A rapid particle concentration method in a sessile droplet has been developed using asymmetric surface acoustic wave (SAW) propagation on a substrate upon which the droplet is placed. The SAW device consisted of a 0.75-mm thick, 127.68 YXaxis- rotated cut LiNbO3 as a substrate. An interdigital transducer electrode (IDT) with 25 straight finger pairs in a simple repeating pattern, 12 mm aperture, and a wavelength of l = 44 μm was patterned on the substrate. The IDT was then driven with a sinusoidal signal at the resonance frequency 8.611 MHz. To investigate the effect of particle type and size on the concentration process, several types of particles were used in this study, including fluorescent particles (1 μm), polystyrene microspheres (3, 6, 20, and 45 μm), and living yeast cells (10- 20 μm). Different RF powers were applied ranging from 120 to 510 mW. The concentration processes occurs within two to twenty seconds, depending on the particle size, type and input radio frequency (RF) power, much faster than currently available particle concentration mechanisms due to the large convective velocities achieved using the SAW device

Friend, James

Li, Haiyan

Yeo, Leslie

A rapid particle concentration method in a sessile droplet has been developed using asymmetric surface acoustic wave (SAW) propagation on a substrate upon which the droplet is placed. The SAW device consisted of a 0.75-mm thick, 127.68 YX-axis- rotated cut LiNbO 3 as a substrate. An interdigital transducer electrode (IDT) with 25 straight finger pairs in a simple repeating pattern, 12 mm aperture, and a wavelength of ? = 44 ?m was patterned on the substrate. The IDT was then driven with a sinusoidal signal at the resonance frequency 8.611 MHz. To investigate the effect of particle type and size on the concentration process, several types of particles were used in this study, including fluorescent particles (1 ?m), polystyrene microspheres (3, 6, 20, and 45 ?m), and living yeast cells (10- 20 ?m). Different RF powers were applied ranging from 120 to 510 mW. The concentration processes occurs within two to twenty seconds, depending on the particle size, type and input radio frequency (RF) power, much faster than currently available particle concentration mechanisms due to the large convective velocities achieved using the SAW devi

Friend, J

Li, H

Yeo, L

RMIT Research Repository

Surface acoustic wave concentration of microparticle and nanoparticle suspensions

University of Queensland eSpace

16th Australasian Fluid Mechanics Conference
Crown Plaza, Gold Coast, Australia
2-7 December 2007
Surface acoustic wave concentration of microparticle and nanoparticle suspensions
James Friend, Haiyan Li and Leslie Yeo
MicroNanophysics Research Laboratory
Monash University, Clayton, Victoria, 3800 AUSTRALIA
mnrl@monash.edu
Abstract
A rapid particle concentration method in a sessile droplet has
been developed using asymmetric surface acoustic wave (SAW)
propagation on a substrate upon which the droplet is placed.
The SAW device consisted of a 0.75-mm thick, 127.68 YX-
axis-rotated cut LiNbO3 as a substrate. An interdigital trans-
ducer electrode (IDT) with 25 straight finger pairs in a sim-
ple repeating pattern, 12 mm aperture, and a wavelength of
λ = 44 µm was patterned on the substrate. The IDT was
then driven with a sinusoidal signal at the resonance frequency
8.611 MHz. To investigate the effect of particle type and size on
the concentration process, several types of particles were used
in this study, including fluorescent particles (1 µm), polystyrene
microspheres (3, 6, 20, and 45 µm), and living yeast cells (10-
20 µm). Different RF powers were applied ranging from 120
to 510 mW. The concentration processes occurs within two to
twenty seconds, depending on the particle size, type and input
radio frequency (RF) power, much faster than currently avail-
able particle concentration mechanisms due to the large con-
vective velocities achieved using the SAW device.
Introduction
The study of surface acoustic waves (SAW) dates from
Rayleigh’s original exposition in 1885 [15], and are most
commonly encountered by humans in earthquakes as ground
roll [2]. Such waves are common to diverse fields of physics,
from studying the geological characteristics of the moon [10] to
how spiders locate their prey [20]. Engineering application of
such waves appeared [5] in telecommunications and signal pro-
cessing soon after a convenient method for forming such waves
atop piezoelectric materials was discovered by White and Volt-
mer in 1965 [23]; microdevices using SAW pervade consumer
and military technology with an average of four in every mobile
phone produced [5].
In contrast, microfluidics has a relatively short history, but also
has an extraordinarily promising future, especially for medical
and biological applications [24]. The transport of fluids on the
micro and nano scales is a particularly thorny problem, with
the effects of viscosity and laminar flow conditions consipiring
against simply shrinking larger devices down to these scales
with the expectation of having something useful [19]. Even
today, despite the variety of methods explored by researchers,
most “microfluidic” systems are still the size of a desktop com-
puter. Electrowetting and electrophoresis are two of the most
prevalent technologies in applied microfluidics [1], and offer
much improved performance in comparison to passive diffu-
sive processes. Further, they are convenient for digital droplet
microfluidics, where the complexity of channel fabrication is
traded for the problems of evaporation and surface-tension-
driven effects in individual nanoliter-order droplets atop a sur-
face [8]. The advantages include an ability to reconfigure or
tailor the device for new applications, the sheer simplicity of
the approach, and ease of loading and unloading fluids. Yet for
mixing, transport and concentration of those droplets they are
a challenge to construct, requiring complex electrode patterns
and fluids or particles within those fluids amenable to manipu-
lation. Worse, for biomedical applications, these technologies
tend to modify the charge on particles within the fluid, affecting
chemical bonding, cellular membranes, opening ion channels,
and causing damage through bubble formation [7, 4].
Alternatives do exist, and one promising approach is the
use of very high-frequency acoustic waves to perform all
of the tasks common in droplet micro/nanofluidics—moving,
splitting, combining, and pinning droplets—along with new
processes[12, 13, 21] enabled via acoustic forces on objects
within the droplets. Rotation of a droplet to perform mixing,
for example, is shown in Fig. 1. Though one can use bulk Love
and other such waves to perform manipulation and sensing [22],
such waves are transmitted throughout the substrate holding the
droplet, and so are subject to losses and diffraction from mount-
ing. Surface acoustic waves are isolated to within 4–5 λ of
the surface upon which they are generated, and so the disper-
sion of such waves is usually far less than bulk waves—the
reason these waves are so popular in electronics applications.
Wixforth [18, 25] and Renaudin [16] have made significant ad-
vances in the use of SAW for droplet microfluidics.
However, the manipulation of micro and nanoparticles within
these droplets represent an important application of any such
technology for biomedical applications, and curiously this as-
pect remains unexplored for droplet structures. Though the
process dates from the study of acoustic particle agglomer-
ation by Kundt [9] eleven years prior to Rayleigh’s SAW
publication, only recently have they found use in microflu-
idics [14, 12, 13, 21]. In this work we describe the discovery
of an ability to perform concentration and dispersion of micro
and nanoparticles inside sessile droplets set atop a lithium nio-
bate substrate via SAW passed into the droplet in an asymmetric
fashion. After discussing the reasons for the concentration pro-
cess, the method is used to concentrate live yeast to illustrate
its possible application to live cells while avoiding lysis and to
counter the typical perception that ultrasound is generally dam-
aging to cells [3].
Method
The SAW is generated on a 0.75-mm thick, 127.68 Y-X-axis-
rotated cut, X-propagating LiNbO3 (Lithium Niobate or LN,
Roditi, London UK) single crystal substrate using a simple in-
terdigital electrode [23] custom-fabricated onto the surface at
the MicroNanophysics Research Laboratory. The wavelength
of the excited SAW is defined by the geometry of the IDT, and
here 25 straight finger pairs in a simple repeating pattern, a
12 mm aperture, and a wavelength of λ = 440 µm were used
to give a fundamental resonance frequency of 8.611 MHz. Dif-
ferent RF powers were used to study the effect of input power
on the concentration process.
The droplet was deposited directly on the LN surface, which is
mildly hydrophyllic. No droplet translation was induced from
acoustic streaming since the input RF power was below the
threshold defined by the power necessary to release the contact
1038
t = 0 t = 1 s
t = 2 s t = 3 s
(a)
(b)
(c)
Figure 1: Surface acoustic waves are attenuated upon (a) con-
tact with a fluid droplet; the energy is reradiated into the fluid
at the Rayleigh angle θR as a compressive wave. Without (b)
asymmetric delivery or absorption of the acoustic radiation into
the droplet there can be no (c) rotation and subsequent mixing
of the droplet. Here, the droplet was placed to one side of the
SAW causing rapid mixing of the water-glycerin (green) mix-
ture.
line pinning present at the edge of the droplet. A micro-pipette
was used to deliver the droplets onto the substrate surface, and
after each run the remains of the droplet and particles was re-
moved from the surface in preparation for the next run.
Fluorescent particles were used to monitor the concentration
process via fluorescence microscopy. The concentration pro-
cess was recorded via high-speed video (Olympus iSpeed cam-
era, Tokyo) used in conjunction with a reflection fluorescent
stereomicroscope (Olympus BXFM, Tokyo, Japan). Wideband
blue light was applied to excite the fluorescent particles and
a fluorescence mirror unit (U-MWB2) with a DM500 dichroic
mirror was used to filter the fluorescent signal from the illumi-
nation. The quality and speed of concentration was determined
using centre-normalized pixel intensity analysis (NPI) of indi-
vidual image frames extracted from the video. Acoustic stream-
ing velocities and the particle velocities were estimated using
Diatrack 3.01 (Semasopht, North Epping, Australia).
Fluoresbrite 1 µm polystyrene (PS) microspheres with an
initial concentration of 4.55×1010 particles/m` in deionized
water were diluted to obtain a concentration of 4.55×106
particles/m`, and then dispersed using high-power 2 MHz ultra-
sound before use. Plain PS microspheres, 3, 10, 20 and 45 µm
in diameter (Polysciences, Inc., USA) were selected to study
the effect of particle size on the behaviour of the SAW device;
all were diluted to provide a consistent sample concentration of
4.99×105 particles/m` in deionized water. Living yeast cells
were used to demonstrate the concentration of bioparticles. In
assessing the manipulation of cells, stock cultures of yeast cells
were grown and maintained on standard agar consisting of 1%
yeast extract, 0.5% neutralized bacteriological peptone, and 1%
glucose solidified with 1.5% agar (w/v). All media were au-
toclaved immediately after preparation at 121 degrees C and
15 psi for 15 min. Yeast cells were grown aerobically to the
required cell density at room temperature. The viability of the
cells was investigated using methylene violet stain which only
penetrates dead yeast cells.
Results
Figure 2 shows the behaviour of 1-µm fluorescent PS particles
as they are convected within the bulk of the drop by SAW-
induced acoustic streaming as individual image frames acquired
by high speed microscopy at 60 fps. The NPI decreases rapidly
toward an asymptotic value of 0.2 in just 10 s, indicating the
concentration process is extremely rapid, and the images of the
process indicate the particles in the droplet collect at the centre
(and bottom in images not shown here). Defining the time taken
for concentration as the period over which the NPI decreases
from a value of 1 to a constant value (0.2 in this case), one can
then quantify the particle concentration process by plotting the
NPI with respect to time. The rate of concentration depends
on the RF power input into the device and specifically the am-
plitude of the SAW propagating across the substrate. Figure 3
indicates that the rate of particle concentration, as described the
temporal variation in the NPI, is enhanced as the input power
is increased to a critical value of approximately 300 mW. Be-
yond this point, the concentration efficiency begins to decrease,
and there is a sharp decrease upon passing about 500 mW. At
these power levels there is virtually no particle concentration,
similar to the situation when low input power (≤120 mW) is
used. The decrease in particle concentration at high power is
attributed to the redispersion of the aggregated particles. The
particle dispersion at high input power is associated with large
acoustic streaming velocities which oppose the dominant par-
ticle aggregation mechanism. At low power, the behaviour is
similar, but the reason is entirely different: the convection ve-
locity is insufficient to drive particle aggregation.
Indeed, the concentration process is a balance between the flow
in the droplet induced via acoustic streaming [17] and the pro-
cess of shear-induced migration [11] responsible for the inward
propagation of the particles towards the center of the droplet
over time. Though the former is entirely expected given the na-
ture of leaky SAW used to drive the motion in the fluid droplet,
the latter is the conclusion of the results shown in Fig. 4. As
the size of the PS particles are changed, the time required to
perform the collection scales with 1/a2, where a represents the
size of the particles, a distinct characteristic of shear-induced
migratory flow. Note that particles at less than 3 µm must ag-
glomerate to larger sizes via a local acoustically-driven process
prior to shear migration, and so the trend is displaced from 3 µm
downwards, and may be observed down to 20 nm (not shown).
The concentration process may be defined in terms of a concen-
1039
(a)
(b)
Time (s)
N
PI
 (a
rb
itr
ar
y 
un
its
)
Figure 2: The (a) concentration of 1 µm PS particles in a sessile
droplet is rapid as indicated by the (b) reduction of the normal-
ized pixel intensity of the video image in ten seconds, corre-
sponding to what is (a) observed with the eye in the images.
N
PI
Time (sec)
Figure 3: Concentration of particles with respect to time, as in-
dicated by the NPI for input power levels from 120 to 510 mW.
tration factor, χ, which represents the ratio of acoustic stream-
ing velocity induced in the fluid to the shear-induced migration
velocity of the particles. Figure 5 plots the change in the par-
ticle concentration at the centre of the droplet, proportional to
the inverse of the NPI, with respect to the concentration factor.
Above a factor χ ∼ χc = 2, the particles always concentrate,
while below that factor, the concentration process fails.
Time (sec)
N
PI
Ti
m
e 
to
 re
ac
h 
st
ea
dy
-s
ta
te
 N
PI
 (s
ec
)
Inverse of (Particle size)2  (1/m2)
Figure 4: (a) Collection of particles (NPI) versus time for sev-
eral different PS particle sizes. Note that (b) plotting the time
of collection versus 1/a2 gives a linear relationship, except for
PS particles less than about 3 µm in diameter.
Critical Concentration Factor
Ch
an
ge
 in
 V
or
te
x 
Vo
lu
m
e 
Fr
ac
tio
n
Figure 5: The concentration process may be described via a crit-
ical concentration factor representing the ratio of the acoustic
streaming velocity [17] induced in the fluid to the shear-induced
migration velocity [11] of the particles towards the center of the
droplet.
The concentration of live yeast is illustrated in Fig. 6, where
the yeast cells remain alive post-concentration according to the
violet dye evaluation process.
Conclusions
To our knowledge, there is no method available for particle con-
centration in the tens of seconds that the system reported in this
work describes. The importance of concentration, especially
1040
Figure 6: The rapid concentration of live yeast cells—a 1 mm
mass is formed at the center of the droplet in 2 seconds.
on the micro and nano scales, cannot be underestimated. Not
only do molecular binding and related chemical processes take
longer as the scale is reduced due to diffusion, in most devices
sensing is an important part of the process and is dependent
on the time required to provide a detectable amount of bind-
ing [6]. Further, in applications of such technology to pathogen
and dangerous chemical detection, an enhancement of the sen-
sitivity and speed of sensing through concentration and rapid
movement of the target molecules or cells would represent a
key advancement. We hope that this approach provides a viable
solution to the problems of diffusive sensing processes.
Acknowledgements
We would like to acknowledge support of this work from
Monash University via the New Staff Member and Small Grant
Schemes.
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