- Email: [email protected]
Microfluidic motion generation with acoustic waves
A
ELSEVIER
Sensors and Actuators
Microfluidic
A 66 ( 1998) 355-360
PHYSICAL
motion generation with acoustic waves Xu Zhu, Eun Sok Kim ‘$
Abstract This paper presents a novel approach for producing microfluidic motion by loosely focused acoustic waves (generated by piezoelectric zinc oxide thin film). Results show that the acoustic waves generated by radio frequency (r.f.) sources with frequencies corresponding to the thickness-mode resonances of the piezoelectric film are very effective in moving liquid around when the waves are loosely focused. The device operates without any significant temperature increase in the liquid, and will be very attractive for mixing or transporting temperaturesensitive fluids. 0 1998ElsevierScienceS.A. All rightsreserved. Kcworcls:
hlicrofluidic
motion;
Acoustic
waves; Piczoelectric
1. Introduction Microfluidic processing systems [ I] need to transport and/or mix two or more fluids in accurately controlled amountsin a reasonableamount of time. Since many microfluidic devices are fabricated in a planar lithographic environment, most of the macroscopic approachesfor fluid mixing [2] (e.g., turbulence, three-dimensionalflow and mechanicalactuation, etc.) are inapplicable. Chaotic advection for liquid mixing has been reported for a microfluidic device [ 31, but addsconsiderableheat to the liquid. Another micromixing technique of using fast diffusion [4] requires small nozzles, which tend to get clogged. One promising technique is to use ultrasonic Lamb waves [ 5,6] to generate masstransport. We have developed an efficient generationof microfluidic motion usingloosely focused acousticwavesby piezoelectric zinc oxide (ZnO) film. High-intensity bursts of acoustic waves (due to loosefocusing to a relatively small spot at an air-liquid surface) produce strong liquid movements in a liquid chamber which is about 1.5 mm X 1.5 mmX 0.5 mm in size, and have beenobservedto bc very effective in mixing liquid. The liquid flow in this small chamber is laminar, according to the Reynolds number (Re) calculated from Re= t L/6)/u
2. Theory Our self-focusing acoustic transducershown in Fig. I is basedon the concept of a constructive wave interference
(1)
where U, S and 11are the characteristic velocity (taken to be IO mm s-’ ), a length scale (about I mm in our case) and * Corresponding
the kinematic viscosity ( 1 mm2s- ’ for water), respectively. The Reynoldsnumber for our caseis about 10,which is much lessthan 2000 (above which the flow is turbulent) andhigher than I (below which the flow is creeping). Thus, the effects of turbulence flow or creeping flow neednot be considered in our case. With a negligible amountof heatgenerationaccompanying the acousticenergy, our transducerdoesnot heatup the liquid in any appreciableamount, and mixing by convection heat transfer is negligible. Thus. the transducerwill be very attractive for mixing temperature-sensitive fluids. This paper describesthe operating principle, design and fabrication of the microfluidic mixer along with our recent observationsof our device’s very large dynamic responses.
author. Tel.: + I 808 956 53 09. Fax: + 1 SO8 956 34 27.
E-ma\l: es~im~wi\‘tki.cn%.\la~vaii.edu 0924-4247/98/$19.00 P/ls09’~-~247(97)01712-3
0
1998 Elsevier Science S.A. All rights reserved.
Fig. I. Cross-sectional view (alone, with typical liquid-flow directions) of the liquid mixer microfabricated on a silicon wafer. Also shown on the left is the electrode pattern of the transducer to form the acoustic-wave sources
for a constructive wave interference in the liquid.
356
X. Zh,
E.S. Kim /Sensors
rind Amrntors
[7,8] (similar to an optical Fresnel lens which blocks certain areas of light to get intensity enhancement). The transducer (when excited with r.f. signals) generates acoustic waves (through the piezoelectric activity of ZnO film), which propagate in the liquid toward the liquid-air interface. For an efficient acoustic-wave generation, the ZnO thickness is chosen to be an odd multiple of a half wavelength (i.e., (211+ l)h/2) which would have put the film at a thickness-mode resonance in an air-backed transducer [ 71. Also, the electrodes of the transducer are laid out such that the acoustic waves will constructively interfere to have a focal point a few hundred micrometers above the liquid-air interface [8]. In this way, we have waves loosely focused near the surface of the liquid. We intentionally choose the loose focusing in order to reduce the acoustic pressure directed upward at the surface to avoid liquid ejection. In addition to the intensified upward acoustic pressure, the transducer generates a lateral acoustic field as indicated by the solid lines with arrowheads in Fig. 1. These acoustic fields (both upward and lateral) cause a strong liquid flow inside the chamber. The lateral acoustic field causes a lateral (tangential to the water-air interface) liquid motion, which is enhanced by the phenomenon known as acoustic streaming [9] (which takes place at the water-air interphase boundary causing a tangential liquid motion along the boundary). As mentioned above. our lensless design borrows its concept from an optical Fresnel lens which blocks certain areas of light to get intensity enhancement. We make only certain areas of the piezoelectric film generate acoustic waves. which arrive at a focal point in phase. The other areas (that would have generated waves with a phase difference of r at the focal point) are designed not to generate any acoustic wave. These are so-called half-wave-band sources. Fig. 2 shows the acoustic-wave transducer with several annular sources. The acoustic waves generated by the successive annular sources are designed to arrive at the focal point v). which is a little above the liquid surface. with finite delays (equal to multiples of the wavelength) by ensuring that the radii (I;,) satisfy the following relation:
A 66 (1998)
35.5-360
f
2=0 Fig. 2. Electrode sources.
patterns
for the transducer
to form
the half-wave-band
obtained as the gradient of an acoustic-wave potential (i.e., H,.= - V,$) , where C$and I’ are the wave potential and the radial distance on the surface, respectively. The potential C$ at any point in water (for the half-wave-band sources shown in Fig. 2) [ 71 is obtained from
where k= 2vlh; R is the distance between a half-wave-band source and the focal point: s is the surface area of the transducer at ; = 0; LL(x’J’,~) is the particle displacement at the Si.,N?-water interface (i.e., the transducer surface adjacent to the water). The acoustic-wave potentials on the air-water surface as a function of the number of half-wave-band source have been numerically calculated, and are shown in Figs. 3-5. As we can see in the Figures, the wave potential has a sharp peak at the focal point on the air-water surface. Also, the more halfwave-band sources are used to generate the acoustic waves, the higher the peak acoustic-wave potential ( and the narrower the potential distribution) becomes. The radial displacement (Q) can be obtained from lfR= -h$/aR, and the numerical calculations of the radial displacements on the air-water surface for the cases of one. three and seven half-wave-band sources are shown in Figs. 6-8. The cross-cut views of Figs. 6-8 are given in Fig. 9 t%
from which we obtain
where n = I, 3, 5, 7... and A,, is the acoustic wavelength in liquid. While it is easy to see that the acoustic waves provide the necessary force to move liquid in the vertically upward direction, lateral motion of liquid needs some explanation, as described below. In particular. a constructive interference of lateral acoustic waves is described in detail. For an isotropic liquid. such as water, the particle displacement I[,. associated with the longitudinal waves can be
Fig. 3. Acoustic-wave potential on the air-water surface when seven half wave-band sources ilre used to generate the acoustic waves.
X. Zh,
ES.
Kim I Smsors
and Acrmtors
Fig. 3. Acoustic-wave potential on the air-water surface when three halfwave-band sources arc used to generate the acoustic waves.
Fig. 5. Acoustic-wave potential on the air-water surface when only one halfwave-band source is used to generate the acoustic waves.
A 66 (1 YW)
355-360
Fig. 7. Lateral particle displacement on the air-water surface when one circular and two annular sources are used to generate the acoustic waves.
Fig. 8. Lateral particle displacement on the air-water surface when one circular and six annular sources are used to generate the acoustic waves.
Fig. 6. Lateral particle displacement on the air-water surface when one circular source at the transducer center is used to generate the acoustic waves.
show the radial displacement along the radial direction from one edge to the center. In these Figures we see no radial displacement at the center of the surface, which makes sense physically since the actuating source is radially symmetric and the liquid is isotropic. Though the radial displacement is zero at the center, there is a relatively large peak of the radial displacement near the center. The peak displacement occurs at about 8 pm radially away from the center (Fig. 6)) when one circular source at the transducer center is used to generate
X104
edge Fig. 9. Cross-cut along the radial diaphragm.
radial position
center
view of Fgs. 6-5 to show the radial particle displacement direction from one edge to the center of the transducer
the acoustic waves. Similar results (with much larger displacement amplitudes) are obtained when we use one circular plus two annular-ring sources (Fig. 7) and one circular plus six annular-ring sources (Fig. 8).
Fig. IO. The radially directed particle displacement at any point in the liquid when one circular and six annular sources ~~re used to generate the acoustic waves.
Thus, we see that the constructive interference of the acoustic waves generated by our transducer also enhances the radial particle displacement, as shown in Fig. 9. The radial displacement is strongly dependent on the number of half-wave-band sources (Figs. 6-8). This kind of lateral displacement is a major driving mechanism for the convection liquid flow in our transducer. Fig. 10 shows the relative radial particle displacement at any point in the chamber. The distribution of the radial displacement near the liquid-Si,N,. interface is similar to that near the liquid-air interface except for the phase difference of YTas shown in Fig. 10. The 5~phase difference makes the particle displacements near those interfaces be directed in opposite directions, as illustrated by the solid lines with arrowheads in Fig. 1. The lateral displacement near the liquid-air interface is directed radially outward, while that near the transducer-water interface is directed radially inward. These lateral displacements (on two different levels of liquid) directed in opposite directions are the major reason for a strong lateral liquid motion.
3. Transducer
Si diSi Fig. I 1. Fabrication
processing
Al L?ima zno
steps for the acoustic-wave
liquid mixer.
Fig. 12. Top-view photograph of tht: t’abricated acoustic-wave liquid mixer. The bottom and top Al electrodes tire patterned into annular rings, while the ZnO is delineated into a solid circle with four bulunce barb eAtending toward the four corners of the square diaphragm.
4. Results and discussion The fabricated devices are tested with the set-up shown in Fig. 13. A continuous sinusoidal wave ( in the range 100-5001 MHz) from a signal generator is fed into a high-speed switch which is turned on/off with a modulating pulse duration set at, in this case,100ns.The r.f. power amplifier then amplifies
fabrication Water with micro-spheres Tryducer 7
The transducer is fabricated on a silicon wafer with the four-mask process shown in Fig. 11. After depositing and patterning 0.8 pm thick LPCVD low-stress silicon nitride. we remove silicon with KOH to form 1 mmX 1 mm membranes. on which 0.5 Km thick Al is evaporated and patterned for an annular bottom electrode. Then a 10 pm thick ZnO layer is sputter-deposited in Ar/O, environment from a ZnO target, followed by 0.5 pm thick Al evaporation and delineation for a top electrode. The segmented top and bottom Al electrodes act as half-wave-band sources as shown in the photograph of the fabricated transducer (Fig. 12).
~.L
Fig. 13. Schelnatic
representation
I‘,
of the test apparatus
for the liquid mixer.
X. Zhu,
E.S. Kim /Sw~sors
and Actuators
the r.f. wave up to OS-10 W level before feeding it to the transducer. We have tested our transducer with water mixed with fluorescent microspheres about 10 pm in diameter. When we apply a series of pulses (at a rate of I-70 kHz, with on-time of 0.1-l fJ-sj of 100400 MHz sinusoidal waves (0.15-10 W level ) to the transducer, we observe active liquid motions. This observation is facilitated by the movements of rhe microspheres in the liquid. The Row patterns of the microspheres are captured with a camera, and shown in Figs. 14 and 15. Fig. 14 shows the flow patterns seen from the top of the device without any tilting, while Fig. 15 shows the flow patterns seen at an angle from the top-view direction. In Fig. 15, we clearly see the loops of microspheres following the liquid flow pattern ( produced by the loosely focused acoustic waves ). The microspheres rotate around such a loop by being vertically pumped upward by the loosely focused acoustic waves from the lower portion of the liquid chamber (or near the transducer), radially moved outward near the air-liquid surface, vertically pumped downward, and then radially moved inward near the transducer. The rotational speed of the microspheres when driven at the resonance frequencies (of the thickness-mode vibration of the piezoelectric film: i.e., 240 and 480 MHz for the fundamental and second-harmonic resonances) is about 30-40 times higher than that when driven at frequencies other than those resonance frequencies. Since the energy used for the liquid motions is very low, there is very little heating in the liquid. This has been confirmed with negligible change in the liquid level (in the cavity that contains only about 2 ~1 of water) even when the device is continuously operated for more than 20 min. Another observation to confirm the negligible heating effect is that it takes almost the same time to evaporate the 2 ~1 of water whether the transducer is in usual operation or not.
A 66 (1998)
3X5-360
Fig. 15. Tilted top-view photograph an angle (from the direct top view) shadows.
of the device under tat taken at such to show the flow loops along with their
The area of the open surface in the transducer cavity (2 ~1 in volume) is about 1.5 mm X 1.5 mm, and the surface tension of the water (contained in the cavity) is large enough to hold the water in the cavity even if the transducer is positioned at 90” upward or upside down. We have operated the transducer in those odd positions. and observed flow patterns very similar to those when the transducer is positioned as shown in Fig. 1. The only difference in those different positions of the transducer is the gravity on the microspheres, which affects the population distribution of the microspheres in the cavity.
5. Conclusions A loosely focused acoustic-beam transducer (using a 0.8 km thick low-stress silicon-nitride membrane and a 10 p,rn thick piezoelectric ZnO film) has been fabricated on a silicon wafer, and shown to be very effective in moving about 2 ~1 of liquid. With fluorescent microspheres in the water, we have observed extremely fast movement of these microspheres when we apply a series of pulses (at l-70 kHz rate with ontime of 100 ns) that contains 240 or 480 MHz sinusoidal waves. This technique of heatless acoustic agitation can advantageously be applied to microfluidic systems where the Reynolds number is small and heating is not desirable.
Acknowledgements
Fig. 14. Top-view photograph of the device under test, which ahuws the flow pattern of the microspheres in water produced by loosely focused acoustic waves.
The authors wish to thank Hasnain Lakdawala of the University of Hawaii and Professor Richard M. White of U.C. Berkeley for help with the numerical calculation and for showing the usefulness of the fluorescent microspheres in various experiments, respectively. This material is based upon work supported by the Defense Advanced Research Projects Agency under contract DABT63-95-C-0039 and NSF CAREER Award #ECS-9501698.
360
X. Zhu, ES. Kim /Setxm
and Ac~wtors
References [II
S. Shoji and M. Esashi, Microflow device and aystems, J. Micromech. Microeng., 4 (1994) 157-171. 121 R.F. Berg and N.P. De Luca, Milliwatt mixer for small fluid samples. Rev. Sci. Instrum., 62 ( 1991) 527-529. [31 I. Evans, D. Liepmann and A.P. Pisano, Planar laminar mixer, IEEE MEMS ‘97. Nagoya, Japan. 1997, pp. 96-101. T&J. Lammerink, M. Elwenspoek and J.H.J. Fluitman, [41 R. Miyake, Micro mixer with fast diffusion, J. MEMS, 2 ( 1993) 238-253. R.M. White and R.T. Howe, Microtransport induced ISI R.M. Moroney, by ultrasonic Lamb waves, Appi. Phys. Lett., 59 ( 1991) 726-774. 161 T.R. Tsao, R.M. Moroney, B.A. Martin and R.M. White, Electrochemical detection of localized mixing produced by ultrasonic flexural waves. IEEE Ultrasonics Symp., Dec. 1991. pp. 937-940. 171 G.S. Kino, Acoustic Waves, Design Imaging and Analog Signal Processing, Prentice-Hall. Englewood Cliffs, NJ, 1987. ISI X. Zhu, E. Tran, W. Wang, ES. Kim and S.Y. Lee, Acoustic wave liquid ejector, Tech. Digest, Solid-State Sensor and Actuator Workshop, Hilron Head, SC, USA, 1996, pp. 250-382. iSI W.L. Nyborg, Acoustic streaming, in W.P. Mason (ed.). Physical Acoustics, Vol. 2B. Academic Press. New York, 1965, pp. 265-283.
Biographies XI{ Z/W! was born in 1970 in Xi’an, China. He received his B.S.E.E. degree in electronics engineering with a major in electrophysics from Tsinghua University, Beijing, China, in 1992. From 1992 to 1994, he worked at Keli High Tech Corp. on the development of GIS software. In 1997 he received an MS. degree in electrical engineering from the University of Hawaii, Manoa. His research focused on piezo-MEMS, acoustic-wave liquid ejectors and mixers. He is currently
A 66 (1998)
355-360
working toward a Ph.D. in electrical and computer engineering at Carnegie Mellon University. His research interest is on surface-micromachined MEMS in standard CMOS processing. Em Sok Kim received B.S. (high honors), M.S. and Ph.D. degrees, all in electrical engineering, from the University of California, Berkeley, in 1982, 1987, and 1990, respectively. His doctoral dissertation demonstrated an integrated micro: phone with more than 300 n- and p-channel FETs on a sirigle silicon chip. In 199 1, he joined the University of Hawaii at Manoa, Honolulu, and is currently an associate professor with the Department of Electrical Engineering and a co-director of the Physical Electronics Laboratory and the Sensors Research Development Center. His research interests include piezoelectric microelectromechanical systems, microsensors and microactuators, film acoustic-wave filters and oscillators, micromachining technologies, etc. He has been awarded two US Patents. He has worked in IBM Research Laboratory, San Jose, CA, NCR Corp., San Diego, CA, and Xicor Inc., Milpitas, CA as a co-op student, design engineer and summerstudent engineer, respectively. Dr Kim serves on the editorial board of the Jom~al ofMicrornechanics mid Microengineering. He has been awarded a Research Initiation Award (FY 91-93) and a Faculty Early Career Development (CAREER) Award (FY 95-99) by the National Science Foundation. He received an Outstanding EE Faculty of the Year Award (voted by UH IEEE student chapter) in May 1996.
ELSEVIER
Sensors and Actuators
Microfluidic
A 66 ( 1998) 355-360
PHYSICAL
motion generation with acoustic waves Xu Zhu, Eun Sok Kim ‘$
Abstract This paper presents a novel approach for producing microfluidic motion by loosely focused acoustic waves (generated by piezoelectric zinc oxide thin film). Results show that the acoustic waves generated by radio frequency (r.f.) sources with frequencies corresponding to the thickness-mode resonances of the piezoelectric film are very effective in moving liquid around when the waves are loosely focused. The device operates without any significant temperature increase in the liquid, and will be very attractive for mixing or transporting temperaturesensitive fluids. 0 1998ElsevierScienceS.A. All rightsreserved. Kcworcls:
hlicrofluidic
motion;
Acoustic
waves; Piczoelectric
1. Introduction Microfluidic processing systems [ I] need to transport and/or mix two or more fluids in accurately controlled amountsin a reasonableamount of time. Since many microfluidic devices are fabricated in a planar lithographic environment, most of the macroscopic approachesfor fluid mixing [2] (e.g., turbulence, three-dimensionalflow and mechanicalactuation, etc.) are inapplicable. Chaotic advection for liquid mixing has been reported for a microfluidic device [ 31, but addsconsiderableheat to the liquid. Another micromixing technique of using fast diffusion [4] requires small nozzles, which tend to get clogged. One promising technique is to use ultrasonic Lamb waves [ 5,6] to generate masstransport. We have developed an efficient generationof microfluidic motion usingloosely focused acousticwavesby piezoelectric zinc oxide (ZnO) film. High-intensity bursts of acoustic waves (due to loosefocusing to a relatively small spot at an air-liquid surface) produce strong liquid movements in a liquid chamber which is about 1.5 mm X 1.5 mmX 0.5 mm in size, and have beenobservedto bc very effective in mixing liquid. The liquid flow in this small chamber is laminar, according to the Reynolds number (Re) calculated from Re= t L/6)/u
2. Theory Our self-focusing acoustic transducershown in Fig. I is basedon the concept of a constructive wave interference
(1)
where U, S and 11are the characteristic velocity (taken to be IO mm s-’ ), a length scale (about I mm in our case) and * Corresponding
the kinematic viscosity ( 1 mm2s- ’ for water), respectively. The Reynoldsnumber for our caseis about 10,which is much lessthan 2000 (above which the flow is turbulent) andhigher than I (below which the flow is creeping). Thus, the effects of turbulence flow or creeping flow neednot be considered in our case. With a negligible amountof heatgenerationaccompanying the acousticenergy, our transducerdoesnot heatup the liquid in any appreciableamount, and mixing by convection heat transfer is negligible. Thus. the transducerwill be very attractive for mixing temperature-sensitive fluids. This paper describesthe operating principle, design and fabrication of the microfluidic mixer along with our recent observationsof our device’s very large dynamic responses.
author. Tel.: + I 808 956 53 09. Fax: + 1 SO8 956 34 27.
E-ma\l: es~im~wi\‘tki.cn%.\la~vaii.edu 0924-4247/98/$19.00 P/ls09’~-~247(97)01712-3
0
1998 Elsevier Science S.A. All rights reserved.
Fig. I. Cross-sectional view (alone, with typical liquid-flow directions) of the liquid mixer microfabricated on a silicon wafer. Also shown on the left is the electrode pattern of the transducer to form the acoustic-wave sources
for a constructive wave interference in the liquid.
356
X. Zh,
E.S. Kim /Sensors
rind Amrntors
[7,8] (similar to an optical Fresnel lens which blocks certain areas of light to get intensity enhancement). The transducer (when excited with r.f. signals) generates acoustic waves (through the piezoelectric activity of ZnO film), which propagate in the liquid toward the liquid-air interface. For an efficient acoustic-wave generation, the ZnO thickness is chosen to be an odd multiple of a half wavelength (i.e., (211+ l)h/2) which would have put the film at a thickness-mode resonance in an air-backed transducer [ 71. Also, the electrodes of the transducer are laid out such that the acoustic waves will constructively interfere to have a focal point a few hundred micrometers above the liquid-air interface [8]. In this way, we have waves loosely focused near the surface of the liquid. We intentionally choose the loose focusing in order to reduce the acoustic pressure directed upward at the surface to avoid liquid ejection. In addition to the intensified upward acoustic pressure, the transducer generates a lateral acoustic field as indicated by the solid lines with arrowheads in Fig. 1. These acoustic fields (both upward and lateral) cause a strong liquid flow inside the chamber. The lateral acoustic field causes a lateral (tangential to the water-air interface) liquid motion, which is enhanced by the phenomenon known as acoustic streaming [9] (which takes place at the water-air interphase boundary causing a tangential liquid motion along the boundary). As mentioned above. our lensless design borrows its concept from an optical Fresnel lens which blocks certain areas of light to get intensity enhancement. We make only certain areas of the piezoelectric film generate acoustic waves. which arrive at a focal point in phase. The other areas (that would have generated waves with a phase difference of r at the focal point) are designed not to generate any acoustic wave. These are so-called half-wave-band sources. Fig. 2 shows the acoustic-wave transducer with several annular sources. The acoustic waves generated by the successive annular sources are designed to arrive at the focal point v). which is a little above the liquid surface. with finite delays (equal to multiples of the wavelength) by ensuring that the radii (I;,) satisfy the following relation:
A 66 (1998)
35.5-360
f
2=0 Fig. 2. Electrode sources.
patterns
for the transducer
to form
the half-wave-band
obtained as the gradient of an acoustic-wave potential (i.e., H,.= - V,$) , where C$and I’ are the wave potential and the radial distance on the surface, respectively. The potential C$ at any point in water (for the half-wave-band sources shown in Fig. 2) [ 71 is obtained from
where k= 2vlh; R is the distance between a half-wave-band source and the focal point: s is the surface area of the transducer at ; = 0; LL(x’J’,~) is the particle displacement at the Si.,N?-water interface (i.e., the transducer surface adjacent to the water). The acoustic-wave potentials on the air-water surface as a function of the number of half-wave-band source have been numerically calculated, and are shown in Figs. 3-5. As we can see in the Figures, the wave potential has a sharp peak at the focal point on the air-water surface. Also, the more halfwave-band sources are used to generate the acoustic waves, the higher the peak acoustic-wave potential ( and the narrower the potential distribution) becomes. The radial displacement (Q) can be obtained from lfR= -h$/aR, and the numerical calculations of the radial displacements on the air-water surface for the cases of one. three and seven half-wave-band sources are shown in Figs. 6-8. The cross-cut views of Figs. 6-8 are given in Fig. 9 t%
from which we obtain
where n = I, 3, 5, 7... and A,, is the acoustic wavelength in liquid. While it is easy to see that the acoustic waves provide the necessary force to move liquid in the vertically upward direction, lateral motion of liquid needs some explanation, as described below. In particular. a constructive interference of lateral acoustic waves is described in detail. For an isotropic liquid. such as water, the particle displacement I[,. associated with the longitudinal waves can be
Fig. 3. Acoustic-wave potential on the air-water surface when seven half wave-band sources ilre used to generate the acoustic waves.
X. Zh,
ES.
Kim I Smsors
and Acrmtors
Fig. 3. Acoustic-wave potential on the air-water surface when three halfwave-band sources arc used to generate the acoustic waves.
Fig. 5. Acoustic-wave potential on the air-water surface when only one halfwave-band source is used to generate the acoustic waves.
A 66 (1 YW)
355-360
Fig. 7. Lateral particle displacement on the air-water surface when one circular and two annular sources are used to generate the acoustic waves.
Fig. 8. Lateral particle displacement on the air-water surface when one circular and six annular sources are used to generate the acoustic waves.
Fig. 6. Lateral particle displacement on the air-water surface when one circular source at the transducer center is used to generate the acoustic waves.
show the radial displacement along the radial direction from one edge to the center. In these Figures we see no radial displacement at the center of the surface, which makes sense physically since the actuating source is radially symmetric and the liquid is isotropic. Though the radial displacement is zero at the center, there is a relatively large peak of the radial displacement near the center. The peak displacement occurs at about 8 pm radially away from the center (Fig. 6)) when one circular source at the transducer center is used to generate
X104
edge Fig. 9. Cross-cut along the radial diaphragm.
radial position
center
view of Fgs. 6-5 to show the radial particle displacement direction from one edge to the center of the transducer
the acoustic waves. Similar results (with much larger displacement amplitudes) are obtained when we use one circular plus two annular-ring sources (Fig. 7) and one circular plus six annular-ring sources (Fig. 8).
Fig. IO. The radially directed particle displacement at any point in the liquid when one circular and six annular sources ~~re used to generate the acoustic waves.
Thus, we see that the constructive interference of the acoustic waves generated by our transducer also enhances the radial particle displacement, as shown in Fig. 9. The radial displacement is strongly dependent on the number of half-wave-band sources (Figs. 6-8). This kind of lateral displacement is a major driving mechanism for the convection liquid flow in our transducer. Fig. 10 shows the relative radial particle displacement at any point in the chamber. The distribution of the radial displacement near the liquid-Si,N,. interface is similar to that near the liquid-air interface except for the phase difference of YTas shown in Fig. 10. The 5~phase difference makes the particle displacements near those interfaces be directed in opposite directions, as illustrated by the solid lines with arrowheads in Fig. 1. The lateral displacement near the liquid-air interface is directed radially outward, while that near the transducer-water interface is directed radially inward. These lateral displacements (on two different levels of liquid) directed in opposite directions are the major reason for a strong lateral liquid motion.
3. Transducer
Si diSi Fig. I 1. Fabrication
processing
Al L?ima zno
steps for the acoustic-wave
liquid mixer.
Fig. 12. Top-view photograph of tht: t’abricated acoustic-wave liquid mixer. The bottom and top Al electrodes tire patterned into annular rings, while the ZnO is delineated into a solid circle with four bulunce barb eAtending toward the four corners of the square diaphragm.
4. Results and discussion The fabricated devices are tested with the set-up shown in Fig. 13. A continuous sinusoidal wave ( in the range 100-5001 MHz) from a signal generator is fed into a high-speed switch which is turned on/off with a modulating pulse duration set at, in this case,100ns.The r.f. power amplifier then amplifies
fabrication Water with micro-spheres Tryducer 7
The transducer is fabricated on a silicon wafer with the four-mask process shown in Fig. 11. After depositing and patterning 0.8 pm thick LPCVD low-stress silicon nitride. we remove silicon with KOH to form 1 mmX 1 mm membranes. on which 0.5 Km thick Al is evaporated and patterned for an annular bottom electrode. Then a 10 pm thick ZnO layer is sputter-deposited in Ar/O, environment from a ZnO target, followed by 0.5 pm thick Al evaporation and delineation for a top electrode. The segmented top and bottom Al electrodes act as half-wave-band sources as shown in the photograph of the fabricated transducer (Fig. 12).
~.L
Fig. 13. Schelnatic
representation
I‘,
of the test apparatus
for the liquid mixer.
X. Zhu,
E.S. Kim /Sw~sors
and Actuators
the r.f. wave up to OS-10 W level before feeding it to the transducer. We have tested our transducer with water mixed with fluorescent microspheres about 10 pm in diameter. When we apply a series of pulses (at a rate of I-70 kHz, with on-time of 0.1-l fJ-sj of 100400 MHz sinusoidal waves (0.15-10 W level ) to the transducer, we observe active liquid motions. This observation is facilitated by the movements of rhe microspheres in the liquid. The Row patterns of the microspheres are captured with a camera, and shown in Figs. 14 and 15. Fig. 14 shows the flow patterns seen from the top of the device without any tilting, while Fig. 15 shows the flow patterns seen at an angle from the top-view direction. In Fig. 15, we clearly see the loops of microspheres following the liquid flow pattern ( produced by the loosely focused acoustic waves ). The microspheres rotate around such a loop by being vertically pumped upward by the loosely focused acoustic waves from the lower portion of the liquid chamber (or near the transducer), radially moved outward near the air-liquid surface, vertically pumped downward, and then radially moved inward near the transducer. The rotational speed of the microspheres when driven at the resonance frequencies (of the thickness-mode vibration of the piezoelectric film: i.e., 240 and 480 MHz for the fundamental and second-harmonic resonances) is about 30-40 times higher than that when driven at frequencies other than those resonance frequencies. Since the energy used for the liquid motions is very low, there is very little heating in the liquid. This has been confirmed with negligible change in the liquid level (in the cavity that contains only about 2 ~1 of water) even when the device is continuously operated for more than 20 min. Another observation to confirm the negligible heating effect is that it takes almost the same time to evaporate the 2 ~1 of water whether the transducer is in usual operation or not.
A 66 (1998)
3X5-360
Fig. 15. Tilted top-view photograph an angle (from the direct top view) shadows.
of the device under tat taken at such to show the flow loops along with their
The area of the open surface in the transducer cavity (2 ~1 in volume) is about 1.5 mm X 1.5 mm, and the surface tension of the water (contained in the cavity) is large enough to hold the water in the cavity even if the transducer is positioned at 90” upward or upside down. We have operated the transducer in those odd positions. and observed flow patterns very similar to those when the transducer is positioned as shown in Fig. 1. The only difference in those different positions of the transducer is the gravity on the microspheres, which affects the population distribution of the microspheres in the cavity.
5. Conclusions A loosely focused acoustic-beam transducer (using a 0.8 km thick low-stress silicon-nitride membrane and a 10 p,rn thick piezoelectric ZnO film) has been fabricated on a silicon wafer, and shown to be very effective in moving about 2 ~1 of liquid. With fluorescent microspheres in the water, we have observed extremely fast movement of these microspheres when we apply a series of pulses (at l-70 kHz rate with ontime of 100 ns) that contains 240 or 480 MHz sinusoidal waves. This technique of heatless acoustic agitation can advantageously be applied to microfluidic systems where the Reynolds number is small and heating is not desirable.
Acknowledgements
Fig. 14. Top-view photograph of the device under test, which ahuws the flow pattern of the microspheres in water produced by loosely focused acoustic waves.
The authors wish to thank Hasnain Lakdawala of the University of Hawaii and Professor Richard M. White of U.C. Berkeley for help with the numerical calculation and for showing the usefulness of the fluorescent microspheres in various experiments, respectively. This material is based upon work supported by the Defense Advanced Research Projects Agency under contract DABT63-95-C-0039 and NSF CAREER Award #ECS-9501698.
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and Ac~wtors
References [II
S. Shoji and M. Esashi, Microflow device and aystems, J. Micromech. Microeng., 4 (1994) 157-171. 121 R.F. Berg and N.P. De Luca, Milliwatt mixer for small fluid samples. Rev. Sci. Instrum., 62 ( 1991) 527-529. [31 I. Evans, D. Liepmann and A.P. Pisano, Planar laminar mixer, IEEE MEMS ‘97. Nagoya, Japan. 1997, pp. 96-101. T&J. Lammerink, M. Elwenspoek and J.H.J. Fluitman, [41 R. Miyake, Micro mixer with fast diffusion, J. MEMS, 2 ( 1993) 238-253. R.M. White and R.T. Howe, Microtransport induced ISI R.M. Moroney, by ultrasonic Lamb waves, Appi. Phys. Lett., 59 ( 1991) 726-774. 161 T.R. Tsao, R.M. Moroney, B.A. Martin and R.M. White, Electrochemical detection of localized mixing produced by ultrasonic flexural waves. IEEE Ultrasonics Symp., Dec. 1991. pp. 937-940. 171 G.S. Kino, Acoustic Waves, Design Imaging and Analog Signal Processing, Prentice-Hall. Englewood Cliffs, NJ, 1987. ISI X. Zhu, E. Tran, W. Wang, ES. Kim and S.Y. Lee, Acoustic wave liquid ejector, Tech. Digest, Solid-State Sensor and Actuator Workshop, Hilron Head, SC, USA, 1996, pp. 250-382. iSI W.L. Nyborg, Acoustic streaming, in W.P. Mason (ed.). Physical Acoustics, Vol. 2B. Academic Press. New York, 1965, pp. 265-283.
Biographies XI{ Z/W! was born in 1970 in Xi’an, China. He received his B.S.E.E. degree in electronics engineering with a major in electrophysics from Tsinghua University, Beijing, China, in 1992. From 1992 to 1994, he worked at Keli High Tech Corp. on the development of GIS software. In 1997 he received an MS. degree in electrical engineering from the University of Hawaii, Manoa. His research focused on piezo-MEMS, acoustic-wave liquid ejectors and mixers. He is currently
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working toward a Ph.D. in electrical and computer engineering at Carnegie Mellon University. His research interest is on surface-micromachined MEMS in standard CMOS processing. Em Sok Kim received B.S. (high honors), M.S. and Ph.D. degrees, all in electrical engineering, from the University of California, Berkeley, in 1982, 1987, and 1990, respectively. His doctoral dissertation demonstrated an integrated micro: phone with more than 300 n- and p-channel FETs on a sirigle silicon chip. In 199 1, he joined the University of Hawaii at Manoa, Honolulu, and is currently an associate professor with the Department of Electrical Engineering and a co-director of the Physical Electronics Laboratory and the Sensors Research Development Center. His research interests include piezoelectric microelectromechanical systems, microsensors and microactuators, film acoustic-wave filters and oscillators, micromachining technologies, etc. He has been awarded two US Patents. He has worked in IBM Research Laboratory, San Jose, CA, NCR Corp., San Diego, CA, and Xicor Inc., Milpitas, CA as a co-op student, design engineer and summerstudent engineer, respectively. Dr Kim serves on the editorial board of the Jom~al ofMicrornechanics mid Microengineering. He has been awarded a Research Initiation Award (FY 91-93) and a Faculty Early Career Development (CAREER) Award (FY 95-99) by the National Science Foundation. He received an Outstanding EE Faculty of the Year Award (voted by UH IEEE student chapter) in May 1996.