A novel heating area design of temperature-jump microfluidic chip for synchrotron radiation solution X-ray scattering

A novel heating area design of temperature-jump microfluidic chip for synchrotron radiation solution X-ray scattering 1 2 first-author 1 corresp Corresponding author. E-mail address: bianfenggang@sinap.ac.cn Corresponding author. E-mail address: bianfenggang@sinap.ac.cn bianfenggang@sinap.ac.cn 1 corresp Corresponding author. E-mail address: wangjie@sinap.ac.cn Corresponding author. E-mail address: wangjie@sinap.ac.cn wangjie@sinap.ac.cn Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Zhangjiang Campus, Shanghai 201204, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Zhangjiang Campus, Shanghai 201204, China University of Chinese Academy of Sciences, Beijing 100049, China University of Chinese Academy of Sciences, Beijing 100049, China In this paper, we present a novel design scheme of temperature-jump (T-jump) area for microfluidic device. Numerical simulation and experimental research of thermal characteristics of the solution in microchannels is completed. Numerical simulation of the temperature-jump microchannel is analyzed to study the heat transfer characteristics by comparing performance of three proposed configurations. Calculation of the power requirement is discussed in the dimensional design of microheater. Temperature sensitive fluorescent dye is applied to investigate the temperature field of microchannel heated by a designed microheater. It is found that the T-jump microfluidic can provide rapid heating for solutions with strong convection heat transfer ability. Synchrotron radiation Solution X-ray scattering Microfluidic chip Temperature-jump 1. Introduction 1 The temperature-jump (T-jump) technique is of great significance for elucidating reaction mechanisms. Perturbed by a rapid temperature change, the reaction system adjusts to a new equilibrium. During the equilibrium shift, the order of reaction steps and the rate of each step can be inferred from intensity of the light absorbed or fluoresced, or from the electrical conductivity [ 1 1 ] . Synchrotron radiation facility, which generated extremely strong X-ray beams, brought dramatic improvement in the kinetic observation by reducing the measurement time [ 2 2 , 3 3 ] . Several approaches can be used to induce the T-jump. Joule heating produced by a fast electrical discharge of capacitor is a common method to rapidly increase the temperature of electrolyte solution [ 4 4 - 6 6 4-6 ] . Non-aqueous solvents, however, are ruled out due to the limit that the sample solution must be of high electrical conductivity. Without strict requirements, microwave heating can increase the temperature of solution which is held in cell mounted in a waveguide cavity [ 7 7 , 8 8 ] . Also, laser technique is widely applied to generate a sudden rise in temperature of solvent by vibration absorption of a near-infrared laser pulse [ 9 9 - 12 12 9-12 ] . These heating apparatus are usually complex and inconvenient when they are coupled with an endstations of synchrotron radiation. Simplifying both device structure and operation are therefore of paramount importance for combing T-jump technique with synchrotron radiation. We have designed and constructed a novel microfluidic thermal system which enables rapid temperature change and is capable of fitting in the beam lines of synchrotron radiation facilities [ 13 13 , 14 14 ] .The design and experimental results of the T-jump area of microfluidic chip will be elaborated in this paper. In the following sections, the design and fabrication of microchannel and heater, experimental method and measurement results are presented. For further understanding about the thermal characterization of the microfluidic chip, a computational fluid dynamics software ANSYS CFX is used to analyze heat transfer performance of microchannel with different configurations. The temperature field in microchannel is characterized by measuring the temperature dependence on fluorescence intensity of fluorescent dye [ 15 15 - 18 18 15-18 ] . 2. Design and fabrication 1 2.1 The microfluidic chip 2 The microfluidic chip is shown schematically in Fig.1 Fig.1 . It consists of the reservoir channel, T-jump channel and sample-detection channel. The microchannels were fabricated in a 2 mm-thick PMMA substrate by micromill technique. The reservoir channel, 1 mm ( w )× 0.8 mm( h ), is in an invariable temperature environment by an external thermoelectric cooler. To ensure a constant flow rate and heating efficiency, the T-jump channel is of a trapezoid- rectangle-trapezoid shape of 11 mm( w )×0.1 mm( h ). The sample-detection channel is a 1 mm( w )×1 mm( h )×30 mm( l ) groove, being sealed on both sides by 20 μm-thick Kapton film that allows X-ray beams to pass through. Adhered to the substrate is a 1 mm-thick PMMA cover plate, with two holes for connecting the channel ends to Telflon tubing and a pumping system (Hamilton, Switzerland) that provides a steady flow rate for the microfluidics. 2.2 The T-jump channel 2 The area where the T-jump channel connects the reservoir channel forms a sudden dimensional change, hence enlarging the area of active heating. Different configurations of the T-jump channel were proposed ( Fig.2 Fig.2 ). Three-dimensional steady state analysis models were built to evaluate performance of the three types of channels, and their heat transfer characteristics were analyzed by fluid dynamics analysis software ANSYS CFX. The Navier-Stokes and continuity equations were solved by coupling with heat transfer equations under the following boundary conditions: (1) constant mass flow rate at microchannel inlet and 1atm pressure at microchannel outlet; (2) constant front wall heat flux and adiabatic wall for others; (3) specific temperature of inlet fluid; (4) no-slip condition at the walls; and (5) incompressible fluid. CFD analysis of the T-jump channels was investigated using water as working fluid. The flow characteristic was evaluated by the Reynolds number ( Re ) calculated by Eq.(1) <math display="block" id="M1"><mrow><mi>R</mi><mi>e</mi><mo>=</mo><mi>ρ</mi><mi>v</mi><mi>D</mi><mo>/</mo><mi>μ</mi><mo>,</mo></mrow></math> where ρ is the fluid density, v is the average fluid velocity, μ is the fluid viscosity and D is the hydraulic diameter defined as <math display="block" id="M2"><mrow><mi>D</mi><mo>=</mo><mtext> </mtext><mn>2</mn><mi>a</mi><mi>b</mi><mo>/</mo><mo stretchy="false">(</mo><mi>a</mi><mo>+</mo><mi>b</mi><mo stretchy="false">)</mo><mo>,</mo></mrow></math> where a is the channel width and b is the channel height. In CFD analysis, the flow behavior is in laminar regime with Reynolds number below 2000. For the range of flow rate used in this work, Re is not larger than 20. The heat transfer characteristic was analyzed the Nusselt number ( Nu ) as: <math display="block" id="M3"><mrow><mi>N</mi><mi>u</mi><mo>=</mo><mi>h</mi><mi>D</mi><mo>/</mo><mi>λ</mi><mo>,</mo></mrow></math> where h is the heat transfer coefficient and λ is the thermal conductivity of fluid. To compare performance of the three microchannel configurations, CFD analysis was conducted under the inlet flow rate of 10 μL/s at 35℃ and heat flux of 70 kW/m 2 . Under these conditions, the heat is transferred to the fluid by forced convection heat transfer. Fig.3 Fig.3 shows the Nusselt number as a function of inlet flow temperature in rectangle, semiellipse and trapezoid T-jump channels. Comparing the three sets of data, one sees that Nu of the trapezoid channel is the highest of all the channels. Although the Nu of trapezoid channel decreases slightly with increasing temperature, it keeps a strong micro-scale effect. The results of simulations at different heat fluxes and flow rates are similar. This shows that the channel geometry has great effect on heat transfer characteristics of microchannels. That the trapezoid channel performs the best in heat transfer can be attributed to the constantly change of cross-sectional area. Due to the steady contraction of cross-sectional area, the flow in trapezoid channel experiences thermally developing laminar flow with significant entrance effect. In addition, the structure leads to some perturbation that aids to improve local heat transfer coefficient in the laminar flow. 2.3 The microheater 2 Considering thermal efficiency and non-disposable recyclability, a Pt film heater line is used. It is independent of the microfluidic chip, fastened on the PMMA substrate. When an electrical current passes through the resistance heater, the heat generated is transferred to the fluid in the microchannel. In this case, two main points, power generation and electromigration, should be taken into account in designing the dimensions of the heater [ 19 19 ] . The amount of heat, Q , transferred to the microfluidics can be estimated by <math display="block" id="M4"><mrow><mi>Q</mi><mo>=</mo><mi>C</mi><mi>m</mi><mi>Δ</mi><mi>T</mi><mo>,</mo></mrow></math> where C is the specific heat capacity, m is the mass flow, and Δ T is the temperature change of fluid. To be specific, it can also be expressed in terms of power requirement: <math display="block" id="M5"><mrow><mi>P</mi><mi>η</mi><mo>=</mo><mi>C</mi><msub><mi>m</mi><mi>r</mi></msub><mi>Δ</mi><mi>T</mi><mo>,</mo></mrow></math> where P <math id="M6"><mo> </mo></math> is the power consumption of heater, η is the thermal efficiency, m r is the mass flow rate of microfluidics. The resistance of the heater, R H , is given by <math display="block" id="M7"><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>=</mo><msup><mi>V</mi><mn>2</mn></msup><mo>/</mo><mi>P</mi><mo>,</mo></mrow></math> where V is the supply voltage. The dimensions of the Pt film heater can be determined by Eq.(7) <math display="block" id="M8"><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>=</mo><msub><mi>ρ</mi><mi>f</mi></msub><mi>l</mi><mo>/</mo><mrow><mo>(</mo><mrow><mi>w</mi><mi>t</mi></mrow><mo>)</mo></mrow><mo>,</mo></mrow></math> where l , w and t are length, width and thickness of the Pt film heater, respectively, and ρ f is resistivity of the Pt film which is different from bulk Pt owing to influence of mean free path of electrons. In this study, the power source for heater was generated by a digital DC power supply (0–60 V), the thermal efficiency η was estimated at 20% conservatively, the Pt film was 200 nm thick, and the mass flow rate of fluid was 10 mg/s. Resistivity of the Pt film was 2.8×10 −7 Ω·m, and specific heat capacity of water is 4200 J·kg −1 ·K −1 . Finally, the temperature increment of the fluid are presented as a function of width-length ratio of the Pt film heater line: <math display="block" id="M9"><mrow><mi>Δ</mi><mi>T</mi><mo>=</mo><mi>α</mi><mi>w</mi><mo>/</mo><mi>l</mi><mo>,</mo></mrow></math> where α is a coefficient determined by <math display="block" id="M10"><mrow><mi>α</mi><mo>=</mo><msup><mi>V</mi><mn>2</mn></msup><mi>η</mi><mi>t</mi><mo>/</mo><mrow><mo>(</mo><mrow><mi>C</mi><msub><mi>m</mi><mi>r</mi></msub><msub><mi>ρ</mi><mi>f</mi></msub></mrow><mo>)</mo></mrow><mtext> </mtext><mo>.</mo></mrow></math> Fig.4 Fig.4 shows temperature increment of the passing flow as a function of the width-length ratio at different applied voltages. The temperature increment rises with the width-length ratio and the applied voltage. The range of temperature increment of the fluid in the channel at a certain range of applied voltage can be decided by the width-length ratio to optimize the heater performance. We chose the width-length ratio was 0.0052 in this design. Another issue is the transport of metallic atoms in conductor, which is driven by the interaction with electrons of passing current. This electromigration decreases reliability of thin-Pt-film electronic component at a current density up to 10 7 A/cm 2 [ 21 21 ] , which shall be the limit of current density of the heater line. Therefore, a zigzag heater line, spaced 100 μm apart with width of 200 μm, was designed and located at the trapezoid T-jump channel ( Fig.5 Fig.5 ). With the use of lift-off technique, 50 nm Cr and 200 nm-thick thin Pt film was deposited on a 1mm-thick glass substrate. The room-temperature resistance of the heater, including lead wires, is roughly 300 Ω. 3. Results and Discussion 1 3.1 Fluorescence measurement 2 Temperature-dependent fluorescence dye, rhodamine B, was used in temperature measurements. The fluorescence intensity of rhodamine B was measured and converted to temperature value according to the calibrated intensity- temperature curve. Prior to the measurement, rhodamine B (Sigma Aldrich, USA) was dissolved into 15 mmol/L Na 2 CO 3 buffer. Filtered with a 0.2 μm syringe filter, the final concentration of rhodamine B was 87 <math id="M11"><mtext>μ</mtext></math> mol/L. Fluorescence imaging was performed using a CCD camera (DP71, Olympus, Japan) and commercial fluorescence microscope (BX51, Olympus, Japan) equipped with a standard green fluorescence filter set. The exposure time was set to a constant manually and the white/black balance control was remained off throughout measurements. In order to correlate the fluorescence intensity with temperature, a microfluidic chip integrated with a Pt100 thermal sensor (Heraeus, Germany) along the microchannel was fabricated. The microfluidic chip was fixed onto a silicone rubber thermostat (homemade) for a constant temperature environment of rhodamine B solution which was injected into the microchannel. The fluorescence images of the channel filled with dye solution were obtained at different temperatures of 25°C–80°C. To eliminate effects of background fluorescence inhomogeneity, background and room temperature images were acquired preferentially during the measurement. For each pixel, the intensity value was subtracted with the background, and normalized with the intensity value at room temperature. 3.2 Fluorescence experimental results 2 As shown in Fig.6 Fig.6 , the correlation between intensity and temperature was established. The normalized intensity at each temperature was averaged from multi-measurement. The data can be fitted by Eq.(10), with R 2 =0.998: <math display="block" id="M12"><mrow><mi>T</mi><mo>=</mo><mtext> </mtext><mn>120.17</mn><mo>−</mo><mn>238.01</mn><mi>I</mi><mo>+</mo><mn>232.77</mn><msup><mi>I</mi><mn>2</mn></msup><mo>−</mo><mn>88.16</mn><msup><mi>I</mi><mn>3</mn></msup><mo>,</mo></mrow></math> where T is the temperature of rhodamine B solution, and I is the normalized fluorescence intensity of rhodamine B solution after background subtraction. The calibrated intensity-temperature relationship was used to explore thermal changes of fluid in the T-jump microchannel. A flow of the rhodamine B solution was continuously injected into the microfluidic chip and dissipated the heat energy provided by external Pt heater as described in Section 2.3. By changing the voltage applied on the Pt heater, various temperature increments of the fluid at each flow rate were obtained from the fluorescent images at the outlet of T-jump microchannel according to the intensity-temperature calibration curve. The fluorescence inside the microchannel was measured at flow rates of 5 and 10 μL/s at 25°C of fluid in the reservoir channel. The temperature change, Δ T , was determined as a function of power input. Fig.7 Fig.7 shows the results at power input of 0.22–2.1 W at 5 and 10 μL/s, with Δ T = 4°C–30°C at the outlet of T-jump microchannel. As can be seen, higher heater power generated larger temperature increment. It should be noted that from the flow rates of 5 μL/s to 10 μL/s, the temperature increment is reduced almost by half at the same power input. Higher flow rate resulted in smaller T-jump owing to the reduction of time in which the fluid contacted with Pt heater. 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DOI: 10.1088/0957-4484/24/50/505708 http://doi.org/10.1088/0957-4484/24/50/505708 In situ TEM and STEM studies of reversible electromigration in thin palladium- platinum bridges 10.1007/s41365-016-0083-9 This work was supported by the National Basic Research Program of China (No. 2011CB911104) . 2016-01-20 2016-02-29 2016-03-02 2016-07-11 2016-08-20 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2016 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-016-0083-9 http://dx.doi.org/10.1007/s41365-016-0083-9 . 2016 Nuclear Science and Techniques 04 27