Microscale Laser-Induced Fluorescence Method for Non-Intrusive Temperature Measurement

Transcription

1 Purdue University Purdue e-pubs Birck and NCN Publications Birck Nanotechnology Center Microscale Laser-Induced Fluorescence Method for Non-Intrusive Temperature Measurement Pramod Chamarthy Birck Nanotechnology Center, Purdue University Steve T. Wereley Birck Nanotechnology Center, Purdue University, Suresh V. Garimella Birck Nanotechnology Center, Purdue University, Follow this and additional works at: Part of the Nanoscience and Nanotechnology Commons Chamarthy, Pramod; Wereley, Steve T.; and Garimella, Suresh V., "Microscale Laser-Induced Fluorescence Method for Non-Intrusive Temperature Measurement" (27). Birck and NCN Publications. Paper This document has been made available through Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information.

2 Proceedings of IMECE27 27 ASME International Mechanical Engineering Congress and Exposition November 11-15, 27, Seattle, Washington, USA IMECE MICROSCALE LASER-INDUCED FLUORESCENCE METHOD FOR NON-INTRUSIVE TEMPERATURE MEASUREMENT Pramod Chamarthy, Steven T. Wereley and Suresh V. Garimella School of Mechanical Engineering and Birck Nanotechnology Center, Purdue University West Lafayette, IN USA (pramodc, wereley, ABSTRACT Ratiometric Laser Induced Fluorescence (LIF) Thermometry is applied for temperature measurements in a T junction, using microscale visualization methods. Rhodamine B (RhB) and Rhodamine 11 (Rh11) are used as the temperature-dependent and temperature-independent dye, respectively. The temperature responses of the two dyes were carefully measured for different concentrations. A novel normalization procedure for the calibration curve is proposed to render the technique system-independent. The mixing plane between a hot and a cold fluid stream for three different temperatures and three different flow rate ratios is visualized using 4X and 1X magnifications. INTRODUCTION Thermal micro-devices such as microchannel heat sinks, µtas, PCR and thermal inkjet printer heads are gaining popularity due to the speed, efficiency and portability that they offer. Lab-on-a-chip devices, which miniaturize and integrate various chemical processes such as mixing, reaction, separation and detection, have revolutionized many aspects of analytical chemistry and biochemistry. It has been demonstrated that the control of fluid temperatures in such devices is essential for enzyme-activated reactions as well as polymerase chain reaction (PCR) amplification of DNA. Microfluidic-based devices are also gaining popularity in electronics cooling applications due to the high heat fluxes achieved by single and two-phase liquid cooling. Hence, it is necessary to develop new techniques which can measure fluid temperatures at such small length scales. Commonly used temperature measurement techniques often prove inadequate for microscale applications. Thermocouples are the most widely used temperature sensors due to their simplicity, reliability, ease of use, and the wide range of temperatures over which they can be used. Even the smaller commercially available thermocouples can still be hundreds of microns in size which is of the same order of magnitude as a microchannel. These thermocouples can be embedded along the microchannel base to measure average heat transfer characteristics [1, 2]; alternatively, micron-scale thermocouples can be fabricated along the channel surface to function as a micro-thermal sensor array [3-5]. Resistance temperature detectors (RTDs) have also been applied to microscale flows by fabricating the resistor element onto the channel surface, the geometry and material of which is decided based on the required accuracy and the temperature. Resistance thermometry has been used to study thermocapillary pumping, transient response during bubble formation and heat transfer characteristics in microchannels [6-8]. The main disadvantage of microfabricated thermocouples and RTDs is that they require precisely controlled fabrication. Moreover the RTDs need precise calibration as the the resistances of the sensor, lead wire and the contact pads can be dependent on temperature. Thermochromic liquid crystals have also been used as a temperature probe due to their unique optical properties which are dependent on temperature within a certain range. TLCs are commercially available as pre-fabricated adhesive sheets, sprayable slurries, or as water resistant micro-capsules. TLC slurries and paints used to make surface measurements can achieve a maximum resolution of ~1 µm while encapsulated TLCs range from 1 to15 µm in size. Hohmann et al. [9] used a thin layer of TLC painted on a surface to study the heat transfer characteristics near an evaporating meniscus with a spatial resolution of ~1 µm. Suspensions of encapsulated TLCs have been used for simultaneous velocity and temperature measurements [1, 11] as well as transient temperature measurements [12, 13]. Infrared thermography has been used to study the boiling characteristics in microchannels by measuring the surface temperatures [14-16] and to study the heat transfer at an evaporating meniscus [17]. The inherent limitation of IR thermography is that it can only be used to measure surface 1 Copyright 27 by ASME

3 temperatures. Also an accurate value of the emissivity of the medium is essential to obtaining meaningful measurements. Molecular Tagging Velocimetry has been used to measure velocity [18] by exciting the molecules in a particular region and measuring the deformation of that region as time progresses. It has been observed that the phosphorescence lifetime of the dye is dependent on the temperature of the fluid, and by measuring the rate of decrease in the intensity, the temperature of the fluid [19, 2] can be deduced. Experimental parameters such as the time delay between pulses and phosphorescence integration time can be optimized to obtain an experimental uncertainty of ±.2ºC over a temperature range of 4ºC, where the uncertainty is dependent on the temperature range. Non-uniform illumination adds to the uncertainty and needs to be reduced by precise calibration. Refractive index variations caused due to a spatially varying temperature fields can also create non-uniformity in the light intensity which introduces errors in the measurement. Among the various methods available, laser-induced fluorescence shows great promise as a method capable of making non-intrusive, whole field measurements inside a volume of liquid. Laser Induced Fluorescence (LIF) Thermometry uses the temperature-dependent fluorescence intensity of dyes to measure temperature. LIF has been used to measure the temperature of reacting species or dyes in flames for combustion studies [21-23] and the temperature of dyes suspended in liquids [24-26]. The two-color LIF method [27-29] can be used to overcome the uncertainty introduced due to non-uniformity in the illumination. Lavielle et al. [3] reported a new technique where the necessity of two dyes can be circumvented by using the ratio of fluorescence from the same dye at different spectral frequencies. Coppeta and Rogers [31] analyzed the temperature dependence of fluorescence characteristics of various dyes. In general, Rhodamine B (RhB) is recommended as the temperaturesensitive dye while Rhodamine 11 (Rh11) is chosen as the temperature-insensitive dye. Studies of this technique to date have been conducted in macroscale environments using laser light to illuminate the dye. Kim et al. [32, 33] have demonstrated microscale resolution for the method using a laser light sheet. In typical microfluidic experiments, the entire volume of the channel is illuminated as it is not possible to achieve such a thin sheet of light. Ross et al. [34] have applied the LIF method with a single dye to measure the temperature in a microfluidic system with an uncertainty ranging from ºC. In the present work, the ratiometric LIF method is applied to measure the temperature in a volume-illuminated microfluidic setup. RhB is used as the temperature-sensitive dye and Rh11 is used as the temperature-insensitive dye. A novel normalization procedure for the calibration curve is proposed to render the technique system-independent. The mixing plane between hot and cold fluid streams for three different temperatures and three different flow rate ratios is visualized at 4X and 1X magnifications. THEORY OF LIF THERMOMETRY Laser Induced Fluorescence is a well known phenomenon and has been used in analytical chemistry for many years. The dependence of fluorescence irradiation, I f, on the concentration of the dye, C, can be expressed using the simple equation f C ( ) I = β φi e ε (1) 1 bc where, β C, Φ, I, ε, and b are the collection efficiency, quantum efficiency, incident irradiation, molar absorptivity and absorption path length, respectively. At low concentrations, when less than 5% of the incident radiation is absorbed by the dye, Equation (1) can be simplified to the following expression, I = β φi εbc (2) f C Since LIF uses fluorescence intensity to calculate temperature, factors such as non-uniform illumination, fluctuation of light, non-uniform dye concentration and photobleaching can all affect the measurements. Uncertainty caused due to illumination can be eliminated using the two-color LIF method, where the fluorescence intensity of a temperature insensitive dye is used to normalize intensity of the temperature-sensitive dye. as The ratio of the fluorescence intensity of the dyes is given I I β ε C φ β ε C φ rhb Crhb rhb rhb rhb rh11 Crh11 rh11 rh11 rh11 = (3) It can be seen that Equation (3) is independent of incident irradiation, I, as well as the absorption path length, b. The absorption spectral intensity ratio and the molar absorptivity ratio are nearly temperature-independent while the quantum efficiency ratio is dependent on temperature. If the fluorescence intensity of both the dyes is not dependent on the ph of the solution, the fluorescence intensity ratio is only dependent on temperature for a fixed concentration ratio. Careful calibration is needed to correlate the fluorescence intensity ratio of the dyes to the temperature of the fluid. This method still suffers from the drawback that the concentrations of both the dyes need to be uniform across the measurement plane. A suitable co-solvent can be added to the solution to aid in the dissolution process to achieve a uniform concentration. EXPERIMENTAL SETUP The experiments were conducted in a standard µpiv setup, shown in Figure 1, which consists of an upright Nikon Eclipse (ME6) microscope and an interline transfer Charge Coupled Device (CCD) camera (Roper Scientific Photometrics, CoolSNAP HQ). A Nikon mercury-arc lamp was used as the illumination source. Metamorph imaging software was used to acquire images. Laser grade rhodamine B and rhodamine 11 (Acros Organics) dissolved in DI water were used for all the experiments. The dyes were mixed in 1 ml of methanol before making the solution to increase their solubility in DI water. For the ratiometric LIF thermometry technique, the image of the Rh11 dye is used to normalize the image of the RhB dye. The two images were acquired with the 2 Copyright 27 by ASME

4 help of a filter wheel attached in front of the mercury arc lamp. The imaging software controlled the filter wheel and was capable of acquiring multiple images alternating between the two filters. An excitation filter of λ~542nm is used for RhB dye while an excitation filter of λ~488nm is used for the Rh11 dye. Emitter 61 nm Computer (Post processing) 12 bit CCD Camera (1376 x 14 pixels) set-up reached equilibrium in approximately 1 to 15 minutes, at which time the thermocouple showed a steady temperature reading. For the validation experiments a simple microchannel was fabricated to visualize the mixing of hot and cold streams at a T junction (Figure 3). The microchannel was cut into a double-sided adhesive tape (Adhesives Research Inc.) and sandwiched between two glass slides. Holes were drilled into the top glass slide for the inlet and outlet ports. The tubing for the hot liquid is immersed in a hot water bath to heat the liquid to different temperatures. The temperature in the hot liquid bath is measured using thermocouples and used as the reference temperature. A small amount of heat loss is expected to occur through the length of the tubing (~2 cm) from the hot water bath till the inlet port. Hence the temperature of the liquid at the T junction is expected to be a few degrees cooler than the temperature of the water bath. Filter Cube Inlet cold fluid Inlet - hot fluid Outlet Microscope Objective Exciter 532 nm Hg lamp Width 5 um Depth 2 um Glass slide Double sided Adhesive tape Test piece Glass slide Figure 1: µlif thermometry experimental setup. The calibration experiments were conducted in a 4 µm square glass microchannel (Vitrocom, Inc.) submerged in a well machined into an aluminum block. The glass channel was filled with the dye solution, sealed on both sides and placed in the well which was then filled with DI water and sealed on top with an acrylic plate. The aluminum block was heated using a thermofoil resistance heater (Minco Products, Inc.) and was enclosed in an insulating material. The setup is shown in Figure 2. 6 mm Glass slide Thermocouple Capillary tube Aluminum well Patch heater Insulator Figure 2: Experimental setup for calibration. The temperature of the water bath was measured using a thermocouple which was used as the reference temperature. Measurements were taken at temperature intervals of 2 C ranging from 2 C to 8 C. At each temperature setting, the Figure 3: Experimental setup for mixing plane visualization. RESULTS AND DISCUSSION Figure 4 shows the fluorescence intensity of the dyes for three different concentrations. It can be seen that the temperature dependence of Rhodamine B (Figure 1(b)) is much stronger than that of Rhodamine 11 (Figure 1(a)). Due to the overlap in the spectral characteristics of the two dyes, a small amount of light emitted by the RhB dye might be present in the Rh11 image and vice versa. This does not affect the measurement technique as the image can still be used for normalization. Figure 5(a) shows the intensity ratio (RhB/Rh11) of the two dyes which is typically used as the calibration curve for temperature measurements. It can be seen that the slope of the calibration curve increases as the concentration of the dyes is increased. This slope is also dependent on the illumination intensity as well as the exposure time of the image. Since the entire channel is illuminated in the volume illumination approach, in contrast to that with a light sheet, changing the depth of the channel will also affect the slope. Hence, a new calibration curve needs to be obtained if any of the above parameters is changed. This can be avoided by normalizing the calibration curve by the intensity ratio value at some particular temperature. The fluorescence intensity at room temperature, 24ºC, is arbitrarily picked to normalize the intensity values for all the concentrations. Figure 2 (b) shows the normalized fluorescence intensity ratio which is used as the calibration curve for the visualization experiments. It can be seen that the normalized intensity-ratio curves are 3 Copyright 27 by ASME

5 independent of concentration of the dyes. This calibration curve is used for the temperature measurements discussed below. Intensity Intensity Rh11-1 mg/l Rh11-5 mg/l Rh11-1 mg/l Temperature (ºC) (a) RhB - 1 mg/l RhB - 5 mg/l RhB - 1 mg/l Temperature (ºC) (b) Figure 4: Fluorescence intensities of the dies for different concentrations: (a) Rhodamine 11, and (b) Rhodamine B. Normalizing the intensity ratios makes the technique system-independent and avoids an extra calibration step for each experimental setup. However, it necessitates a knowledge of the intensity ratio at some particular temperature for that experimental setup. If the temperature of the fluid at some location is known, then the intensity ratio at that point can be used for normalization. The feasibility of this method for temperature measurement in microfluidic applications is demonstrated through quantitative visualization of the temperature field in a T junction. In all the images, cold liquid at room temperature is flowing from left to right in the image and hot liquid is being added through the branch and flows to the right. Since the temperature of the cold liquid is known, the intensity ratio of the cold liquid is used to normalize all the images. The temperature of the hot liquid and the flow rate ratios were varied to obtain different mixing profiles. temperature of the water bath is given below each figure. In Figures 6(a)-6(c) and 7(a)-7(c) the flow rate of the cold fluid is 1 ml/min and the flow rate of the hot fluid is.32 ml/min. In Figures 6(d)-6(f) and 7(d)-7(f) the flow rates of the cold fluid and the hot fluid are 1 ml/min. In Figures 6(g)-6(i) and 7(g)- 7(i) the flow rate of the cold fluid is.32 ml/min and the flow rate of the hot fluid is 1 ml/min. Intensity ratio (RhB/Rh11) Normalized intensity ratio RhB & Rh11 1mg/L RhB & Rh11 5mg/L RhB & Rh11 1mg/L Temperature (ºC) (a) RhB & Rh11-1 mg/l RhB & Rh11-5 mg/l RhB & Rh11-1 mg/l Temperature (ºC) (b) Figure 5: (a) Fluorescence intensity ratio of (RhB/Rh11) for different concentrations of the dyes. (b) Normalized fluorescence intensity ratio (RhB/Rh11). It is noted that the temperature of the hot fluid at the T junction would be at least a few degrees less than the temperature of the hot water bath, due to the losses occurring in the tubing between the water bath and the measurement region. This temperature drop would be lower at higher flow rates. This is confirmed by comparing Figures 7(c), 7(f) and 7(i). Since the flow rate of the hot fluid in 7(c) is 3 times smaller than that in 7(f) the temperature of the fluid in 7(c) is considerably less that that in 7(f). The temperatures of the hot fluid in 7(f) and 7(i) are the same as they have the same flow rates. Figures 6 and 7 show the mixing profiles at the T junction for a 4X and 1X magnification, respectively. The 4 Copyright 27 by ASME

6 (a) 31ºC (b) 41ºC (c) 52ºC (d) 3ºC (e) 4ºC (f) 49ºC (g) 31ºC (h) 38ºC (i) 53ºC 2ºC 24ºC 29ºC 35ºC 41ºC 49ºC 58ºC Figure 6: Microscale temperature visualization using the LIF method for a 4X objective. Cold fluid is flowing from left to right and hot fluid is being pumped through the branch. The temperature shown below each image is the inlet temperature of the hot fluid. 5 Copyright 27 by ASME

7 (a) 31ºC (b) 41ºC (c) 52ºC (d) 31ºC (e) 39ºC (f) 51ºC (g) 3ºC (h) 39ºC (i) 51ºC 2ºC 24ºC 29ºC 35ºC 41ºC 49ºC 58ºC Figure 7: Microscale temperature visualization using the LIF method for a 1X objective. Cold liquid is flowing from left to right and hot liquid is being pumped through the branch. The temperature shown below each image is the temperature of the hot fluid. 6 Copyright 27 by ASME

8 CONCLUSIONS The ratiometric LIF method was applied to measure the temperature in a volume-illuminated microfluidic setup. A novel normalization procedure for the calibration curve was proposed to render the technique system-independent. The mixing plane between the hot and cold fluid for three different temperatures and three different flow rate ratios were visualized using 4X and 1X magnifications. Work is currently underway to conduct a detailed uncertainty analysis for this approach. The accuracy of the technique in the horizontal and vertical direction, which depends on the experimental parameters such as magnification, will be explored. REFERENCES 1. 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