T3P.I07

MICROFLUIDIC FLOW METER AND VISCOMETER UTILIZING FLOW-INDUCED VIBRATION ON AN OPTIC FIBER CANTILEVER

Po-Yau Jul, Chien-Hsiung Tsai2, Lung-Ming Fu2 and Che-Hsin Linl. •

lNational Sun Vat-sen University, Kaohsiung, Taiwan 2National Pingtung University of Science and Technology, Pingtung, Taiwan

ABSTRACT This paper presents a microfluidic flow sensor for

the detections of velocity and viscosity, especially for ultra-low viscosity applications. An etched optic fiber

with the diameter of 9 !im is embedded in a microfluidic chip to couple laser light into the microfluidic channel. An flow induced vibration causes periodic flapping motion of the optic fiber cantilever. Through the frequency analysis, the fluidic properties including the flow rate and the viscosity can be detected and identified. Results show that this developed sensor is capable of sensing liquid samples with the flow rates from 2.5 mllmin to 15.0 mllmin and the viscosities from 0.306 cP to 1.2 cP. In addition, air samples(0.0148 cP) with various flow rates can also be detected using the developed sensor. The developed flow sensor provides a simple and straight forward method for sensing flow characteristics in a microfluidic channel.

KEYWORDS flow meter, viscometer, flow-induced vibration

INTRODUCTION Flow rate detection in microfluidic channel have

been attracted lots of researchers in MEMS fields in the past years. A number of mature techniques such as LDV(Laser Doppler Velocimetry)[I], PIV(Particle Image Velocimetry)[2, 3], differential pressure flow sensors[ 4], thermal flow sensors[ 5], cantilever-base gas-flow sensors[6, 7] and resonating flow sensors[8] have been reported. These sensors can detect fluid flow rate in microfluidic channel with high precision. However, these flow meters cannot analysis the flow behaviors of non-Newtonian liquids such as compressible fluids due to the instable hydrodynamic properties of these fluids.

On contrast, flow induced vibration is capable of measuring the flow behaviors of non-Newtonian fluids. The phenomenon of flow-induced vibration occurs due to the surface interaction of solid and liquid. In general, the vortex-shedding can be induced when the flow rate or the Reynolds numbers of the flow is high enough. Two-dimensional lapping motions for thin structures such as flag[9] or cantilever[ I 0] along the axial flow field will be induced.[II] Typically, the resonance frequency of the cantilever structure is correlated with

the flow rate and viscosity of the surrounding liquid. Recently, MEMS-based sensors have been utilized

for measuring hydrodynamic liquid properties such as flow rate, viscosity, flow direction. In general, micromachined cantilevers are usually used as the major sensing part for these measurements.[lO, 12] However, these sensors relied on complicated fabricated processes or delicate equipment which is not suitable for practical applications. This study presents a simple method for detecting the flow rate and viscosity in one sensor. An flow induced vibration causes periodic flapping motion of an etched optic fiber cantilever. Flow rate and viscosity of the fluid samples can be detected with this simple approach.

(A) Etched optic fiber

I

Optic fiber cantilever

(8)

'\ Flow induced Flow direction vibration

Fig. 1: (Aj, (Bj Schematic showing the working principle of this study. (Cj Images showing the optic fiber cantilever and the flow induced vibration on the optical cantilever.

CHIP DESIGN AND PRINCIPLE Figure 1 presents the basic working principle of

this work. In order to enhance the flexibility of the optic fiber cantilever, single-mode optic fibers were chemically etched to 9 !-tm in diameter. The etched optical fiber is then inserted into a microfluidic channel and used as the sensing element by transferring the periodical optical signal into the electric signals. The flow along with the fiber-cantilever may cause hydrodynamic drag-force on the cantilever and result in pressure differences at the both sides of the fiber­cantilever. Flow-induced vibrations occur when the fluid flow rate is high enough. The flapping motion caused by the flow-induced vibration at the optic fiber­tip is then used for inspecting the flow properties. Flow rate and the viscosity of the fluids inside the microfluidic channel can be obtained via analyzing the flapping motions of the optic fiber cantilever.

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Fig. 2: The relationship of measured fiber diameters and etching time.

FABRICATION Prior to the chip fabrication process, single-mode

optical fiber (SMF28e+, Coming, USA) with the core

diameter of 8.2 Ilm and the outer diameter of 125±0.7

Ilm was used to produce the optic fiber cantilever. After stripping the plastic buffer layer of the fibers, the stripped fibers were then immersed into BOE solution (Buffered oxide etch ant) to reduce the diameter of the optic fiber. Figure 2 presents the relationship between the diameter of the etched fiber and etching time. The insets show the SEM images of the tips of optical­fibers before and after chemical etching. In the present

study, the diameter of 9 11m for the etched optic fiber was chosen due to less optical loss.

(a) Glass cleaning

(b) PR spin-coating

(c) Lithography

(e) Glass Etching and PR stripping

(f) Cover glass drilling

(g) Fusion bonding

(d) Developing and hard baking (h) Etched fiber insertion Fig. 3: A simplified fabrication process of the proposedflow sensor.

In the present study, the microfluidic chip was fabricated using a fast process for producing microfluidic systems on soda-lime glass.[13] Figure 3 illustrates the fabrication process of the proposed MEMS-based flow sensor. A single microfluidic channel with a sudden expansion channel was

produced with a wet chemical etching process.(Fig. 2a-2e) Note that an insertion guide was designed on the microchip for easier optical fiber insertion. A cover glass slide drilled with fluid via holes was covered on the etched slide then thermally bonded. (Fig. 2f-2g) Etched optic fiber was then finally inserted into the microfluidic channel via the insertion guide and then fixed with epoxy resin .

Figure 4(a) shows the picture of the chip after fabrication. The dimension of the microchip device was 7.5 cm in length and 1.5 cm in width. The

microchannel was 100 !lill in width with a sudden expansion channel. Note that the depth of fabricated microfluidic channel was about 70 Ilm. Figure 4(b) shows the closed-up view of the fiber-cantilever with feeding the detection light source while Fig. 4(c) presents a bright-view picture showing the details for the channel dimension.

(C)

Fig. 4: (a) A picture showing the fabricated sensor (b) Closed-up view of the inserted optic fiber (c) Closed-up view showing the dimension of the microchannel.

EXPERIMENT ALS Figure 5 shows the experimental setup for testing

the developed flow sensor. The test was operated under an optical microscope (E-400, Nikon, Japan). Since commercial syringe pump was droved with the stepper motor which may cause pulsate flow injection and alert the experimental results. The fluid was injected into the chip via a syringe that driven by a home-made pneumatic pump such that steady flow rate can be provided. Green laser (532 nm, 10 mW, OP Mount Inc., Taiwan) which was coupled into the optic fiber through an lOX objective and the optical signals were collected using an APD module (avalanche photodiode, C5460-0 I, Hamamatsu, Japan). A data acquisition card (PCI-6024E, National Instruments, USA) was used to collected the optical signals from the vibrating optic

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fiber. Meanwhile, high speed digital camera (1200 fps, EX-F1, Casio, Japan) was adopted to simultaneously record the vibration motion of the optic fiber.

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'.

Micro Fig 5: Scheme illustration of the experimental setup for evaluating the developedflow sensor.

RESULTS AND DISSCUSSION Figure 6 shows the measured optical signals in

frequency domain result of measuring water sample with the flow rate of 6 mUmin. Note that the sampling frequency for acquiring the optical signal was set at 100 kHz. The inset of Fig. 6 shows the time domain signals (raw optical responses) collected by the APD module. The frequency domain data was transferred from the raw signals (time domain signals) using Fourier transformation. Results show that significant vibration frequencies were resolved after the FFT computation. In this case, the frequency spectrum comprised three characteristic peaks at 93.3 Hz (first order), 186.3 Hz (second order) and 280.5 Hz (third order). In addition, there was no noise signal was resolved after the FFT computation.

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Detected flow rate: 6ml/min

Detected liqUid: D.I.water

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Frequency (Hz) Fig. 6: Measured optical signals in frequency domain while the inset presenting the raw signals (time domain) at 1 second.

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Fig 7: The relationship of Reynolds numbers (flow rate) and the 1'1 order vibration frequency for DJ water.

Figure 7 presents the results of detecting water with various flow rates. The data points present the first vibration mode of the optic fiber under various flow rates. Note that the dynamic range for measuring the flow rate of water sample is from the Reynolds number of 250 to 1250, corresponding to the flow rate of 3 mUmin to 15 mUmin. Although the detectable range for flow rate sensing is not wide, the sensitivity is high

of up to around 40 IJU(min'Hz)

800 13.2 kHz Air flow rate: 1000 ml/min 700 :;-600 �500 CIJ '0 ::::I 400 :-= Q.300 E c( 200

15

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Detection time: 20sec Sampling rate: 100kHz

26.58 kHz 39.42kHz

1�=097251 00 2 00 400 600 800 1000 Air flow rate (ml/min)

50

Fig 8: (up) The frequency domain signal for measuring air flow at a flow rate of 1000 mLimin. (bottom) The relationship of air flow rate and the 1"1 order vibration frequency.

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Figure 8 shows the experimental results for the results of detecting an ultralow viscous flow of air. Results show that the frequency spectrum comprised three characteristic values at 13.2 kHz (first order), 26.5 kHz (second order), and 39.4 kHz (third order) while the air flow with the flow rate of 1000 mUmin. Figure 9 presents the results for measuring fluids with different viscosity. The flow rate for this test was set at 4.5 mUmin and only the first vibration frequency for each case was used to plot this figure. The developed sensor has successfully detected various fluids including air (0.0148 cP), acetone (0.306 cP), methanol (0.544 cP), or water (1.000 cP), and ethanol (1.200 cP). Results indicate that the measured frequencies of the flow­induced vibration can be a unique index for detecting viscosity of flow, especially for the ultra-small viscosity samples.

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=0 . 9989 :::l -4Air g 102 -3Acetone II... �Methanol LL t:, D.l.water

10.2 10-1 Viscosity (cP)

Fig. 9: The comparison of different fluid viscosities measured by the proposedflow sensor.

CONCLUSIONS This work has successfully developed a

microfluidic flow sensor utilizing the principle of flow induced vibration on an optical cantilever. The developed flow sensor provided a simple and straight forward way to detect flow rate and viscosity using flow-induced vibration phenomena. Results showed that the sensitivity for measuring the flow rate was up to 4.5 mUmin. The detectable viscosity range could be from ultra-low viscous fluid of gas to medium viscous fluid of water. The flow sensor developed in this study provided a simple and low-cost way to resolved fluids with various flow rates and viscosities.

ACKNOWLEDGMENT The authors would like to thank the financial

supports from National Science Council of Taiwan.

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[4]0. Berberig, et aI., "The Prandtl micro flow sensor (PMFS): a novel silicon diaphragm capacitive sensor for flow-velocity measurement," Sensors and

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[6] Y. H. Wang, et aI., "MEMS-based gas flow sensors," Microfluidics and Nanofluidics, vol. 6, pp. 333-346, 2009.

[7] L. M. Fu, et al., "MEMS-based gas flow sensors," Microfluidics and Nanofluidics, vol. 6, pp. 333-346, 2009.

[8] C. Harrison, et aI., "MEMS sensors for density­viscosity sensing in a low-flow microfluidic environment," Sensors and Actuators a-Physical, vol. 141, pp. 266-275, 2008.

[9] S. Taneda, "Waving Motions of Flags," Journal Of

The Physical Society oOf Japan, vol. 24, 1968. [10] V. Lien and F. Vollmer, "Microfluidic flow

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[11] C.-K. Lin, "Flapping Motion Of a Planar Jet Impinging on a V-shaped plate," Journal of Aircraf, vol. 30, p. 6, 1993.

[12] S. Kim, et aI., "Fluidic applications for atomic force microscopy (AFM) with microcantilever sensors," Experiments in Fluids, vol. 48, pp. 721-736, 2010.

[13] C. H. Lin, et al., "A fast prototyping process for fabrication of micro fluidic systems on soda-lime glass," Journal of Micromechanics and Microengineering, vol. 11, pp. 726-732, 2001.

CONTACT Che-Hsin Lin, Department of Mechanical and Electro­mechanical Engineering, National Sun Yat-sen University, 804 Kaohsiung, Taiwan, ROC; Tel : +886-7-5252000-4240; E-mail: chehsin@mail.nsysu.edu.tw

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