Simultaneous Measurement of Velocity and Dissolved Oxygen Concentration Field in Microchannel using Oxygen Sensitive Particles

10TH INTERNATIONAL SYMPOSIUM ON PARTICLE IMAGE VELOCIMETRY – PIV13 Delft, The Netherlands, July 2-4, 2013

Simultaneous Measurement of Velocity and Dissolved Oxygen

Concentration Field in Microchannel using Oxygen Sensitive Particles

Hyun Dong Kim1, Seung Jae Yi1 and Kyung Chun Kim1

1

School of Mechanical Engineering, Pusan National University, Busan, Korea kckim@pusan.ac.kr

ABSTRACT

This paper reports a technique for measuring the velocity and dissolved oxygen (DO) concentration fields simultaneously in a micro-scale water flow using oxygen sensitive particles (OSParticles) and a conventional micro particle image velocimetry (μ-PIV) method. The OSParticles were fabricated using a dispersion polymerization method by synthesizing platinum (II) octaethyporphrin (PtOEP) with polystyrene (PS), and used as tracer particles and oxygen sensors. An ultra violet light-emitting diode (UV LED) with a wavelength of 385 nm was used as the excitation light source, and phosphorescence images of OSParticles were captured on a CMOS high speed camera. The interrogation window concept was used to measure the DO concentration in water from the dispersed phosphorescence intensity distribution of OSParticles. The Stern-Volmer equations in the interrogation windows were obtained from in-situ calibration. Water containing OSParticles with DO values of 0% and 100% were injected into a Y-shaped microchannel using a double loading syringe pump. The velocity and DO concentration field over the entire channel area were quantified

1. Introduction

Dissolved oxygen (DO) is one of the essential substrates in aerobic microbial processes, such as fermentation, cultivation of microorganisms and production of industrial chemicals. In the micro-bioreactor technology, it is essential to supply sufficient oxygen in many bioreaction processes due to the low solubility of oxygen in culture broths (aqueous solutions), which is worsened by laminar flow and difficult mixing circumstances [1-4].

Conventionally, the DO is measured using electro-chemical sensors, such as polarographic and galvanic sensors, which measure the change in current generated by oxidation and de-oxidation. In recent years, an ultra-microelectrode array (UMEA) and optode sensor technique measuring the fluorescence intensity, which varies with the DOC, were developed to measure the DOC on the micro scale. On the other hand, electro-chemical sensors have poor accuracy in low DOC and an optode sensor can only measure the concentration at a particular point. Therefore, they are unsuitable for measuring the DOC field and diffusion coefficient. To overcome these drawbacks, the laser induced fluorescence (LIF) method using an oxygen indicator molecule was studied [5-7].

Particle-based oxygen concentration monitoring or pressure measurement techniques have been evaluated by some research groups. Koo et al. (2004) measured the change in oxygen concentration near C6 glioma cells using a PEBBLE (Photonic Explorer for Biomedical use with Biologically Localized Embedding) sensor fabricated using ruthenium complex and silica nanoparticles[8]. Abe et al. (2004) developed the PIV-PSP hybrid system using pressure sensitive particles (PSParticles), which were micro-balloons made from silicon dioxide coated with a ruthenium complex, to obtain the oxygen concentration field and velocity field around a nitrogen gas jet. Although this experiment provided proof of concept, they could only measure the oxygen concentration over a limited dynamic range (0 ~ 1 %) [9]. Kimura et al. (2006, 2010) developed dual luminophore polystyrene microspheres (OSBeads) exhibiting both oxygen-sensitive platinum porphyrin luminescence and pressure-insensitive silicon porphyrin luminescence, and measured the velocity and pressure field [10, 11].

On the other hand, DOC field measurements in liquid phase flow with oxygen quenchable particles have never been attempted. Considering that the oxygen transfer phenomenon is strongly related to the internal flow characteristics in some micro-bioreactor systems, it is important to obtain information on the DOC and velocity field simultaneously for the efficient design and analysis of micro-systems. This paper discusses the simultaneous measurement of the velocity and DOC fields in a micro-scale water flow using oxygen sensitive functional particles.

2. Experimental setup for simultaneous measurement

Simultaneous measurements of the DOC and velocity fields in the Y-shaped microchannel were taken using the experimental setup shown in Fig. 1. The microchannel was fabricated using micro electromechanical systems (MEMS) technologies (photolithography, reactive ion etching, and anodic bonding). The microchannel was made from glass to

Figure 1 Experimental setup for simultaneous measurements of the DOC and velocity field in the microchannel prevent oxygen permeation into the microchannel wall and to visualize oxygen diffusion and flow field. The inlet and outlet ports were connected to NanoPort (UPCHURCH SCIENTIFIC) and sealed with epoxy. The microchannel consisted of two inlet branches (200 μm width) and one outlet channel (400 μm width) with a channel depth of 75 μm. The microchannel was loaded on the microscope (Olympus BX51) and a 385 nm UV LED was attached to the microscope to excite the OSP in the water sample. The image data was captured on a CMOS high speed camera (Photron, Fastcam SA1.1) with a 20x objective lens (LUCPlan FL N, N/A=0.45, Olympus) at 250 frames per second. The luminescence from the particle was filtered through a 590 nm long-pass optical filter. The temperature of the DI water sample was maintained at 23 ± 0.2℃ throughout the experiment to minimize the temperature effect on the phosphorescence intensity.

3. Calibration procedure

A series of in-situ calibration steps were performed using the same experimental setup with the same flow rate prior to the main experiment to ensure accuracy of the measurements and obtain quantitative results. Several different DOC water samples (0 %, 9 %, 25 %, 49 %, 61 %, 74 %, and 87 %) were sucked into the Y-shaped microchannel to acquire the corresponding intensities. Sodium sulfite (Na2SO3, Sigma Aldrich, USA) was used to change the DOC. The DOC in the water sample can be contaminated easily by the oxygen in air if the sample is opened to the atmosphere during a sample change. Therefore, Teflon tubes connected to two branch channels were placed into the sample water in a beaker first, and the sample water was then sucked through a tube connected to the outlet microchannel using a double-loading syringe pump (KDS270, KD Scientific, USA).

The calibration and main experiment was performed at a water flow rate of 1 µℓ/min. The corresponding Reynolds number based on the hydraulic diameter was 0.07. During calibration, 1,000 images of the microchannel with OSP contained in the water flow were acquired for each DOC sample. The syringe pump was kept switched on while recording the image data. The effect of a temperature difference on the phosphorescence intensity was ignored because calibration was always carried out at a constant room temperature and the time required for the experiment was just a few seconds.

The relationship between the fluorescence intensity and oxygen concentration can be described by the Stern-Volmer equation as follows: ] [ 1 0 _{K DO} I I sv   (1) where KSV is the Stern-Volmer constant, I0 and I are the fluorescence in the absence and presence of oxygen, respectively,

and [DO] is the DOC.

Fig. 2 presents a flow chart of the calibration procedure to obtain the Stern-Volmer plots of the phosphorescence intensity of the OSP and DOC, and Fig. 3 presents raw images of the PtOEP-doped OSP captured for the in-situ calibration in the microchannel along with its ensemble-averaged phosphorescence intensity distribution using 1000 images at three different DOC levels. A difference in intensity was clearly observed with the naked eye between DOC levels of 0 % and 40 %. On the other hand, the intensity difference was not appeared as discernible between DOC values of 40 % and 75 %. Owing to the non-uniform distribution of the excitation light intensity by UV-LED illumination via complex microscopic optics, the corresponding ensemble-averaged phosphorescence intensity showed a non-uniform distribution, even though the DOC level was constant in the entire microchannel. With increasing DOC

Figure 2 Calibration procedure for the phosphorescent intensity and DOC

(a)

(b)

(c)

Figure 3 Raw images of the OSP in the microchannel and its ensemble-averaged intensity distribution at different DOCs (a) 0% (b) 40% (c) 87%

level, the overall intensity decreased and the locations of the high intensity region appeared consistently at the same position in the microchannel for all calibration cases.

4. Result and discuss

Fig. 4 shows a raw image of OSP obtained from the main experiment. Water with a 100% DOC was injected in the upper branch, and water with a 0% DOC was injected in the lower branch. The intensity of the OSP in the region of 0 % DOC water flow was much higher than that of the 100 % DOC water flow. From the merging point of the Y-shaped microchannels, the diffusion layer should be developed at the interface between the two different DOC water streams. The diffusion layer was difficult to recognize by the naked eye because of the non-uniform distribution of illumination and the dispersed nature of the OSP.

Surprisingly, the DO diffusion process could be recognized in the instantaneous and ensemble-averaged DOC field, as shown in Fig. 5 by converting the intensity distribution to a DOC using the map of the Stern-Volmer constants yielded from the in-situ calibration. An increase in diffusion layer thickness downstream from the merging point of the Y-shape microchannel was observed. In the instantaneous DOC field, local fluctuations were observed in the DOC due mainly to the non-uniform distribution of OSP, as shown in Fig. 5 (a). For example, a local spot cannot be avoided if the number of particles in a particular interrogation window is much higher than that of the neighbors. Nevertheless, ensemble averaging can filter out such peaks, as shown in Fig. 5 (b).

With the phosphorescence images of the OSP, the velocity field was calculated from the 32 x 32 pixel interrogation window using the conventional two-frame cross correlation algorithm using the time-resolved Micro-PIV technique. The DOC can be obtained where the OSP are placed in a water flow. Fig. 6 shows the instantaneous and ensemble-averaged velocity field extracted from the OSP images in the Y-shaped microchannel. The velocity field was obtained successfully despite the intensity variations due to a difference in DOC between the upper and lower branches. Because the Reynolds number is 0.7, the measured velocity field in the microchannel showed a parabolic velocity profile, which is typical in laminar 2D channel flow, despite the DOC being different. The maximum velocity was approximately 3.5 mm/s. Although the water samples were injected at the same flow rate, there was a slight difference in velocity at the two branches of the Y-shaped microchannel, as shown in Fig. 6. As a result, the centerline of the DO diffusion layer was inclined toward the lower wall along the streamwise direction.

The correlation depth, Zcorr, of the volume illuminated two dimensional micro PIV has been described as a function of the

focusing characteristics of the recording optics and lens properties [12].





where ε is the threshold weighting function value, dp is the particle diameter, M and NA are magnification and numerical

Figure 4 Phosphorescent image of the OSP when D.I. water with a 0% DOC and 100% DOC meet in a Y-shaped microchannel

(a)

(b)

Figure 5 Distribution of the DOC calculated from the Stern-Volmer constant and instantaneous phosphorescence intensity (a) instantaneous DOC field (b) ensemble -averaged DOC field.

(a)

(b)

Figure 6 Velocity vector field extracted from the phosphorescence image of a particle (a) instantaneous velocity field (b) ensemble-averaged velocity field

of immersion medium. Using eq. (2) for the micro PIV experiment with NA = 0.45, ε = 0.01, n0 = 1, dp = 2.69 μm, λ =

0.647 μm and M = 20, Zcorr was found to be approximately 21 μm. As discussed in the calibration procedure, the water

samples with the OSP were sucked through the tubes immersed in the water samples to prevent oxygen contamination during the tubing processes. The slight differences in water sample heights in the beakers and tube lengths might cause a slight difference in pressure at the two inlet branch ports followed by a small difference in velocity profile.

5. Conclusion

A simultaneous measurement technique of velocity and DOC fields using OSP and conventional simple μ-PIV in a microchannel was developed successfully. In-situ whole field calibration has performed successfully using 1,000 images of particles under a range of DOCs in the microchannel using the intensity based method. Qualitative visualization of two different phosphorescence intensities due to the different DOC water streams was clearly observed. Using the Stern-Volmer constants obtained from the in-situ calibration for each calibration window, instantaneous DOC field and ensemble-averaged DOC field was measured successfully within a 0.5 % random error.

Using the OSP as PIV tracers, the instantaneous and ensemble-averaged velocity field in the microchannel were measured simultaneously with the DOC field based on the conventional two frames cross correlation technique. The maximum velocity was approximately 3.5 mm/s under this experimental condition and a parabolic velocity profile was obtained. Although the flow rate of D.I. water in each branch channels of the Y-shape microchannel was the same, there is a slight velocity difference at the merging point. As a result, the axis of the DOC diffusion layer was inclined toward the lower wall along the streamwise direction. The uniformity of the diameters and phosphorescence characteristics of the OSP are the prime requirements for precise measurements of the DOC in microfluidics. Moreover, the non-uniformity of UV illumination over the measurement field of view should be minimized.

ACKNOWLEDGEMENT

This study was supported financially by the National Research Foundation (NRF) of Korea through a grant funded by the Korea government (MEST) (no. 2011-0030663 and no. 2011-0000199). The authors gratefully acknowledge this support. REFERENCES

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