscillation.The withamaximumamplitudeof7nm.Itthenrelaxesina dynamicalresponse ofthefilmunder asingle2mWex- single oscillation, meaning that the damping is strong. citationis7 nm. At resonance, theamplitudeof the os- The period of this damped oscillation is estimated to cillationmust behigher.Assuminga lineardependance 0.25 s (see Fig. 2) which corresponds to a pseudo fre- oftheamplitudewiththeappliedpower,theamplitude quency of 4 Hz. We have made the same observations of the oscillation would be of the order of hundreds of Towardsanoptofluidicpump? 3 nm at 70 mW, i.e. close to the film thickness. This ex- The liquid flow is in the nano-liter range. It could be plains why the film breaks instantaneously. precisely adjusted by changing the laser power. It has to be noted that the resonance or antireso- Connectingtheframewithtworeservoirs,usingmi- nancefrequencydoesn’tdependontheopticalpowerin crofluidic valves as the ones that have been demon- theexploredpowerrange.Ithardlyvarieswiththecon- strated recently [17], the film would behave as an centration, or the temperature, or the frame geometry. optofluidic pump. Since it vibrates at resonance, the We have also checked that this resonance phenomenon opticalpower couldbevery lowwhichmaybe a crucial doesn’t depend on the wavelength of the laser, leading issue in biologicalsystems where opticalpower isdetri- alwaysto aresonancefrequency of23.5Hz. Thisisalso mental to cells in general [18]. Such a system would be true with white light. cheap, and even sun light can be used. Moreover, since itmightbedifficulttomakeathinfilmfromanyliquid, thisfilmcanbeenclosedinasofttransparentmembrane 4 Discussion as soon as this membrane is soft enough to vibrate under light excitation.The resonance frequency may then be different from the lowresonance frequency we found When a force is applied on the air/liquid interface the here. surface tension acts as a spring. Then the oscillation frequency ν of the film should be of the order of ν=1/2π(α/m)1/2 (1) 5 Conclusion α being the surface tension coefficient, m being homo- geneoustoamass.Here,thefilmmassisabout7×10−7 Wehaveshownthatevenalowpowerlasercandramat- kg. With α=0.03 Nm−1 [13], one finds ν =30 Hz. This ically change the dynamical behavior of a liquid film. At low power, a modulated laser beam enhanced the is the correct order of magnitude for the resonance we lifetime of a thin soap by a factor of two, whereas at found. However, this is just an estimation of it. A two higher power,the film breaks nearly instantly.Thefilm timeshighermassforexample,accordingtoequation1, vibrates in a symmetric mode and behaves as a vibrat- would lead to a 21 Hz resonance frequency, closer to ing membrane in a diaphragm pump. the value we found. The resonance frequency probably Such a mechanical action on a liquid interface at depends onthefilmsurfaceandonitschemicalproper- resonancewithoutanyinvasiveoperationcouldalsofind ties. For example, the interfacial visco-elasticity of the issuesinbiologysuchasinthe-in-or-outpumpingofliq- liquid film can be studied by its mechanical response, uids in biologicalcells. It may also be of timely interest with exchanges between surface and bulk from the film in dermatologyto remove angioma,in opthalmologyto and also from the meniscus between the film and the distroy floaters, or in cataract operation. frame. Moreover, it is difficult to increase significantly the surface tensionof a liquidfilm, however vibratinga fiberglass with a higher surface tension could to be an Acknowledgments alternative. The authors thank J. R. Thébaut for technical help. Thefilmindeedvibratesonasymmetricmode(also This work has been performed within the EU COST called squeezing mode), because the laser bends each action MP 1205 Advances in Optofluidics: integration interface in a symmetrical way. The film inflates when of optical control and photonics with microfluidics. the laseris onanddeflates when off, likein a breathing mode. It is different from the asymmetric modes (also called bending mode) usually seen in soap films under References acoustic excitationwhere both interfaces are bended in anasymmetricway[14–16].Here,wecan’texcitebend- ing modes, we only excite breathing modes which may [1] A.Ashkin,J.M.Dziedzic,Phys.Rev.Lett.30,139 (1973). have more practical applications. 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