Table Of ContentESJphysics2017
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 un-
der 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. The liquid within the [2] A.Casner,J.P.Delville,Phys.Rev.Lett.90,144503
(2003).
film ispumped inand outwhen thelaserismodulated.
[3] S.M. Barnett,R.Loudon,J.Phys.B39,S671(2006).
Evenatverylowpower(2mW),withnaturallight,the
[4] O.Emile, J.Emile, Phys.Rev.Lett.106, 183904 (2011).
filmisvibratinglikeamembraneinadiaphragmpump. [5] O.Emile, J.Emile, F.Casanova,EPL101,34005 (2013).
4 OlivierEMILEandJanineEMILE
[6] O.Emile,J.Emile,Optofluid.MicrofluidNanofluid.1,27
(2014).
[7] A.Ashkin,Science210, 1081(1980).
[8] N.G.Astrath,L.C.Malacarne,M.L.Baesso,
G.V.Lukasievicz,S.E.Bialkowski,Nat.Commun.5,4363
(2014).
[9] A.L.Klein,W.Bouwhuis,C.W.Visser,H.Lhuissier,
C.Sun,J.H.Snoeijer,E.Villermaux,D.Lohse,
H.Gelderblom,Phys.Rev.Appl.3,044018 (2015).
[10] A.Casner,J.P.Delville,Phys.Rev.Lett.87,054503
(2001).
[11] O.Emile,J.Emile,LabChip,14,3525(2014).
[12] J.Emile,F.Casanova,G.Loas,O.Emile,SoftMatter8,
7223 (2012).
[13] M.Cocera, O.Lopez,J.Estelrich,J.L.Parra,A.dela
Maza,Spectrosc. Int.J.16,235(2002).
[14] A.Boudaoud,Y.Couder,M.BenAmar,Phys.Rev.Lett.
82,3847 (1999).
[15] S.K.Acharige, F.Elias,C.J.Derec, SoftMatter10,8341
(2014).
[16] J.Seiwert, J.Pierre, B.Dollet,J.Fluid.Mech.788,183
(2016).
[17] M.Boyd-Moss,S.Baratchi,M.DiVenere, K.Khoshmanesh,
LabonaChip16,3177 (2016).
[18] E.K.Sackmann,A.L.Fulton,D.J.Beebe, Nature507, 181
(2014).
