Surface Acoustic Wave Driven Microfluidics – A Review
Author
Luong, TD, Nguyen, NT
Published
2010
Journal Title
Micro and Nanosystems
DOI
https://doi.org/10.2174/1876402911002030217
Copyright Statement
© 2010 Bentham Science Publishers. This is the author-manuscript version of this paper.
Reproduced in accordance with the copyright policy of the publisher. Please refer to the journal
website for access to the definitive, published version.
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Surface Acoustic Wave Driven Microfluidics
Trung-Dung Luong and Nam-Trung Nguyen*
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue,
Singapore, 639798
*Address correspondence to this author at the School of Mechanical and Aerospace Engineering, Nanyang
Technological University, 50 Nanyang Avenue, Singapore, 639798
E-mail: mntnguyen@ntu.edu.sg
Tel: (+65) 67904457
Fax: (+65) 67911859
Abstract: This paper presents a systematic overview on the recent works on surface acoustic wave (SAW)
driven microfluidics. SAW microfluidics is based on acoustic streaming induced by leaky SAW radiation into a
liquid. The development of this field attracts attention from microfluidic research community due to its rapid
actuation, programmable capability, simple and yet efficient operation. In our paper, SAW microfluidic
applications are categorized into droplet-based applications and continuous-flow applications. Droplet is
actuated into unique behaviours depending on the applied SAW power. A wide range of droplet based
applications have been employed utilizing these behaviours. In continuous-flow system, applications are further
categorized based on the interaction of travelling SAW and standing SAW with the bulk liquid. Finally, future
perspectives of SAW driven microfluidics are discussed.
Keywords: surface acoustic wave, microfluidics, droplet-based, continuous-flow, mixing, sorting, pumping,
concentrating
1
1. INTRODUCTION
Over the past few decades, surface acoustic wave devices have been successfully employed in various
applications. Common usage of SAW ranges from radio frequency (RF) communication [1] to chemical and
biochemical sensors [2] to optical modulators [3]. In recent years, SAW has been attracting attentions from
microfluidics research communities. SAW microfluidic working mechanism is based on the efficient energy
transferred from the megahertz (MHz) wave into the liquid, which is due to the mismatch of sound velocities in
the substrate and fluid. Compared to other actuation mechanisms in microfluidics such as surface gradient [4],
thermo capillarity [5], electrowetting [6], magnetism [7], the apparent advantages of SAW are large force, fast
operation and simple electrodes network. Current technologies make the fabrication of SAW devices relatively
cost-effective. Moreover, the simple integration of SAW devices into microscale system promises an excellent
solution for fluid miniaturization platforms. Many works have been reported in the literature on SAW
microfluidics. Reviews on these works were published recently by Yeo and Friend et al. [8], Fu et al. [9] and
Wixforth et al. [10]. However, the main focus of the above papers is SAW droplet-based microfluidics. The
recent works on SAW based continuous-flow applications have not been discussed in depth.
Recent developments of SAW microfluidics are systematically discussed throughout this paper. Generally,
SAW devices consist of interdigitated electrodes (IDT) patterned on a piezoelectric substrate as shown in Fig.
(1a). Upon applying a RF signal to the IDT, the piezoelectric substrate contracts and expands due to the
redistribution of charges. Continuous deformations lead to the launching of SAW, which has both longitudinal
and transverse vibrations along the propagation of the waves. The combined effect causes the point near the
surface to move in an ellipse which is in the plane normal to the surface and parallel to the wave propagation.
SAW wavelength is determined by the transducer pitch
d2
=
λ
, Fig. (1b). The central frequency of the
SAW is determined by the relation:
0
f
λ
/cf =
,where is the wave propagation speed of SAW in the
substrate. These electrodes are fabricated onto the substrate using standard lithography and lift off/wet etching
process. Different piezoelectric materials are used in traditional SAW devices. In SAW microfluidic application,
single crystal lithium niobate (LiNbO
3
) substrate, however, is preferred due to the relatively high
electromechancial coupling coefficient.
c
2
Interdigitated electrode
Piezoelectric substrate
=2d
Fig. (1). SAW device principle: (a) SAW generation and coupled surface movement; (b) interdigitated electrode
concept.
When SAW comes into contact with a liquid along its path, the leaky SAW, with decaying amplitude, is
launched into the bulk liquid. The refraction angle of the radiated wave could be calculated based on Snell's law:
, where and are the sound speeds in the substrate and in the liquid, respectively.
Acoustic radiance pressure is also built up inside the liquid and results in internal streaming following the wave
propagation. However, internal flow circulation depends on the nature of the experimental setup. As shown in
Figure (2), the circulation is in a clockwise direction [10] for droplet and a counter clockwise direction for liquid
in a confined channel [11]. This so-called acoustic streaming effect is the foundation for application of SAW in
microfluidics. In this review paper, SAW microfluidics is categorized and discussed according to droplet-based
and continuous-flow applications. In the discussion of droplet-based applications, reported works are further
categorized based on droplet behaviour in different power regimes. Continuous-flow based applications are
discussed under the categorization of travelling SAW and standing SAW.
(
sl
1
/sin cc
R
−
=
θ
)
s
c
l
c
3
Fig. (2). Acoustic streaming in (a) droplet and (b) in a confined channel.
2. SAW DROPLET-BASED APPLICATIONS
Under different power load, behaviours such as droplet vibration, droplet motion, droplet jetting and droplet
atomization are observed. In this section, published works on SAW-based droplet manipulation are reviewed
according to the different droplet behaviours. As shown in Fig. (3), when the RF power is applied, acoustic
streaming is induced due to leaky wave radiated into the droplet. At low power, the droplet vibrates at its
position, Fig. (3a). Wide range of applications utilizing this effect includes droplet mixing, droplet heating,
particle patterning and particles concentration. Increasing the power makes acoustic streaming strong enough to
result in a significant inertial force and drive droplet in the wave propagation direction [Fig. (3b)]. Droplet
actuation has been used for pumping, sample collecting and sample dispensing. Several methods for droplet
positioning are proposed for a well controlled SAW droplet platform. Further increasing the power makes
acoustic streaming so violent that the liquid is jetted into the air as shown in Fig. (3c). Droplet jetting is suitable
for ink-jet printing applications. When the applied power is too high, strong capillary waves at the droplet
surface overcome capillary stress and result in atomization of droplet [Fig. (3d)]. This effect has been exploited
to synthesize arrays of nanoparticles.
4