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Localized ultrahigh frequency acoustic fields induced micro-vortices for submilliseconds microfluidic mixing

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We present an acoustic microfluidic mixing approach via acousto-mechanically induced micro-vortices sustained by localized ultrahigh frequency (UHF) acoustic fields. A micro-fabricated solid-mounted thin-film piezoelectric resonator (SMR) with a frequency of 1.54 GHz has been integrated into microfluidic systems. Experimental and simulation results show that UHF-SMR triggers strong acoustic field gradients to produce efficient and highly localized acoustic streaming vortices, providing a powerful source for microfluidic mixing. Homogeneous mixing with 87% mixing efficiency at a Peclet number of 35520 within 1 ms has been achieved. The proposed strategy shows a great potential for microfluidic mixing and enhanced molecule transportation in minimized analytical systems.
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IN-LINE TRAPPING AND ROTATION OF BIO-PARTICLES
VIA 3-D MICRO-VORTICES GENERATED BY LOCALIZED
ULTRAHIGH FREQUENCY ACOUSTIC RESONATORS
Meihang He, Weiwei Cui, Hongxiang Zhang, Yang Yang, Wei Pang*,
Hemi Qu, and Xuexin Duan*
State Key Laboratory of Precision Measuring Technology and
Instruments, Tianjin University, Tianjin, 30072, China
ABSTRACT
acoustofluidics.
This paper reports a minimized bio-particles
manipulation tool utilizing localized ultrahigh
frequency acoustic resonators (UHF-AR). The
resonator works at frequency of 1540 MHz, which is
high enough to break through scale effect and trigger
localized acoustofluidic streaming (in formation of 3-D
vortices) within microfluidic systems. Especially, the
vortices can be precisely tuned in principle, providing
versatile manipulations of microparticles. In this paper,
online trapping of particles with diameter from 50 nm
to tens of micrometers has been demonstrated.
Furthermore, single micro-scaled bioparticle can be
well confined and locally rotated by the vortices.
Compared with other acoustic tweezers, the UHF-AR
acoustofluidic tool owns advantages of low power
consumption, minimized size and IC-compatible
fabrication, which enable it convenient to integrate
with compliable lab-on-a-chip devices.
KEYWORDS
Micro-vortices, ultrahigh frequency, trap, rotation,
microfluidics
INTRODUCTION
Trapping and local manipulation of single
bioparticle offers many possible applications in
regenerative medicine, tissue engineering, neuroscience,
and biophysics [1-3]. For
decades, various technologies
have been developed to realize particles trapping in
liquids, including optical tweezers, acoustic tweezers,
magnetic tweezers, and electric field traps (electrophoresis
and dielectrophoresis) [4-7]. Among them,
acoustic
methods have proven their utility with contactless
manipulation of microscale objects including
separation, queuing, concentration
and rotation
mainly
relying on the acoustic radiation effects [5,8].
However, these technologies suffer limitations in
throughput, trapping efficiency, and bio-compatibility.
Acoustofluidic streaming has been explored for
bioparticle manipulations due to its excellent bio-
compatibility and low cost [9]. However, precise
control of bioparticles using streaming remains as
challenge because of lack of effective strategy to
trigger highly localized and well-tunable streaming
field.
Localized ultrahigh frequency acoustic fields
generated by thickness extensional mode acoustic wave
resonators have been reported to trigger micro-vortices
within microfluidic systems [10], providing an effective
strategy to break through above limitations in the field of
In this study, we further developed such micro-
vortices to be an effective tool to trap and rotate bio-
particles with diameter range from 50 nm to tens of
micrometers. This approach utilizes acoustofluidic
streaming forces, which is in contrary to previous reported
micro-particle manipulations using acoustic radiation
forces.
Especially, the formation and velocity of micro-
vortices are demonstrated to be well tuned by altering the
acoustic power, flow velocity, and microchannel height.
Frequency effect and scale effect that domain the
acoustofluidic field have been discussed, revealing the
significance of the use of UHF-AR in field. By tuning the
micro-vortex, versatile manipulations of particles of
different sizes have been demonstrated.
DEVICE DESIGN AND METHODS
The UHF-AR consists of a thin piezoelectric film
sandwiched by top electrode and bottom electrode. The
frequency of the device is mainly determined by the
piezoelectric (PZ) film as =

, whered is the
thickness, and the other parameters are determined by the
materials of the film [11]. To obtain GHz frequency, the
thickness of PZ layer is designed to be one micrometer or
thinner. In this work, UHF-AR works at resonant
frequency of 1540 MHz with 1 um piezoelectric film. The
fabrication process has been presented in detail in our
previous published literatures [12,13]. The characterized
width of the UHF-AR varies from nanometers to hundreds
of micrometers, which is suitable to integrate with
microfluidic systems. The PDMS microchannel is
fabricated with standard soft lithography process [11].
Figure 1 illustrates the UH-AR bio-particle
manipulation system which includes a microchannel and
resonator device located at the center of the channel. The
resonator is excited by a radio frequency signal generator
(Keysight, N5171B) and a power amplifier (Mini-Circuits,
ZHL-5W-422+), by which the applied frequency and
power can be altered. The microchannel is set as 40 μm in
height and 600 μm in width.
978-1-5386-2732-7/17/$31.00 ©2017 IEEE 1789 Transducers 2017, Kaohsiung, TAIWAN, June 18-22, 2017
W3P.071
Figure 1: Scheme of the single-beam acoustic
microfluidic system: particles are introduced through
microchannel and trapped by micro-vortices around
the resonator, followed by manipulations via tuning the
system parameters.
GENERATION OF MICRO-VORTEX
Even though acoustic streaming has been widely
studied including its generation mechanism and
applications, localized micro-vortex featured with tunable
strength and formation are still difficult to be realized
because of limited choices or optimization strategies of
current acoustic devices and acoustofluidic technologies.
The main limitations of these tools are their low
frequency and poor minimization capacity. One of the
most effective principles to trigger acoustic streaming
arises from the viscous attenuation on the boundaries.
And the body force generated on the vibrating surface
scales with
[13]. The most used acoustic frequency
ranges from several MHz to tens of MHz. Ultrahigh
frequency (30~300 MHz) SAWs have been utilized to
trigger microvortex within microchannels by breaking
through the scale of acoustofluidics. Compared with
previous acoustic tools, the UHF-AR presented in this
work provides a much greater body force considering its
GHz frequency. As figure 2 shows, the body force is at
the scale of 10
12
N/m
3
, and decreases sharply along the
direction away from the device surface. Furthermore,
because of its thickness extensional mode, the acoustic
energy is emitted from the resonating region, i.e. the
pentagon in the inserted SEM image of the device in
figure 2. Thus, the acoustic energy is naturally focused
within the micro-patterned area. As the inserted image in
figure 2 shows, the ultra-strong body force generated
within the pentagon area actuates the fluid away from the
device, forming a single-beam streaming. Micro-vortices
are then formed around the acoustofluidic streaming beam
due to the pressure gradients. The height of the channel
plays a determinative role in the formation of micro-
vortices due to the scaling effect of the acoustofluidics
[14]. When ultrahigh frequency acoustic wave travels into
the fluid, it would decay within several micrometers.
While, the microscaled height of microchannels would
affect the energy coupled into the liquid. As figure 2
shows, the area confined by the body force curve
represents the acoustic energy to drive fluid.
Microchannles with height more than 40 um would not
influence the coupled energy. While, lowering the
microchannel height would reduce the coupled energy and
further result in weaker micro-vortex.
Figure 2: Distribution of acoustic fields-triggered body
force in the z-direction labeled in figure 1. The inset in
the up-left is the SEM photo of UHF-AR device; the
right figure presents the formation of micro-vortices
induced by the pressure gradients beside the single-
beam acoustofluidic streaming.
PRINCIPLE OF MICROPARTICLE TRAP
Here the driving mechanism is a combination of
inertial force (for attracting a particle) and viscous force
(for rotating a particle). Vortex centers with minimal
flow velocity have been shown to be stable trapping
positions for objects by the finite elements simulation
result in figure 3 (a). The velocity value is acquired on
the labeled line, and the direction is indicated by the
arrows. In our case, the dynamics can be understood by a
combination of viscous force along the flow direction
(F
V
= 3πηvD, where η denotes fluid viscosity, v denotes
the flow velocity and D is the ‘effective’ diameter of
particle), and inertial force directing along the velocity
gradient (F
I
=Re·F
V
, where Re denotes Reynolds
number). As figure 3 (b) shows, the viscous force (black
arrow) drags the microparticles into the vortex-streaming
lines, and the inertial force (blue arrow) pushes the
trapped particles into the center of the vortex. When
trapped in the vortex-streaming center, the microparticle
would keep stable as the result of the equilibrium of the
inertial force from the surroundings. Meanwhile, the
viscous force working on the surface rotates the
microparticle as figure 3 (b) shows. As the viscous force
is proportional to the velocity of the streaming line
around the microparticle surface, higher power would
lead to a higher rotation speed since it triggers stronger
vortex.
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Figure 3: Principles of micro/nanoparticles trapping
and rotation using micro-vortices triggered streaming
forces (inertial force and viscous force).
EXPERIMENTS
Micro/nanoscaled polystyrene beads suspended in
PBS buffer (10 mM, pH=7.4) with diameter of 9 μm, 1
μm, and 200 nm are respectively injected into the system
with a given flow rate of 2 μL/min. The applied power is
set as 500 mW. Online trapping of the
micro/nanoparticles within the microfluidic system are
monitored lively using a microscope (Olympus, 24 fps).
When particles flow above the UHF-AR, the device is
powered on with a 500 mW RF signal at the resonating
frequency of 1540 MHz. The trapping process lasts for
about 10 s, and then power off the UHF-AR. The flow
rate keeps stable in the experiment. By diluting the
suspension, the concentration of microbeads with
diameter of 15μm can be controlled in a low level, and
trapping of single particle within a controllable period is
easy to achieve. When a single microbead is trapped by
the UHF-AR, we reduce the flow rate to zero, and lower
the power to 1 mW. Low power and static flow help to
create a stable environment for better localized rotation
characterizations. As nanoparticle and biomolecule
trapping remains as a challenge, nanoparticle with
diameter of 50 nm and FITC with diameter of several
nanometers are respectively introduced into the
microsystem to test the limitation of UHF-AR trapper.
And the parameters set on the nanoparticle trap are
optimized to obtain the best trapping effect.
RESULTS AND DISCUSSIONS
Figure 4 presents the experimental results of online
trapping of micro/nanoparticles. Particles with diameter
above 200 nm have been successfully trapped by the
UHF-AR (figure 4 (a)-(c)). The trapping region is well
patterned around the edges of UHF-AR, of which the
reason is that the micro-vortex is effectively generated at
the edges of the resonator due to the large acoustic
pressure gradients. The particles passing through the
vortex region can be rapidly trapped, while those don't
enter the effective trap region would flow away. After
powering off the device, the trapped particles pattern
flows away as the figure 4 (a)-(c) shows.
As figure 4 shows, the flowing particles can be
effectively trapped by the micro-vortices around the
device, and the minimum size of trapped particle is
demonstrated down to 50 nm, showing good capability of
this method for trapping and concentration of
nanoparticles. For the rotation experiment, the applied
power of the resonator is reduced to 1 mW to decrease the
strength of micro-vortices for better observation and
characterizations. Polystyrene microbeads with diameter
of 15 μm are attracted within the micro-vortices and
rotated by the viscous force as shown in Figure 5. As the
rotation of microscaled matters is driven by a streaming
force as discussed above, and the applied power is low,
this technology is bio-friendly, and suitable for cell
manipulations.
Figure 4: Online trapping and concentration of
micro/nanoparticles: (a) 9 μm; (b) 1 μm; (c) 200 nm.
(d)-(f) present the trapping effect of nanoparticles with
different size and the minimum is 50 nm. In the
experiments, fluids were introduced from the right to
the left, and when the acoustic device is powered off,
the trapped particles would be released from the
micro-vortices.
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Figure 5: In-situ rotation of microbeads with diameter
of 15 μm.
CONCLUSIONS
We demonstrated a 3-D micro-vortex based
microfluidic system for online trapping and rotation of
micro/nano-scaled bio-particles by using a 1540 MHz
acoustic resonator. In this system, the micro-vortex is well
controlled to trap particles from the fluid, and locally
rotate single bead by optimizing the applied power, flow
rate, and microchannel height. Nanoparticles with
diameter of 50 nm has been effectively trapped, showing
the potential for bioparticle and biomolecule enrichment.
Considering the MEMS fabrication process of the UHF-
AR, the acoustofluidic device can be integrated to build
up complex micro-analytical systems, enabling the vortex
streaming to be a practical tool in biological research.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial
support from the Natural Science Foundation of China
(NSFC No. 51375341, 61674114), the 111 Project
(B07014), and the Tianjin Applied Basic Research and
Advanced Technology (14JCYBJC41500).
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CONTACT
*X.X. Duan, email: xduan@tju.edu.cn
*W. Pang, email: weipang@tju.edu.cn
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... The nonlinear attenuation of oscillating displacements in dispersive media results in a body force (F B ) at the z-axis, which pushes the liquid in the direction of acoustic wave propagation and then generates a stable liquid flow (acoustic streaming) 56 . Since F B ∝ ω 4 (ω is the angular frequency), the strength of F B generated by UHF BAWs is much stronger, which is due to the enhanced local energy density [57][58][59] . In addition, the rather small footprint of the UHF device (dozens to millions of square microns) results in a more focused acoustic wave beam 60 . ...
... Here, we qualitatively discuss the influence of the boundary conditions and the force status of the trapped particles by simulation. Once the device was fixed, the strength and distribution of the acoustic streaming were mainly determined by the geometry of the microchannel 59,63,64 . We then calculated the distribution of the acoustic field in 2D models under different geometric confinements, as shown in Supplementary Fig. S3. ...
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Concentration and separation of particles and biological specimens is a fundamental function of micro/nanofluidic systems. Acoustic streaming is an effective and biocompatible way to create rapid microscale fluid motion and induce particle capture, though the >100 MHz frequencies required to directly generate acoustic body forces on the microscale have traditionally been difficult to generate and localize in a way that is amenable to efficient generation of streaming. Moreover, acoustic, hydrodynamic and electrical forces as typically applied have difficulty manipulating specimens in the sub-micron regime. In this work, we introduce highly focused travelling surface acoustic waves (SAW) at high frequencies between 193-636 MHz for efficient and highly localized production of acoustic streaming vortices on microfluidic length scales. Concentration occurs via a novel mechanism, whereby the combined acoustic radiation and streaming field results in size-selective aggregation in fluid streamlines in the vicinity of a high-amplitude acoustic beam, as opposed to previous acoustic radiation induced particle concentration where objects typically migrate towards minimum pressure locations. Though the acoustic streaming is induced by a travelling wave, we are able to manipulate particles an order of magnitude smaller than possible using the travelling wave force alone. We experimentally and theoretically examine the range of particle sizes that can be captured in fluid streamlines using this technique, with rapid particle concentration demonstrated down to 300 nm diameters. We also demonstrate that locations of trapping and concentration are size-dependent, which is attributed to the combined effects of the acoustic streaming and acoustic forces.
Article
We demonstrate an acoustofluidic device using Lamb waves (LWs) to manipulate polystyrene (PS) microparticles suspend-ed in a sessile droplet of water. The LW-based acoustofluidic platform used in this study is advantageous in that the device is actuated over a range of frequencies without changing the device structure or electrode pattern. In addition, the device is simple to operate and cheap to fabricate. The LWs, produced on a piezoelectric substrate, attenuate inside the fluid and create acoustic streaming flow (ASF) in the form of a poloidal flow with toroidal vortices. The PS particles experience direct acoustic radiation force (ARF) in addition to being influenced by the ASF, which drive the concentration of particles to form a ring. This phenomenon was previously attributed to the ASF alone, but the present experimental results confirm that the ARF plays an important role in forming the particle ring, which would not be possible in the presence of only the ASF. We used a range of actuation frequencies (45-280 MHz), PS particle diameters (1-10 μm), and droplet volumes (5, 7.5, and 10 μL) to experimentally demonstrate this phenomenon.
Article
This study presents a novel acoustic mixer comprising of a microfabricated silicon nitride membrane with a hole etched through it. We show that the introduction of the through hole leads to extremely fast and homogeneous mixing. When the membrane is immersed in fluid and subjected to acoustic excitation, a strong streaming field in the form of vortices is generated. The vortices are always observed to centre at the hole, pointing to the critical role it has on the streaming field. We hypothesise that the hole introduces a discontinuity to the boundary conditions of the membrane, leading to strong streaming vortices. With numerical simulations, we show that the hole's presence can increase the volume force responsible for driving the streaming field by 2 orders of magnitude, thus supporting our hypothesis. We investigate the mixing performance at different Peclet numbers by varying the flow rates for various devices containing circular, square and rectangular shaped holes of different dimensions. We demonstrate rapid mixing within 3 ms mixing time (90% mixing efficiency at 60 μl min(-1) total flow rate, Peclet number equals 8333 ± 3.5%) is possible with the current designs. Finally, we examine the membrane with two circular holes which are covered by air bubbles and compare it to when the membrane is fully immersed. We find that coupling between the holes' vortices occurs only when membrane is immersed; while with the bubble membrane, the upstream hole's vortices can act as a blockage to fluid flow passing it.
Article
Micro/nano scale biosensors integrated with the local adsorption mask have been demonstrated to have a better limit of detection (LOD) and less sample consumptions. However, the molecular diffusions and binding kinetics in such confined droplet have been less studied which limited further development and application of the local adsorption method and imposed restrictions on discovery of new signal amplification strategies. In this work, we studied the kinetic issues via experimental investigations and theoretical analysis on microfabricated biosensors. Mass sensitive film bulk acoustic resonator (FBAR) sensors with hydrophobic Teflon film covering the non-sensing area as the mask were introduced. The fabricated masking sensors were characterized with physical adsorption of bovine serum albumin (BSA) and specific binding of antibody and antigen. Over an order of magnitude improvement on LOD was experimentally monitored. An analytical model was introduced to discuss the target molecule diffusion and binding kinetics in droplet environment, especially the crucial effects of incubation time, which has been less covered in previous local adsorption related literatures. An incubation time accumulated signal amplification effect was theoretically predicted, experimentally monitored and carefully explained. In addition, device optimization was explored based on the analytical model to fully utilize the merits of local adsorption. The discussions on the kinetic issues are believed to have wide implications for other types of micro/nano fabricated biosensors with potentially improved LOD. Copyright © 2015 Elsevier B.V. All rights reserved.