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Microfluidic device based on surface acoustic wave
Sensors and Actuators B 118 (2006) 380–385
Microfluidic device based on surface acoustic wave D. Beyssen ∗ , L. Le Brizoual, O. Elmazria, P. Alnot Laboratoire de Physique des Milieux Ionis´es et Applications UMR-CNRS, Universit´e Henri Poincar´e Bvd des Aiguillettes BP 239, 54506 Vandoeuvre les Nancy, France Available online 23 May 2006
Abstract Microfluidic systems can be implemented for miniaturization of chemical and biological processes on a sub-millimeter scale. In this study, surface acoustic waves (SAW) were used to actuate small droplet (from 2 to 20 l) on planar surface of a piezoelectric substrate. In order to improve the droplet displacements, a hydrophobic film was deposited on the LiNbO3 substrate. We have deposited by plasma enhanced chemical vapor deposition a-CFX film. All those films were compared to a spin coated PVDF, the best hydrophobic film was realised for PG = 100 W and PR = 400 mT. This film has a sliding force around 85 N. We studied the effect of different viscosities by using water/glycerol mixtures. This device propelled a water droplet at 40 mm/s, while the velocity of a glycerol droplet will not exceed 2 mm/s. The study of the droplet motion by a high-speed acquisition camera demonstrates a periodical phenomenon at a frequency of 120 Hz for pure water. © 2006 Elsevier B.V. All rights reserved. Keywords: Microfluidic; SAW; Hydrophobic surface; Droplet actuation; Sliding force
1. Introduction Recently, micro electro mechanical systems (MEMS) have generated a rising interest in fluid actuation. Because of their wide range of applications from inkjet printer to Lab-on-Chip, fluid actuation in microchannels is more studied and used in microfluidic devices. But, with the miniaturisation of the channels sometimes the measured flows are very different from those predicted by the classical theory, established by Poiseuille. The variations can reach 50%, which is unacceptable when you work for example on biological applications. As an alternative, of actuation in microchannels, moving individual small drops on planar surface appear more suitable for operations of proportioning, of mixture, or chemical reactions. With a gauged volume from nanolitres to microlitres, each drop behaves like a microreactor. One can handle the drops on a surface, move them, divide them or mix them with another drop [1]. In this study, we work on surface acoustic wave (SAW) droplet actuation on planar surface. SAW devices have been widely used in RF signal processing and band pass filter applications, and have thus become the basis of a huge industry in mobile communication [2]. Moreover recently published works have demonstrated the interest of surface acoustic waves in microfluidic systems. Specifically,
Katsumi Chono et al. have used SAW in atomization system [3] while, Strobl et al. have worked on a mixing system and a fluidic actuator based on SAW [4,5]. Therefore, we can use SAW to actuate droplet and that can be interesting, SAW can be used to the localisation of liquid droplet on the substrate [6]. Our aim is to use a SAW device to actuate a small droplet (from 2 to 20 l) on the planar surface of a LiNbO3 substrate. Nevertheless, the question that arises here is, how a wave of few nanometers in amplitude can induce a droplet motion of few millimetres of diameter. In fact the acoustic wave generated by the inter-digited transducer (IDT) in contact with a liquid, radiates a longitudinal wave into the droplet. If the RF power is sufficient, the wave attenuation due to the viscosity forces creates an acoustic pressure gradient in the droplet along the direction of the wave propagation. This pressure gradient produces a force in the same direction and induces the fluid flow. The non-linear phenomenon that transforms the wave attenuation in a steady fluid flow is called acoustic streaming [7]. In this study, we determined the hydrophobic properties of each film. Secondly we measured the evolution of the droplet velocity as a function of the dynamic viscosity. Finally, the solid/liquid contact angle during droplet motion is measured in order to understand the droplet motion. 2. Experimental set-up
∗
Corresponding author. Tel.: +33 3 83 68 41 63; fax: +33 3 83 68 49 33. E-mail address: [email protected] (D. Beyssen).
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.04.084
Our SAW devices were designed on a 128◦ rotated Y-cut X-propagating LiNbO3 2 wafer. Each IDT has 50 pairs of fin-
D. Beyssen et al. / Sensors and Actuators B 118 (2006) 380–385
Fig. 1. Experimental set-up with the RF power supply (Rhode & Schwarz) and the RF amplifier (Mini-Circuits ZHL-SW-1) connected to the LiNbO3 Wafer.
gers with a spatial period of 100 m and an aperture of 5 mm. These devices exhibit two center frequencies at 39.92 (in X direction) and 36.40 MHz (in a direction perpendicular to X) along which the corresponding surface wave velocities are 3992 and 3640 m/s. The RF signal from a standard signal generator (Rhode & Schwarz) was amplified by a RF power amplifier (Mini-Circuits ZHL-SW-1) before being applied to the SAW device (Fig. 1). The films a-CFX was deposited using plasma enhanced chemical vapor deposition reactor. We have fixed the deposition time to 4 min and the gas flow to 100 sccm. The pressure was varied from 200 to 500 mTorr and the RF power used were 100, 150 and 200 W. The samples were characterized by XPS and AFM techniques. The composition of CFX coatings were measured using a Kratos XPS instrument functioning with an actived charge neutralizer and having a monochromatized Al K␣ X-rays source. The films surface morphologies were observed with a Scientific Park TMCP AFM in noncontact mode. Moreover, the PVDF–TrFE film (PolyVinyliDene Fluoride–TriFluoroethylene) has been realised by spin-coating (3000 turn/min) and present a thickness of 3 m. This microfluidic device permits the control of the droplet displacement in the both perpendicular directions. The evolution of the contact angle was observed with a high-speed acquisition camera FTA (360 frames/s) and the droplet velocity was measured with a conventional CCD camera (25 frames/s).
3. Results and discussions The contact angle hysteresis θ = θ a − θ r is a very important parameter when the droplet motion is considered. This can be seen from Eq. (1) originally presented by Furmidge [8], which
381
Fig. 2. Sliding force values for a-CFX films vs. reactor pressure at fixed generator power. For information, the value for PVDF.
expresses the required force to move a droplet on the surface. θa + θr Fs = 2Rγlv sin (cos θr − cos θa ) (1) 2 where R is the radius of the droplet (m), γ lv the surface tension liquid/vapor (N/m), θ a and θ r the advancing and receding angle. The sliding force can be measured easily by tilting the surface with a droplet of known volume and recording the angle when the droplets start to move. We have measured the sliding force for each hydrophobic film. The mobility of the droplet on a surface can be improved by reducing the contact angle hysteresis. Therefore the required surface must be as smooth and homogeneous as possible. We studied as a function of the discharge parameter the sliding force (Fig. 2). The best a-CFX film was realised for PG = 100 W and PR = 400 mT. This film should be compared to PVDF film, which presents also a low sliding force. As can be seen those two coatings present approximately the same sliding force (FS = 85 N). Those two previous fluorinated coating were used to improve the surface properties of the device. The fluctuation of sliding property can be explained by surface roughness and/or surface chemical composition [9]. We have performed AFM and XPS measurements in order to investigate the surface properties of our films. In general, it has been observed that an increased surface roughness leads in a higher contact angles and consequently in a lower sliding force determined by Furmidge. The AFM measurements show that roughness of the films is very weak and comparable for each film. Indeed, root mean square surface roughness does not exceed and is around 0.5 nm. The XPS wide spectra indicate the presence of fluorine, carbon and very small amount of oxygen and nitrogen in all the coats (Fig. 3). The Fig. 4 shows the C1s XPS spectrum in the energy range of 280–300 eV of film deposited under (400 mT, 100 W) conditions. These C1s spectra were decomposed with “XPSpeak” program into six Gaussian peaks, corresponding to CF3
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D. Beyssen et al. / Sensors and Actuators B 118 (2006) 380–385
Fig. 3. Wide XPS spectrum of the film deposited under 400 mT and 100 W realised by PECVD process with CHF3 precursor.
(293.8 eV), CF2 (292.1 eV), CF (290 eV), C–CFn (287.6 eV), C–C (285.6 eV) and C pollution (283.3 eV). More precisely, we shall be interested in the percentage of –CF3 groups because the good sliding properties of surfaces with high –CF3 content has already been observed and explained [9]. Finally, the CFX film that possesses the higher content (4%), correspond to the coating realised under (400 mT, 100 W). The lower sliding force measured for the previous film is attributed to an increase of the –CF3 groups at the surface of the film. In this system, we studied the effect of different viscosities by using water/glycerol mixtures. Those values at 25 ◦ C varied from 0.89 mPa.s for pure water, to 934 mPa.s for pure glycerol. We notice that the threshold power PTh , required to move the droplet, increases with viscosity. As an example, the forward
Fig. 4. C1s XPS-spectrum of film deposited under (400 mT, 100 W) conditions. The six Gaussian peaks, correspond to CF3 (293.8 eV), CF2 (292.1 eV), CF (290 eV), C–CFn (287.6 eV), C–C (285.6 eV) and C pollution (283.3 eV).
Fig. 5. Droplet velocity evolution vs. dynamic viscosity (water/glycerol mixture used) on CFX film.
power measured PF , is nearly 34 dBm (2.518 W) for pure glycerol and 28 dBm (0.631 W) for pure water. Secondly the droplet velocities strongly depend on the liquid viscosity. Typically, a water droplet can be propelled to 40 mm/s, while the velocity of a glycerol droplet will not exceed 2 mm/s (Fig. 5). The energy
Fig. 6. Evolution of advancing and receding contact angle, θ a and θ r during motion droplet (5 l) on PVDF-TrFE film. (a) RF Power = 0, (b) P < PT : vibration of droplet free surface, (c) P = PT : beginning of droplet motion, (d) P = PT : droplet motion at constant velocity, (e) Evolution of θ a and θ r during droplet displacement.
D. Beyssen et al. / Sensors and Actuators B 118 (2006) 380–385
Fig. 7. Periodical distortion of droplet: step a: droplet adopts a spherical shape on our hydrophobic coating (t = 0 s), step b: the droplet stand up and the solid/liquid interface decrease (t = 0.0028 s), step c: the droplet top bend to the side, in the direction of SAW propagation (t = 0.0056 s), step d: the three-phase contact line advances and the droplet adopts a spherical shape again (t = 0.0084 s).
loss due to the viscous friction may increase with viscosity and reduces the droplet velocity. Because of the excellent properties of PVDF, we measured the evolution of the advancing contact angle θ a and the receding contact angle θ r during the motion of the water droplet on our PVDF surface (Fig. 6a). Firstly, absorbed energy by the droplet is transformed in vibration energy (Fig. 6b). Secondly the droplet was deformed by the acoustic pressure (Fig. 6c) and began to move (Fig. 6d). The Fig. 4e shows the measured values of those angles in each steps (a)–(d). The increase of the contact angle hysteresis θ = θ a − θ r is directly related to the actuation force produced by the SAW. Next, we have investigated with a highspeed camera the distortion of the droplet caused by acoustic pressure in order to decompose the droplet motion. In fact during its displacement, a water droplet (volume = 2 l) get out of shape with a cyclical way (Fig. 7). We have measured as can be seen in Fig. 7, three periodical steps in the droplet distortion. The periodicity or frequency of the droplet behaviour was determined with a high-speed acquisition camera FTA (360 frames per second equivalent to 1 frame per 0.0028 s). We note that a multiple of the frequency could be recorded due to the stroboscopic effect of the frame rate acquisition of the camera. The frequency of this distortion cycle is close to 120 Hz. Moreover, in order to describe with precision this phenomenon, we have measured the height “h” and the diameter “d” of the droplet during its displacement (Fig. 7). We consider the initial height h0 and diameter d0 , of the droplet and we plotted the normalized height h/h0 and the normalized diameter d/d0 evolution as a function of the droplet displacement duration on Fig. 8. We can notice that the droplet suffers periodically strong deformations. The droplet stand up more than once and half her initial height because of longitudinal wave emitted into the droplet when the SAW is attenuated at the solid/liquid interface.
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Fig. 8. Evolution of normalized height h/h0 and diameter d/d0 during water droplet motion (2 l).
More precisely, if the SAW is in contact with a liquid and if the SAW has any displacement component normal to the surface, this is the case of the Rayleigh wave, the SAW power is leaking into the fluid in form of a longitudinal wave. Therefore, this leaky SAW (LSAW) decay exponentially with distance at the solid/liquid interface [4]. Because the sound velocity in fluids VL is always smaller than the SAW-velocity in the solid substrate VS , longitudinal wave radiates into the liquid with diffraction angle of: θ = arcsin
VL VSolid
(2)
The pressure applied by the wave on the droplet free surface contributes to its distortion. Uchida et al. [10] have measured this acoustic streaming force after a longitudinal wave propagation into the liquid of 10 mm. Her SAW device were performed on 128◦ rotated Y cut X propagating LiNbO3 with an optimal frequency of 50 MHz. In our case the droplet lateral size is around 1 mm or less. Firstly, this force is in the order of 100 N when the SAW amplitude is near than one nanometer. When the RF power is sufficient to actuate a droplet, the SAW amplitude is also near than few nanometer [11]. This force is comparable as the sliding force required to move a droplet of known volume on a particular coating. We notice that between the first and the third step, the droplet minimizes its own interface with the solid (Fig. 7). This favours its displacement. We have measured on a droplet, presenting a viscosity five times higher than water, the vibration frequency. We observe that the droplet free surface vibrates at a frequency of 20 Hz. The step of three-phase contact line displacement appears six times less quickly as the same phenomenon with a pure water droplet, which reduces drastically the droplet velocity displacement on the surface. Next, the advancing distance of three phase contact lines has been determined as a function of distortion cycle number (Fig. 9). We note that the covered distance by the
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D. Beyssen et al. / Sensors and Actuators B 118 (2006) 380–385
tion) where temperature is periodically controlled, for example 95 ◦ C → 50 ◦ C → 70 ◦ C → 95 ◦ C. In fact, PCR allows, by temperature cycle, to duplicate DNA or RNA sequences. Acknowledgements We would like to thank J.J. Ehrahardt, J. Lambert and C. Tiusan of Universit´e Henri Poincar´e (Nancy) respectively for XPS and AFM measurements on our samples. References
Fig. 9. The advancing distance of three phase contact lines as a function of distortion cycle number.
three-phase contact line for one period or one distortion cycle is always in the range of 100–150 m. The observed variations were linked to the surface state, in fact impurity such as dust may deters the three-phase contact line displacement. Moreover after 13 cycles, the droplet has moved from a distance equivalent to her diameter. This corresponding to a droplet velocity of 40 mm.s−1 , which is equal to the velocities measured in Fig. 3 with the digital camera. 4. Conclusion We have demonstrated that SAW can be used to actuate small amounts of liquids of viscosities extending over a large range (from 1 to 1000 mPa s). The PVDF layer appears better than a-CFX for droplet displacement as the former presents a slightly lower sliding force than the latter. Moreover, the force exerted by SAW on the droplet has been already measured and it is comparable to the sliding force FS . We clearly measured a three-step motion of the droplet for water and during its displacement, the water droplet get out of shape with a cyclical way at a frequency of 120 Hz. This technique to move and control small amounts of liquid by employing SAW is an alternative to micro-channels and will result in a new generation of instrumentation for biological or chemical analyses. Indeed, this technique allows displacement, merger, separation of droplets like the EWOD technique (ElectroWetting On Dielectric), which is the more used for droplet microfluidic on open surface. Moreover, when two droplets merge, the mixture of these two substances is realised quickly [12] compared to diffusion process, thanks to internal mixture in the droplet caused by SAW streaming. And recently, Kondoh et al. were shown that SAW-liquid droplet system could be used as heating system or thermocycler [13]. By example, it’s an alternative for PCR (polymerase chain reac-
[1] P. Tabeling, Introduction a` la microfluidique, Editions Belin, Paris, 2003, 238–253. [2] C.K. Campbell (Ed.), Surface Acoustic Wave Devices for Mobile and Wireless Communications, Academic Press, 1998. [3] K. Chono, N. Shimizu, Y. Matsui, Jpn J. Appl. Phys. 43 (5b) (2004) 2987–2991. [4] C.J. Strobl, A. Rathgeber, A. Wixforth, Planar microfluidic processors, IEEE Ultrason. Symp. 1 (2002) 255–258. [5] S. Alzuagua, W. Daniau, J.F. Manceau, S. Ballandras, F. Bastien, Displacement of droplets on a surface using ultrasonic vibration/World Congress on Ultrasonics, September 7, Paris, France, 2003. [6] S. Alzuaga, S. Ballandras, F. Bastien, W. Daniau, B. Gauthier-Manuel, J.F. Manceau, A large scale X–Y positioning and localisation system of liquid droplet using SAW on LiNbO3 , in: 2003 IEEE International Ultrasonics Symposium, Honolulu, Hawaii, USA, 2003. [7] W.L.M. Nyborg, W.P. Mason, Acoustic streaming, Phys. Acoust., Part B 1 (1965) 265. [8] C.G.L. Furmidge, J. Colloid Sci. 17 (1962) 309–324. [9] J.H. Wang, J.J. Chen, R.B. Timmons, Chem. Mater. 8 (1996) 2212–2214. [10] T. Uchida, T. Suzuki, Investigation of acoustic streaming excited by surface acoustic wave, IEEE Ultrason. Symp. (1995) 1081–1084. [11] A. Renaudin, P. Tabourier, Plateforme SAW d´edi´ee a` la microfluidique discr`ete pour applications biologiques, in: Congr`es SHF “Microfluidique 2004”, Toulouse, 2004, pp. 12–17. [12] A. Wixforth, Acoustically driven planar microfluidics, Superlattices Microstruct. 33 (2003) 389–396. [13] J. Kondoh, N. Shimizu, Y. Matsui, M. Sugimoto, S. Shiokawa, Development of thermocycler for small liquid droplets, in: 2005 IEEE Ultrasonics Symposium, Vol. 2, Rotterdam, 2005, pp. 1023–1027.
Biographies Beyssen Denis was born in 1979 in Brive-la-Gaillarde, France. He received the Master degree in Physics and Chemistry from the University of Limoges. Presently, he is a Ph.D. student in Microsystems at the Laboratoire de la Physique des Milieux Ionis´es et Applications (LPMIA) in Nancy, France. His current research concerns fluid actuation by Surface Acoustic Wave on piezoelectric substrates.
Laurent Le Brizoual was born in La Rochelle, France in 1973. He received the Ph.D degree in material science in the Universit´e de Nantes, France. His dissertation was dedicated to Ti–Si–N films for diffusion barrier between copper and silicon. He joined the Laboratoire de la Physique des Milieux Ionis´es et Applications (LPMIA) in Nancy, France, as a permanent staff member in 2000. His current interests are in piezoelectric materials for SAW devices and their applications.
D. Beyssen et al. / Sensors and Actuators B 118 (2006) 380–385 Omar ELMAZRIA was born in Casablanca on May 5, 1968. He received his M.S. degree in Industrial computer science and opto-electronic from Universities of Metz, Nancy I, and Polytechnic Institute of Lorraine, in 1993 and his Ph.D degree in Electronic in 1996 from Metz University, France. In 1997 He joined the University of Nancy I as Associate Professor of Electronic and telecommunication and as Professor respectively in 1997 and 2003. His current research concerns the development of electrical application of a diamond thin film including diamond SAW device. He is a member of the IEEE UFFC Societies and the club EEA France.
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Patrick ALNOT graduated in chemical engineering at Ecole Nationale des Industries Chimiques in Nancy in 1981. In between 1979 and 1983, he went during 2 years at Liverpool University (GB) then 1 year at the Central Laboratory of IBM at San Jose (US). He got his PhD in Surface Science in 1988. Next and that during 11 years he worked at THALES group on III–V semiconductors surfaces. In 1989 within THALES group, he became the head of a research group involved in characterizations of devices such as TEGFET, HBT, MESFET and electronic materials. In 1994 he leaved THALES and joined University of Nancy as a Professor in Physics and Electronics. He is actually the head of Microsystems and Micro sensors group within the LPMIA and vice-chaiman of the research council of the University. He is the author and co-author of more than 70 publications in the field of surface science, physics of semiconductor materials, Microsystems and Micro sensors.
Microfluidic device based on surface acoustic wave D. Beyssen ∗ , L. Le Brizoual, O. Elmazria, P. Alnot Laboratoire de Physique des Milieux Ionis´es et Applications UMR-CNRS, Universit´e Henri Poincar´e Bvd des Aiguillettes BP 239, 54506 Vandoeuvre les Nancy, France Available online 23 May 2006
Abstract Microfluidic systems can be implemented for miniaturization of chemical and biological processes on a sub-millimeter scale. In this study, surface acoustic waves (SAW) were used to actuate small droplet (from 2 to 20 l) on planar surface of a piezoelectric substrate. In order to improve the droplet displacements, a hydrophobic film was deposited on the LiNbO3 substrate. We have deposited by plasma enhanced chemical vapor deposition a-CFX film. All those films were compared to a spin coated PVDF, the best hydrophobic film was realised for PG = 100 W and PR = 400 mT. This film has a sliding force around 85 N. We studied the effect of different viscosities by using water/glycerol mixtures. This device propelled a water droplet at 40 mm/s, while the velocity of a glycerol droplet will not exceed 2 mm/s. The study of the droplet motion by a high-speed acquisition camera demonstrates a periodical phenomenon at a frequency of 120 Hz for pure water. © 2006 Elsevier B.V. All rights reserved. Keywords: Microfluidic; SAW; Hydrophobic surface; Droplet actuation; Sliding force
1. Introduction Recently, micro electro mechanical systems (MEMS) have generated a rising interest in fluid actuation. Because of their wide range of applications from inkjet printer to Lab-on-Chip, fluid actuation in microchannels is more studied and used in microfluidic devices. But, with the miniaturisation of the channels sometimes the measured flows are very different from those predicted by the classical theory, established by Poiseuille. The variations can reach 50%, which is unacceptable when you work for example on biological applications. As an alternative, of actuation in microchannels, moving individual small drops on planar surface appear more suitable for operations of proportioning, of mixture, or chemical reactions. With a gauged volume from nanolitres to microlitres, each drop behaves like a microreactor. One can handle the drops on a surface, move them, divide them or mix them with another drop [1]. In this study, we work on surface acoustic wave (SAW) droplet actuation on planar surface. SAW devices have been widely used in RF signal processing and band pass filter applications, and have thus become the basis of a huge industry in mobile communication [2]. Moreover recently published works have demonstrated the interest of surface acoustic waves in microfluidic systems. Specifically,
Katsumi Chono et al. have used SAW in atomization system [3] while, Strobl et al. have worked on a mixing system and a fluidic actuator based on SAW [4,5]. Therefore, we can use SAW to actuate droplet and that can be interesting, SAW can be used to the localisation of liquid droplet on the substrate [6]. Our aim is to use a SAW device to actuate a small droplet (from 2 to 20 l) on the planar surface of a LiNbO3 substrate. Nevertheless, the question that arises here is, how a wave of few nanometers in amplitude can induce a droplet motion of few millimetres of diameter. In fact the acoustic wave generated by the inter-digited transducer (IDT) in contact with a liquid, radiates a longitudinal wave into the droplet. If the RF power is sufficient, the wave attenuation due to the viscosity forces creates an acoustic pressure gradient in the droplet along the direction of the wave propagation. This pressure gradient produces a force in the same direction and induces the fluid flow. The non-linear phenomenon that transforms the wave attenuation in a steady fluid flow is called acoustic streaming [7]. In this study, we determined the hydrophobic properties of each film. Secondly we measured the evolution of the droplet velocity as a function of the dynamic viscosity. Finally, the solid/liquid contact angle during droplet motion is measured in order to understand the droplet motion. 2. Experimental set-up
∗
Corresponding author. Tel.: +33 3 83 68 41 63; fax: +33 3 83 68 49 33. E-mail address: [email protected] (D. Beyssen).
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.04.084
Our SAW devices were designed on a 128◦ rotated Y-cut X-propagating LiNbO3 2 wafer. Each IDT has 50 pairs of fin-
D. Beyssen et al. / Sensors and Actuators B 118 (2006) 380–385
Fig. 1. Experimental set-up with the RF power supply (Rhode & Schwarz) and the RF amplifier (Mini-Circuits ZHL-SW-1) connected to the LiNbO3 Wafer.
gers with a spatial period of 100 m and an aperture of 5 mm. These devices exhibit two center frequencies at 39.92 (in X direction) and 36.40 MHz (in a direction perpendicular to X) along which the corresponding surface wave velocities are 3992 and 3640 m/s. The RF signal from a standard signal generator (Rhode & Schwarz) was amplified by a RF power amplifier (Mini-Circuits ZHL-SW-1) before being applied to the SAW device (Fig. 1). The films a-CFX was deposited using plasma enhanced chemical vapor deposition reactor. We have fixed the deposition time to 4 min and the gas flow to 100 sccm. The pressure was varied from 200 to 500 mTorr and the RF power used were 100, 150 and 200 W. The samples were characterized by XPS and AFM techniques. The composition of CFX coatings were measured using a Kratos XPS instrument functioning with an actived charge neutralizer and having a monochromatized Al K␣ X-rays source. The films surface morphologies were observed with a Scientific Park TMCP AFM in noncontact mode. Moreover, the PVDF–TrFE film (PolyVinyliDene Fluoride–TriFluoroethylene) has been realised by spin-coating (3000 turn/min) and present a thickness of 3 m. This microfluidic device permits the control of the droplet displacement in the both perpendicular directions. The evolution of the contact angle was observed with a high-speed acquisition camera FTA (360 frames/s) and the droplet velocity was measured with a conventional CCD camera (25 frames/s).
3. Results and discussions The contact angle hysteresis θ = θ a − θ r is a very important parameter when the droplet motion is considered. This can be seen from Eq. (1) originally presented by Furmidge [8], which
381
Fig. 2. Sliding force values for a-CFX films vs. reactor pressure at fixed generator power. For information, the value for PVDF.
expresses the required force to move a droplet on the surface. θa + θr Fs = 2Rγlv sin (cos θr − cos θa ) (1) 2 where R is the radius of the droplet (m), γ lv the surface tension liquid/vapor (N/m), θ a and θ r the advancing and receding angle. The sliding force can be measured easily by tilting the surface with a droplet of known volume and recording the angle when the droplets start to move. We have measured the sliding force for each hydrophobic film. The mobility of the droplet on a surface can be improved by reducing the contact angle hysteresis. Therefore the required surface must be as smooth and homogeneous as possible. We studied as a function of the discharge parameter the sliding force (Fig. 2). The best a-CFX film was realised for PG = 100 W and PR = 400 mT. This film should be compared to PVDF film, which presents also a low sliding force. As can be seen those two coatings present approximately the same sliding force (FS = 85 N). Those two previous fluorinated coating were used to improve the surface properties of the device. The fluctuation of sliding property can be explained by surface roughness and/or surface chemical composition [9]. We have performed AFM and XPS measurements in order to investigate the surface properties of our films. In general, it has been observed that an increased surface roughness leads in a higher contact angles and consequently in a lower sliding force determined by Furmidge. The AFM measurements show that roughness of the films is very weak and comparable for each film. Indeed, root mean square surface roughness does not exceed and is around 0.5 nm. The XPS wide spectra indicate the presence of fluorine, carbon and very small amount of oxygen and nitrogen in all the coats (Fig. 3). The Fig. 4 shows the C1s XPS spectrum in the energy range of 280–300 eV of film deposited under (400 mT, 100 W) conditions. These C1s spectra were decomposed with “XPSpeak” program into six Gaussian peaks, corresponding to CF3
382
D. Beyssen et al. / Sensors and Actuators B 118 (2006) 380–385
Fig. 3. Wide XPS spectrum of the film deposited under 400 mT and 100 W realised by PECVD process with CHF3 precursor.
(293.8 eV), CF2 (292.1 eV), CF (290 eV), C–CFn (287.6 eV), C–C (285.6 eV) and C pollution (283.3 eV). More precisely, we shall be interested in the percentage of –CF3 groups because the good sliding properties of surfaces with high –CF3 content has already been observed and explained [9]. Finally, the CFX film that possesses the higher content (4%), correspond to the coating realised under (400 mT, 100 W). The lower sliding force measured for the previous film is attributed to an increase of the –CF3 groups at the surface of the film. In this system, we studied the effect of different viscosities by using water/glycerol mixtures. Those values at 25 ◦ C varied from 0.89 mPa.s for pure water, to 934 mPa.s for pure glycerol. We notice that the threshold power PTh , required to move the droplet, increases with viscosity. As an example, the forward
Fig. 4. C1s XPS-spectrum of film deposited under (400 mT, 100 W) conditions. The six Gaussian peaks, correspond to CF3 (293.8 eV), CF2 (292.1 eV), CF (290 eV), C–CFn (287.6 eV), C–C (285.6 eV) and C pollution (283.3 eV).
Fig. 5. Droplet velocity evolution vs. dynamic viscosity (water/glycerol mixture used) on CFX film.
power measured PF , is nearly 34 dBm (2.518 W) for pure glycerol and 28 dBm (0.631 W) for pure water. Secondly the droplet velocities strongly depend on the liquid viscosity. Typically, a water droplet can be propelled to 40 mm/s, while the velocity of a glycerol droplet will not exceed 2 mm/s (Fig. 5). The energy
Fig. 6. Evolution of advancing and receding contact angle, θ a and θ r during motion droplet (5 l) on PVDF-TrFE film. (a) RF Power = 0, (b) P < PT : vibration of droplet free surface, (c) P = PT : beginning of droplet motion, (d) P = PT : droplet motion at constant velocity, (e) Evolution of θ a and θ r during droplet displacement.
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Fig. 7. Periodical distortion of droplet: step a: droplet adopts a spherical shape on our hydrophobic coating (t = 0 s), step b: the droplet stand up and the solid/liquid interface decrease (t = 0.0028 s), step c: the droplet top bend to the side, in the direction of SAW propagation (t = 0.0056 s), step d: the three-phase contact line advances and the droplet adopts a spherical shape again (t = 0.0084 s).
loss due to the viscous friction may increase with viscosity and reduces the droplet velocity. Because of the excellent properties of PVDF, we measured the evolution of the advancing contact angle θ a and the receding contact angle θ r during the motion of the water droplet on our PVDF surface (Fig. 6a). Firstly, absorbed energy by the droplet is transformed in vibration energy (Fig. 6b). Secondly the droplet was deformed by the acoustic pressure (Fig. 6c) and began to move (Fig. 6d). The Fig. 4e shows the measured values of those angles in each steps (a)–(d). The increase of the contact angle hysteresis θ = θ a − θ r is directly related to the actuation force produced by the SAW. Next, we have investigated with a highspeed camera the distortion of the droplet caused by acoustic pressure in order to decompose the droplet motion. In fact during its displacement, a water droplet (volume = 2 l) get out of shape with a cyclical way (Fig. 7). We have measured as can be seen in Fig. 7, three periodical steps in the droplet distortion. The periodicity or frequency of the droplet behaviour was determined with a high-speed acquisition camera FTA (360 frames per second equivalent to 1 frame per 0.0028 s). We note that a multiple of the frequency could be recorded due to the stroboscopic effect of the frame rate acquisition of the camera. The frequency of this distortion cycle is close to 120 Hz. Moreover, in order to describe with precision this phenomenon, we have measured the height “h” and the diameter “d” of the droplet during its displacement (Fig. 7). We consider the initial height h0 and diameter d0 , of the droplet and we plotted the normalized height h/h0 and the normalized diameter d/d0 evolution as a function of the droplet displacement duration on Fig. 8. We can notice that the droplet suffers periodically strong deformations. The droplet stand up more than once and half her initial height because of longitudinal wave emitted into the droplet when the SAW is attenuated at the solid/liquid interface.
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Fig. 8. Evolution of normalized height h/h0 and diameter d/d0 during water droplet motion (2 l).
More precisely, if the SAW is in contact with a liquid and if the SAW has any displacement component normal to the surface, this is the case of the Rayleigh wave, the SAW power is leaking into the fluid in form of a longitudinal wave. Therefore, this leaky SAW (LSAW) decay exponentially with distance at the solid/liquid interface [4]. Because the sound velocity in fluids VL is always smaller than the SAW-velocity in the solid substrate VS , longitudinal wave radiates into the liquid with diffraction angle of: θ = arcsin
VL VSolid
(2)
The pressure applied by the wave on the droplet free surface contributes to its distortion. Uchida et al. [10] have measured this acoustic streaming force after a longitudinal wave propagation into the liquid of 10 mm. Her SAW device were performed on 128◦ rotated Y cut X propagating LiNbO3 with an optimal frequency of 50 MHz. In our case the droplet lateral size is around 1 mm or less. Firstly, this force is in the order of 100 N when the SAW amplitude is near than one nanometer. When the RF power is sufficient to actuate a droplet, the SAW amplitude is also near than few nanometer [11]. This force is comparable as the sliding force required to move a droplet of known volume on a particular coating. We notice that between the first and the third step, the droplet minimizes its own interface with the solid (Fig. 7). This favours its displacement. We have measured on a droplet, presenting a viscosity five times higher than water, the vibration frequency. We observe that the droplet free surface vibrates at a frequency of 20 Hz. The step of three-phase contact line displacement appears six times less quickly as the same phenomenon with a pure water droplet, which reduces drastically the droplet velocity displacement on the surface. Next, the advancing distance of three phase contact lines has been determined as a function of distortion cycle number (Fig. 9). We note that the covered distance by the
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tion) where temperature is periodically controlled, for example 95 ◦ C → 50 ◦ C → 70 ◦ C → 95 ◦ C. In fact, PCR allows, by temperature cycle, to duplicate DNA or RNA sequences. Acknowledgements We would like to thank J.J. Ehrahardt, J. Lambert and C. Tiusan of Universit´e Henri Poincar´e (Nancy) respectively for XPS and AFM measurements on our samples. References
Fig. 9. The advancing distance of three phase contact lines as a function of distortion cycle number.
three-phase contact line for one period or one distortion cycle is always in the range of 100–150 m. The observed variations were linked to the surface state, in fact impurity such as dust may deters the three-phase contact line displacement. Moreover after 13 cycles, the droplet has moved from a distance equivalent to her diameter. This corresponding to a droplet velocity of 40 mm.s−1 , which is equal to the velocities measured in Fig. 3 with the digital camera. 4. Conclusion We have demonstrated that SAW can be used to actuate small amounts of liquids of viscosities extending over a large range (from 1 to 1000 mPa s). The PVDF layer appears better than a-CFX for droplet displacement as the former presents a slightly lower sliding force than the latter. Moreover, the force exerted by SAW on the droplet has been already measured and it is comparable to the sliding force FS . We clearly measured a three-step motion of the droplet for water and during its displacement, the water droplet get out of shape with a cyclical way at a frequency of 120 Hz. This technique to move and control small amounts of liquid by employing SAW is an alternative to micro-channels and will result in a new generation of instrumentation for biological or chemical analyses. Indeed, this technique allows displacement, merger, separation of droplets like the EWOD technique (ElectroWetting On Dielectric), which is the more used for droplet microfluidic on open surface. Moreover, when two droplets merge, the mixture of these two substances is realised quickly [12] compared to diffusion process, thanks to internal mixture in the droplet caused by SAW streaming. And recently, Kondoh et al. were shown that SAW-liquid droplet system could be used as heating system or thermocycler [13]. By example, it’s an alternative for PCR (polymerase chain reac-
[1] P. Tabeling, Introduction a` la microfluidique, Editions Belin, Paris, 2003, 238–253. [2] C.K. Campbell (Ed.), Surface Acoustic Wave Devices for Mobile and Wireless Communications, Academic Press, 1998. [3] K. Chono, N. Shimizu, Y. Matsui, Jpn J. Appl. Phys. 43 (5b) (2004) 2987–2991. [4] C.J. Strobl, A. Rathgeber, A. Wixforth, Planar microfluidic processors, IEEE Ultrason. Symp. 1 (2002) 255–258. [5] S. Alzuagua, W. Daniau, J.F. Manceau, S. Ballandras, F. Bastien, Displacement of droplets on a surface using ultrasonic vibration/World Congress on Ultrasonics, September 7, Paris, France, 2003. [6] S. Alzuaga, S. Ballandras, F. Bastien, W. Daniau, B. Gauthier-Manuel, J.F. Manceau, A large scale X–Y positioning and localisation system of liquid droplet using SAW on LiNbO3 , in: 2003 IEEE International Ultrasonics Symposium, Honolulu, Hawaii, USA, 2003. [7] W.L.M. Nyborg, W.P. Mason, Acoustic streaming, Phys. Acoust., Part B 1 (1965) 265. [8] C.G.L. Furmidge, J. Colloid Sci. 17 (1962) 309–324. [9] J.H. Wang, J.J. Chen, R.B. Timmons, Chem. Mater. 8 (1996) 2212–2214. [10] T. Uchida, T. Suzuki, Investigation of acoustic streaming excited by surface acoustic wave, IEEE Ultrason. Symp. (1995) 1081–1084. [11] A. Renaudin, P. Tabourier, Plateforme SAW d´edi´ee a` la microfluidique discr`ete pour applications biologiques, in: Congr`es SHF “Microfluidique 2004”, Toulouse, 2004, pp. 12–17. [12] A. Wixforth, Acoustically driven planar microfluidics, Superlattices Microstruct. 33 (2003) 389–396. [13] J. Kondoh, N. Shimizu, Y. Matsui, M. Sugimoto, S. Shiokawa, Development of thermocycler for small liquid droplets, in: 2005 IEEE Ultrasonics Symposium, Vol. 2, Rotterdam, 2005, pp. 1023–1027.
Biographies Beyssen Denis was born in 1979 in Brive-la-Gaillarde, France. He received the Master degree in Physics and Chemistry from the University of Limoges. Presently, he is a Ph.D. student in Microsystems at the Laboratoire de la Physique des Milieux Ionis´es et Applications (LPMIA) in Nancy, France. His current research concerns fluid actuation by Surface Acoustic Wave on piezoelectric substrates.
Laurent Le Brizoual was born in La Rochelle, France in 1973. He received the Ph.D degree in material science in the Universit´e de Nantes, France. His dissertation was dedicated to Ti–Si–N films for diffusion barrier between copper and silicon. He joined the Laboratoire de la Physique des Milieux Ionis´es et Applications (LPMIA) in Nancy, France, as a permanent staff member in 2000. His current interests are in piezoelectric materials for SAW devices and their applications.
D. Beyssen et al. / Sensors and Actuators B 118 (2006) 380–385 Omar ELMAZRIA was born in Casablanca on May 5, 1968. He received his M.S. degree in Industrial computer science and opto-electronic from Universities of Metz, Nancy I, and Polytechnic Institute of Lorraine, in 1993 and his Ph.D degree in Electronic in 1996 from Metz University, France. In 1997 He joined the University of Nancy I as Associate Professor of Electronic and telecommunication and as Professor respectively in 1997 and 2003. His current research concerns the development of electrical application of a diamond thin film including diamond SAW device. He is a member of the IEEE UFFC Societies and the club EEA France.
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Patrick ALNOT graduated in chemical engineering at Ecole Nationale des Industries Chimiques in Nancy in 1981. In between 1979 and 1983, he went during 2 years at Liverpool University (GB) then 1 year at the Central Laboratory of IBM at San Jose (US). He got his PhD in Surface Science in 1988. Next and that during 11 years he worked at THALES group on III–V semiconductors surfaces. In 1989 within THALES group, he became the head of a research group involved in characterizations of devices such as TEGFET, HBT, MESFET and electronic materials. In 1994 he leaved THALES and joined University of Nancy as a Professor in Physics and Electronics. He is actually the head of Microsystems and Micro sensors group within the LPMIA and vice-chaiman of the research council of the University. He is the author and co-author of more than 70 publications in the field of surface science, physics of semiconductor materials, Microsystems and Micro sensors.