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Wireless Controlled Local Heating and Mixing Multiple Droplets Using Micro-Fabricated Resonator Array for Micro-Reactor Applications

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This work reported a wireless controlled micro-actuator system for rapid heating and mixing of multiple droplets using integrated arrays of micro-fabricated 2.5 GHz solid-mounted thin-film piezoelectric resonators (SMRs) and a millimeter-scale omnidirectional antenna. An equivalent circuit is proposed to analyze the mechanism of the heating, mixing of the SMR and the wireless communication system. The heating and mixing rate can be tuned by adjusting the input power as well as the transmission distance between the transmitting antenna and the receiving antennas. A heating rate up to 3.7 °C per second and ultra-fast mixing of the droplet was demonstrated with the wireless microsystem. In addition, two types of circuits, H-shaped and rake-shaped, were designed and fabricated for parallel operating actuator array and controlling the power distribution with the array. Both uniform and gradient heating of the multiple droplets are achieved, which can be potentially applied for developing high-throughput wireless micro-reactor system.
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Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000.
Digital Object Identifier 10.1109/ACCESS.2017.Doi Number
Wireless Controlled Local Heating and Mixing
Multiple Droplets Using Micro-fabricated
Resonator Array for Micro-reactor Applications
Zhan Wang, Hongxiang Zhang, Yang Yang, Hemi Qu, Ziyu Han, Wei Pang, Xuexin Duan1
1State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China
Corresponding author: Xuexin Duan (e-mail: xduan@tju.edu.cn).
The authors gratefully acknowledge financial support from the Natural Science Foundation of China (NSFC no. 61674114), Tianjin Applied Basic Research
and Advanced Technology (14JCYBJC41500), and the 111 Project (B07014).
ABSTRACT This work reported a wireless controlled micro-actuator system for rapid heating and mixing
of multiple droplets using integrated arrays of micro-fabricated 2.5 GHz solid-mounted thin-film
piezoelectric resonators (SMRs) and a millimeter-scale omnidirectional antenna. An equivalent circuit is
proposed to analyze the mechanism of the heating, mixing of the SMR and the wireless communication
system. The heating and mixing rate can be tuned by adjusting the input power as well as the transmission
distance between the transmitting antenna and the receiving antennas. A heating rate up to 3.7 °C per
second and ultra-fast mixing of the droplet was demonstrated with the wireless microsystem. In addition,
two types of circuits, H-shaped and rake-shaped, were designed and fabricated for parallel operating
actuator array and controlling the power distribution with the array. Both uniform and gradient heating of
the multiple droplets are achieved, which can be potentially applied for developing high-throughput
wireless micro-reactor system.
INDEX TERMS Chemical reactors, Piezoelectric actuators, Wireless communication
I. INTRODUCTION
Developing droplet micro-reactors are of great interests for a
variety of (bio)chemical applications such as drug discovery
[1], [2], biomedical assay [3], combinatorial chemistry [4],
and other lab-on-chip applications [5]-[8]. Because of the
limited volume, high efficient heating and mixing in such
small droplet is rather challenging. Micro-fabricated heater
[9], [10] and mixer [11], [12], as well as their integrations
[13], have been developed for this end. Acoustic resonators
are reported as one of the represented active actuators to heat
the droplets [14], [15] due to the dissipation of the acoustic
energy into the liquid. In a recent work, we have
demonstrated the rapid heating and mixing in liquid droplets
using a MEMS fabricated solid-mounted film bulk acoustic
resonator (SMR) [13], [16] which has a clear advantage of
CMOS compatibility, high effective electromechanical
coupling, high solution stability, and is able to generating
micro-vortices at device-liquid interface to enhance the
droplet mixing [17]-[19].
Most of the existing micro-reactor system uses wire
connections from the devices to the external power source.
However, when it comes to harsh or closed environment, as
well as narrow space, the conventional wired connections are
limited. In addition, for developing high-throughput micro-
reactors, manipulation of multiple droplets is required [20]-
[22]. For these reasons, there has been growing interests in
developing wireless heating techniques. Microwave [23],
[24], induction [25], and LC circuit heating [26], [27] have
been reported so far. However, these methods have various
practical challenges, including long-distance control, limited
to certain types of liquid, hard for minimization or
implantation, and unable to heat individual droplet. The
methods of microwave and induction heating are hard to
achieve arrays. And all of these methods cannot achieve
wireless mixing of droplets. Thus, developing wireless driven
micro-devices for developing high throughput droplet micro-
reactors is on strong demand. Furthermore, to the author’s
knowledge, wireless controlled local heating and mixing
multiple droplets has been rarely reported.
In this work, we developed a novel wireless controlled
micro-reactor system for high throughput heating and mixing
multiple droplets by integrating arrays of SMRs with a
millimeter-scale omnidirectional antenna on a single PCB
board. The resonator is powered by electromagnetic-field
coupling, thus no hard wire, energy storage or harvesting
components are required. An equivalent circuit of the
2169-3536 (c) 2017 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See
http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI
10.1109/ACCESS.2017.2766270, IEEE Access
Z Wang et al: Wireless Controlled Local Heating and Mixing Multiple Droplets
2 VOLUME XX, 2017
wireless system is proposed to explain the mechanism of
electromagnetic-field coupling. Wireless passive heating and
mixing of 1 µl water droplet is experimentally demonstrated.
The heating and mixing rate are studied with different
powers applied to the transmitting antenna as well as the
transmission distance between transmitting and receiving
antennas. Uniform and gradient heating of multiple droplets
were achieved by using two types of circuits, H-shaped and
raked-shaped, which proves another advantage of using
wireless techniques.
II. WIRELESS MICRO-REACTOR DESIGN
A.
EXPERIMENTAL SETUP
The schematic diagram of the experimental setup for wireless
droplet heating and mixing is shown in Figure 1 (a). A solid
mounted resonator (SMR) with a resonant frequency of 2.50
GHz and millimeter-scale omnidirectional receiving antenna
(Molex, 047980001) were integrated on a single PCB board.
The PCB is fabricated with FR-4 epoxy glass substrate. The
substrate dielectric constant is 4.5 and the thickness of PCB
is 1.6 mm. A commercial monopolar antenna was connected
to RF signal generator (Agilent EXG Analog Signal
Generator N5171B) and power amplifier (MODEL NO.
ZHL-5W-422+) as a transmitting antenna. Wireless passive
actuation of SMR was achieved via electromagnetic-field
coupling between transmitting and receiving antennas. The
illustration of SMR is shown in Figure S3 in the
supplementary material.
Fig. 1. (a) Schematic diagram of the wireless controlled SMR as actuator
for heating and mixing of microscale droplet. The inset is an image of
the device. (b) The equivalent circuit model of wireless passive
actuation.
B.
MECHANISM OF WIRELESS PASSIVE ACTUATION
The equivalent circuit model of the wireless passive actuator
is shown in Figure 1(b), which composes the source circuit,
the electromagnetic field coupled circuit and the Butterworth
Van Dyke (BVD) equivalent circuit of SMR. Ps and Zs refer
to the input power and input impedance. Z1 and Z2 refer to
the characteristic impedance of coaxial transmission line,
while A1 and A2 represent the transmitting antenna and
receiving antenna, respectively. C0, Cm and Lm represent the
static capacitance, the motional capacitance and the motional
inductance. Re, R0 and Rm represent the resistance of the
metal electrodes, the resistance related to dielectric losses and
the motional resistance associated with acoustic losses,
respectively. The complex Poynting vector (S) of electric
dipole can be decomposed into two parts: radial Poynting
vector (Sr) and polar Poynting vector (Sθ),
23
2
2
002
sin 1
1
8
rl
S I j
r kr

 


 
 


, (1a)
2
2
2
00 23
cos sin 1
1
16
kl
S j I r kr









, (1b)
Here, η0, I0, l, θ, λ and k represent the wave impedance in air,
current amplitude of electric dipole, length of electric dipole,
polar angle, wave length (~0.12 m calculated in air and ~0.07
m in FR-4 at the resonance frequency of 2.5 GHz) and wave
number. r refers to the distance between transmitting antenna
and receiving antenna, indicating that the wireless actuation
of SMR can be controlled by adjusting the transmission
distance.
In the near field of the antenna (kr<1), high-order term of
1/kr play a dominant role, then the equation (1) can be
simplified as
2 3 5
2
2
002
sin 1 1
8
rr
l
S j I S j
r kr r
 
   
 
 
, (2a)
25
2
2
00 23
cos sin 1 1
16
kl
S j I S j
r kr r


 
 
 
 
,(2b)
In this condition, the power is oscillating between
electromagnetic field and antenna and does not radiate. The
received power (Prec) of receiving antenna can be deduced as
5
1
rec rec
S
P d P r
 
Ss
. (3)
In the far field (kr>1), high-order term of 1/kr can be
ignored, then the equation (1) can be simplified as
22
2
0022
sin 1
8
rr
l
S I S
rr

 


, (4a)
0S
, (4b)
Thus, the received power (Prec) of receiving antenna can be
deduced as
. (5)
In our wireless system, the transmitting frequency of
antenna is 2.5 GHz. r<1.91 cm is the near field of the antenna
and r>1.91 cm is the far field.
C.
MECHANISM OF DROPLET HEATING AND MIXING
In the BVD equivalent circuit, Rm represents the load of the
resonator, which refers to the liquid droplet. The mechanism
of droplet heating and mixing is further analyzed in
2169-3536 (c) 2017 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See
http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI
10.1109/ACCESS.2017.2766270, IEEE Access
Z Wang et al: Wireless Controlled Local Heating and Mixing Multiple Droplets
3 VOLUME XX, 2017
supplementary material. The heat generated by fluid flow of
per unit mass can be written as
2
0
Q c v

, (6)
The heat conduction equation in the flow field is given by
2
pT
C v T k T Q
t

   


, (7)
Where ρ, c0, β and v represent he density of the liquid, sound
velocity in liquid, the attenuation coefficient and the
vibration velocity of liquid. Cp and k are the isobaric heat
capacity and the thermal conductivity.
D.
SIMULATED AND MEASURED RESULTS
Equation (7) demonstrates that the droplet is heated and
mixed simultaneously and the temperature distribution is
uniform within a very short time.
The heating performance of the SMR in water is simulated
by finite element analysis (COMSOL) according to (6) and
(7), as shown in figure 2 (a). The wireless heating capacity
was investigated with water droplets at different input power
as well as transmission distance. To characterize the acoustic
heating, the droplet temperature was measured with an
inserted thermocouple. All of the experiments were carried
out at 20 °C. Figure 2 (b) shows the temperature profiles of a
1 µl water droplet under the stimulation of various input
power from 0.5 W to 8 W, while the transmission distance
was fixed at 6 mm. The temperature increases during the
initial 10 s. The heating rates are calculated to be 0.5, 1.0, 2.2,
3.0 and 3.7 °C /s for the input power of 0.5, 1.0, 2.0, 4.0 and
8.0 W respectively. After 10 s stimulation, the temperature
reaches to a relatively steady state, which is compatible to the
results of droplet heating using wire-connected SMR. Here,
we also provide the comparison of the expected theoretical
curve and the measured heating profile (see supplementary
material Figure S4). The input power of the experiment is 8
W and the distance is 6 mm. It is rather difficult to build the
simulation model of wireless heating system. Thus, we
directly apply the power of receiving antenna with 251 mW,
which is calculated by the input power (8 W) multiplied by
the efficiency of power transfer from transmitting antenna to
receiving antenna (3.14% in this case). The differences
between theoretical and experimental curves could be
induced by many reasons, for example the liquid
volatilization, impedance mismatch and environmental
interface. In addition, the temperature change of the water
droplet is also dependent with the transmission distance
according to (3) and (5). As the transmission distance
changing from 1 mm to 10 mm (Figure 2 (c)), the maximum
temperature is negatively correlated to the distance, ranging
from 60 °C at 1 mm to 16 °C at 10 mm. The experimental
data of the maximum temperature change versus
transmission distance at the fixed input power of 8 W is
plotted in Figure 2 (d). In the case of near field, the
maximum temperature change can be fitted by the function
of
5
( 10)
a
Tx

, (8)
Here a is the fitted parameter, and x represented the nearest
distance between antennas. Thus, the distance between
antennas is the sum of x and antennas’ radii. The length of 10
mm represents the sum of antennas’ radii. Equation (8) is
deduced by (3). The fitting result is well matched with the
theory described above (Figure 2(d)). And the differences
between experimental and fitted curves are caused by the
liquid volatilization, heat conduction of droplet, etc.
In our developed wireless controlled micro-reactor system
thorough microscale acoustic devices, some of the energy is
converted to heat and some of the energy is used to mix the
droplet. Thus, the heating efficiency of our wireless
controlling system is less efficient comparable to microwave,
induction and LC circuit heating. The heating efficiency of
our system is presented in supplementary material (page 3).
As the mixing of the droplet is the challenging issue and
plays a very important role for improving the reaction
efficiency in droplets. We believe our system has the
advantages compared with the microwave, induction or LC
circuit which cannot achieve local mixing of the droplets.
Fig. 2. (a) COMSOL simulation of the energy conversion when SMR
contacted with water, while the color gradient represents temperature
distribution. (b) Temperature change of a 1 µl DI water droplet versus
time with different input power, from 0.5 W to 8 W. (c) Maximum
temperature change of a 1 µl DI water droplet versus power with
different transmission distance, from 1 mm to 10 mm. (d) Experimental
data and fitted curve of the maximum temperature change versus
distance at the fixed input power of 8 W.
As predicted by the theory, the mixing rate is also related
with the input power and the transmission distance. A series
of mixing experiments were then applied. One drop of
quantum dots (QDs, CdSSe/ZnS Water Soluble, PL 600 nm)
solution was introduced to the water droplet to facilitate the
monitoring of the mixing process. Figure 3 (a) shows that
when no input power was added to the SMR, the
fluorescence distribution remained almost unchanged within
2 s, which is due to the slow diffusion by the Brownian
motion. Whereas, when applying power, vigorous multiple
micro-vortices were generated immediately in the droplet,
leading to a rapid mixing within the droplet (see Figure 3 (b)
and the supplementary Video 1).
2169-3536 (c) 2017 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See
http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI
10.1109/ACCESS.2017.2766270, IEEE Access
Z Wang et al: Wireless Controlled Local Heating and Mixing Multiple Droplets
4 VOLUME XX, 2017
Fig. 3. Fluorescent microscope images showing the mixing of quantum
dot solution and water droplet (a) without input power and with input
power at 2 W and fixed wireless transmission distance at 1 cm. (c-d)
Normalized mixing index as a function of time with (c) different input
power (fixed distance 1 cm) and (d) different distance (fixed input power
8 W). Blank group refers to an infinite distance.
To quantify the mixing efficiency, the normalized mixing
index (NMI) is introduced [28], [29] (see supplementary
material). Experimental results demonstrated that the NMI is
related to the input power (Figure 3 (c)) as well as the
transmission distance (Figure 3 (d)). NMI increases quickly
from 0 to 0.9, within 0.5 s with the input power at 1 W and
fixed distance at 1 cm, or within 0.3 s with the distance over
2 cm and fixed input power at 8 W, which both guarantee an
effective mixing in the droplet.
III. CIRCUITS DESIGN
A unique advantage to have the wireless controlled heating
and mixing is the capability to achieve multiple droplets
manipulations. To demonstrate this, two kinds of SMR arrays
with H-shaped and rake-shaped circuits were applied
respectively.
A.
H-SHAPED CIRCUIT
For the H-shaped circuit, an array of four identical 2.5 GHz
SMR actuators were placed on the circuit board and
connected to the receiving antenna which is located at the
center of the board. Figure 4 (a) shows the schematic
diagram of the H-shaped wireless SMR array, which consists
of one single antenna and four SMRs in parallel connection.
The symmetrical circuit structure ensures the symmetrical
distribution of electromagnetic field in space. Figure 4 (b)
shows the simulated distribution of the electric field on the
integrated PCB board which indicates the uniform
distribution of the energy. Thus, the four SMRs will have the
same heating and mixing efficiency. Experimentally, the
heating rate and mixing index of the four SMRs were
measured respectively, under the same stimulations.
Experimental results prove that the rather homogeneity
droplet heating (Figure 4 (c)) and mixing (Figure 4 (d)) by
the four SMRs. The maximum temperature changes of the
four SMRs are 23.4, 24.1, 24 and 24.1 °C respectively (under
the stimulation of fixed input power of 8 W and transmission
distance of 4 mm). The error of the maximum temperature is
less than 1 °C. While the normalized mixing index of the
four SMRs all increased above 0.8 within 1.5 s under the
input power of 2 W and transmission distance fixed at 1 cm.
Thus, the wireless H-shaped SMR array can be potentially
applied for high-throughput multiple droplets uniform
heating and mixing.
Fig. 4. (a) Schematic diagram of the H-shaped SMR array integrated with
a single antenna. (b) The electric field distribution nearby on integrated
PCB board, while the color-bar represents the normalized electric field
intensity. Temperature change (c) and normalized mixing index (d) of
the four droplets located on top of the SMRs.
B.
RAKE-SHAPED CIRCUIT
Besides the uniform heating, non-uniform heating the multi-
droplets was also demonstrated by redesigning the circuit. A
rake-shaped circuit with five ports arranged in parallel was
applied to connect five individual SMRs with the receiving
antenna (Figure 5(a)). In this case, the electromagnetic field
propagates mainly in the gaps (insulation medium) between
the rake-shaped conductor and external ground conductor
plane, which will result a non-uniform distribution of the
electromagnetic field: strongest electromagnetic field exists
on both sides and weakest one exists in the middle. Therefore,
the two actuators arranged on both sides (port 1 and 5) will
get the most energy while the SMR in the middle (port 3) get
the least energy. The remaining two actuators (port 2 and 4)
will get the energy between the most and the least. This rake-
shaped design of the circuit provides the possibilities to
realize a temperature gradient in the droplet array. According
to the nature of electromagnetic wave propagation in air
(Maxwell equations) and the boundary conditions on the
conductor, the distribution of electromagnetic field in space
containing such a circuit was solved through numerical
calculation by COMSOL. Figure 5 (b) shows the simulated
distribution of the electric field on the integrated PCB board.
The solution domain of electric field in the simulation model
is shown in supplementary material (Figure S5). The results
indicate that the distribution of electric field with the five
ports is non-uniform: port 1 and 5 are the strongest and port 3
is the weakest. It proves the theory analysis above. The
different power distribution of the five SMRs will result
different vibration intensity and heating efficiency of these
SMRs (see supplementary material Video 2). Figure 5 (c)
plots the real-time temperature profile of the droplets heated
2169-3536 (c) 2017 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See
http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI
10.1109/ACCESS.2017.2766270, IEEE Access
Z Wang et al: Wireless Controlled Local Heating and Mixing Multiple Droplets
5 VOLUME XX, 2017
by the five actuators with the fixed input power at 8 W and
transmitting distance of 4 mm. It shows that the heating
efficiency of the two SMRs on both sides is the highest and
the middle one is the lowest, which further demonstrates the
theory analysis and simulation results. The maximum
temperature changes of these SMRs is shown in Figure 5 (d)
and the temperature differences between adjacent SMRs are
almost the same (about 5 °C). Besides, finite element
analysis gives the transmission coefficients (Scattering
parameters) in 2.5 GHz from input (antenna) port to SMR
ports. Thus we can evaluate the power applied to each SMR
port, which is proportional to the square of transmission
coefficient. The normalized power distribution of these SMR
ports is shown in Figure 5(d) and consistent with the gradient
of the maximum temperature changes. Also, this theoretical
power distribution is well matched with the experimental
result (see supplementary material Figure S6).
Fig. 5. (a) Photo of the rake-shaped SMR array integrated with a single
antenna. (b) The simulated electric field distribution on the integrated
PCB board, while the color-bar represents the normalized electric field
intensity. (c) Temperature change of five droplets heated by different
SMRs with the fixed input power of 8 W and transmitting distance of 4
mm. (d) Plot of the maximum temperature changes of the five droplets
and the normalized power distribution of the five ports calculated by
COMSOL.
IV. CONCLUSION
In conclusion, for the first time, wireless controlled MEMS
piezoelectric resonator array for heating and mixing of
multiple liquid droplets is demonstrated. An equivalent
circuit model is proposed to describe the wireless coupling
and the mechanism of the heating and mixing by SMR. The
system performs a heating rate up to 3.7 °C /s and a rapid
mixing within 0.3 s in the water droplet. The heating and
mixing can be further tuned by adjusting the input power or
the wireless transmission distance. Both uniform and non-
uniform (gradient) wireless heating of multiple droplets were
demonstrated by applying H-shaped and rake-shaped
arranged SMR array, which shows great potential as a
versatile platform for developing high-throughput micro-
reactors.
ACKNOWLEDGMENT
The authors gratefully acknowledge financial support from
the Natural Science Foundation of China (NSFC no.
61674114), Tianjin Applied Basic Research and Advanced
Technology (14JCYBJC41500), and the 111 Project
(B07014).
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... Micro-sized water droplets have been widely used in multiple research fields, such as analytical chemistry [1], [2], precision engineering [3]- [7], biological sciences [8]- [12], etc. [13]- [17] In these droplet-based applications, for smoothing the edge of a substrate processed by wet etching [18], digitally studying pHsensitive enzymatic activities [19], [20] and precisely actuating intelligent structures made up of hydrogel and graphene [21]- [26], it is critical to produce ultramono-sized droplets with an exact pH value. However, currently, it is still a big challenge to achieve the goal explained above. ...
... Fig. 4(a) A2-A5 shows the elongating droplet returned to the circle shape in 1.4 s. After removing the negative electrode from the droplet, the V d can be calculated by measuring D d and using (2). To compare the relationship between D d and the D e , in our experiments, seven droplets (1.4 uL, 1490 μm) were electrolyzed and the corresponding D e was set to 700 μm-1300 μm with increments of 100 μm. ...
... For the particle that was electrolyzed for 60 mins, the pH value reached 12.2 and the droplet size was 340 μm. The volume of this droplet was only 0.016 μL (1.1% of the initial volume) when calculated using (2). The experiment reveals that this method is capable of adjusting the pH values and sizes for a droplet at the same time. ...
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In this paper, a strategy to modify each micro-droplet’s volume and synchronously adjust its pH value as required based on the electrolysis reaction in silicone oil is demonstrated. A pair of platinum electrodes fixed onto the jaws of a vernier caliper was used to modify the micro-droplet’s volume and adjust its pH value by simply adjusting the distance between the two electrodes. To get a micro-droplet with desirable volume and pH values, three models, the relationship between the droplet’s volume and its diameter when the droplet was placed on a fluorinated ethylene propylene (FEP)–covered glass substrate, the relationship between the distance of the two electrodes and the size of the resulted micro-droplet, and the relationship between the pH value and the micro-droplet’s consuming rate, were built through the least square method. In our experiments, a droplet (5% sodium chloride solution, 1.4 uL, pH = 7) could consume 98.9% of its initial volume and form a new droplet with a volume of 0.016 µL and pH of 12.2. In addition, to validate that this method is also suitable in the acid and alkaline solutions, 0.001 mol/L NaOH and H2SO4 solutions were respectively operated using the same procedure. Both the volume and pH value could be controlled, which proved the potential application of our proposed method in analytical chemistry, precision engineering, etc.
... Recently, thermal functionality has been gaining high interest due to the increasing requirements of thermal tasks performed by functional materials and devices. In numerous domains, such as multi-zoned thermal forming, [1,3] thermal-driven shape morphing, [4,5] encrypted heat messaging, [2] active therapy, [6] thermal actuators, [7] the thermo-chromatic, [8][9][10][11] wearable devices, [12,13] the thermal-haptic, [13] micro-reactors, [14,15] camouflage, [16,17] and damage detection, [18] the highly flexible thermal manipulability is notably anticipated. Specifically, there are four major requirements for thermal functional materials. ...
... Introducing the dynamically regulated thermal fields for fluid control to realize complex micro-reaction devices is another potential application. [14,15] Particularly, the CTP method is highly suitable for encrypted messaging under natural light and presents highly freewheeling infrared patterns. [2] ...
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Manipulation of heat distribution in the material is a long-term goal that has been pursued in the field of thermal functional materials. Various thermal manipulation methods have been successfully developed, some of which have realized gorgeous thermal patterns. However, existing thermal functional materials are either difficult to avoid introducing massive external heaters and cables or it is hard to achieve dynamic thermal manipulation with a high thermal resolution and accuracy. In addition, the complicated manufacturing process also limits the wide application of thermal functional materials. Herein, a computed thermal patterning method is proposed, which can dynamically achieve a freewheeling thermal manipulation in the highly versatile and easily manufactured multi-layered material. This method first introduces the principle of tomography into the thermal manipulation by treating heat beams as light energy via a multi-angled rhythmical superposition, enabling the human characters to be written, paintings to be drawn, movies to be played, without embedding any external heaters or cables. A particular thermal diffusion problem in the tomographic process is solved by developing an inverse thermal diffusion optimization. Experimental cases demonstrate the great potential of this method in multi-zoned thermal forming, encrypted messaging, 3D thermal printing, and morphing.
... Compared with the above two strategies, thin-film-type device fabricated by MEMS technology as another avenue has the advantages of smaller size (~ µm) and higher frequency (~ GHz) (Pang et al. 2012;Zhang et al. 2017), which might contribute to high level of integration for on-chip applications and large acoustic streaming force for higher vortices velocity (~ m/s). However, in the realm of acoustic mixing, the research on GHz acoustic mixing (Cui et al. 2016;Qu et al. 2017;Wang et al. 2017) is still limited. ...
... Different from PZT acoustic mixing (Madison et al. 2017), thin-film acoustic micromixer with higher frequency would produce larger body force to form violent acoustic streaming. On the other hand, the micromixing at open boundary (Qu et al. 2017;Wang et al. 2017) and in microchannel (Cui et al. 2016) has been reported. Differently, we investigated the flow velocity of in-situ microdroplet-mixing of different scales under closed boundary. ...
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Rapid and reliable micromixing requires continuous improvement to renovate more powerful microfluidics chip for chemosynthesis, biological assay, and drug purification. In this work, we realized rapid in-situ mixing in droplets on a closed electro-microfluidic chip. Electrowetting and 2.5 GHz acoustic wave streaming were coupled into a monolithic chip for the manipulation and active mixing of microdroplets, respectively. Finite-element analysis simulation provided three-dimensional illustrations of turbulent flow pattern, fluid velocity, and vortices core locations. We carried out mixing experiments on different scales from nanoscale molecules to microscale particles, accelerating mixing efficiency by more than 50 times compared with pure diffusion. In the enzyme catalytic reaction experiment for biological assay demonstration, mixing efficiency of biological samples improves by about one order of magnitude compared with conventional 96-well-plate assay. Limited temperature rising of mixing in microdroplets validates biological safety, which guarantees potentials of the chip in various biochemical analyses and medical applications.
... As for the strategies combining the thermal effect and other physical fields, the thermal-electrical methods have a strict requirement on fluid conductivity because of the introduction of electric field (Ng et al. 2009;Zhang et al. 2016). And the heating efficiency of fluid in the acoustic-thermal approaches is low because of the extra energy consumption for fluid mixing Wang et al. 2017). So aiming at these three issues, it is meaningful and important to propose an efficient, economic and high-throughput thermal-based approach with a low dependence on fluid characteristics for continuous micromixing and microreaction, and to reveal the mixing mechanism behind it. ...
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