Infrared Based Techniques in Microfluidic Applications: History
Please note this is an old version of this entry, which may differ significantly from the current revision.

A body at a temperature around ambient values emits electromagnetic radiation in the infrared (IR) band of the electromagnetic spectrum, i.e., with a wavelength from 700 nm to 1 mm. Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques and can be used in microfluidic applications.

  • thermometry
  • gas microflow
  • microsystem
  • microfluidics

1. Principle of Infrared Imaging

A body at a temperature around ambient values emits electromagnetic radiation in the infrared (IR) band of the electromagnetic spectrum, i.e., with a wavelength from 700 nm to 1 mm. Based on this phenomenon, infrared thermography (IRT) consists in transforming the energy radiated from the body into an electronic signal by means of a radiometer (the infrared sensor of the IR cameras). The signal is then converted into an image that maps the different infrared radiation levels represented in colors or grayscale.
Planck’s law describes the energy distribution from a blackbody at a given temperature as a function of the emission wavelength. However, some bodies may have a radiative behavior far from the blackbody one. By knowing the emissivity, that is the ratio of the radiation emitted by the body to the radiation emitted by the blackbody at the same temperature, it is possible to determine the temperature of a specific body. The precise knowledge of the emissivity of materials is then essential for temperature measurements using IRT, with the difficulty that the emissivity can be a function of both the wavelength and the direction of radiative emission. Also, great care must be taken to include additional parameters in the data analysis, such as reflections of thermal radiations from nearby sources, to ensure a reliable estimate of the temperature. A calibration process is necessary to consider the influence of all the surrounding elements on the energy detected by the sensor. As a rule of thumb, it can be argued that low reflectivity materials due to their high emissivity are better suited for IR imaging.
The performances of an infrared system are evaluated in terms of thermal sensitivity (for current devices it can be less than 20 mK), scan speed (that can be higher than 1600 Hz), image resolution (up to tens of thousands of pixels) and intensity resolution (up to 16-bits) [1]. It is an efficient tool for mapping the surface temperature of solids or liquids, for which radiation can be considered as a surface phenomenon, as the radiation emitted by the molecules that do not belong to a thin surface layer is directly absorbed by the body itself.
In the case of gases, radiation is really a volumetric phenomenon, as the medium is transparent to radiation emitted by all the gas molecules. Nonpolar gases, such as O2 or N2, do not emit radiation and are essentially transparent to incident thermal radiation. It is, however, different for polar gaseous molecules, such as CO2, H2O, NH3, or hydrocarbon gases, which emit and absorb over a wide temperature range [2]. In addition, gaseous radiation is complicated by the fact that, unlike radiation from a solid or a liquid, which presents a continuous emission spectrum, gaseous radiation is concentrated in specific wavelength intervals that are called bands. Each molecule at the gaseous state has its own radiative emission spectrum which requires the gathering of a complex database in order to associate infrared radiative emission to temperature [3]. For internal gas flows, it is generally more straightforward to measure the radiations of the walls and to deduce, with appropriate hypotheses, information on the temperature distribution within the gas.
Despite these difficulties, IRT has several advantages as it is non-intrusive and contactless. In the case of internal fluid flows, however, direct fluid IRT cannot be easily implemented, since most materials are not transparent to IR radiation. In addition, even specific materials partly transparent to some IR radiations emit their own radiation, which can lead to the detection of a wrong signal or to an image with a low signal over noise ratio.

2. Applications of IRT to Liquid Microflows

At microscale, the limitations are due to diffraction phenomena: the spatial resolution of a pixel cannot be lower than the wavelength of the IR signal (i.e., lower than about 5 µm). On the other hand, the acquisition frequency depends on the image size, the integration time and the type of data storage, and quite high frequencies of the order of some kHz can be obtained for images of limited size. Many studies on IR measurements in liquids microflows have been published, and some of them propose solutions to overcome spatial and temporal limits.

2.1. Liquid Temperature Estimation through Measurement of the Wall Temperature

Hetsroni et al. [4] measured external surface temperatures in small channels (1070 µm in diameter) filled with water flowing at Reynolds numbers ranging from 10 to 400. The methodology they proposed allows to limit systematic errors caused by the radiations of the surrounding elements. Once calibrated, the radiometer used in their study gave a typical noise equivalent to a temperature difference of 0.07 K, which was lower than the sensitivity of the system. A detailed study leads to a value of standard uncertainty for the temperature wall of 0.29 K. In a similar way, Patil and Narayanan [5] employed the IR technique to measure the wall and near-wall temperatures of water flowing through a silicon microchannel 50 µm wide and 135 µm deep. They worked at a temperature of about 45 °C and were able to determine the local temperatures with an uncertainty of 0.60 and 1.33 K for Reynolds numbers of 297 and 251, respectively.
Liu et al. [6] employed IR imaging techniques to study the effects of viscous dissipation in quartz glass microtubes with inner diameters of 19.9 µm and 44.2 µm. De-ionized water was the working fluid. Depending on the Reynolds number Re based on the microtube radius, the temperature difference between the inlet and outlet of the fluid varied from 1 to about 10 °C. The temperature inside the fluid was deduced from surface temperature measurements of the outer wall with corrections considering convection in the fluid and conduction in the microchannel wall. 
Hetsroni et al. [7] combined the IR technique with high speed flow visualization to simultaneously measure the surface temperature and the flow pattern of two-phase flows in microchannels in order to explore the relationship between temperature surface, heat flux and bubble generation during boiling. The working fluids were air-water and steam-water flowing in a series of parallel triangular microchannels with hydraulic diameters of 103, 129 and 161 µm. The authors were able to map temperatures in the range of 50 to 120 °C with a radiometer sensitivity of 0.1 K by averaging them over a time interval of 0.04 s.
Haber et al. [8] studied by IR imaging the fast and exothermic reaction of tetraethoxysilane hydrolysis by mapping temperature profiles in a micro-reactor made of two parallel rectangular microchannels (one for the reaction and one for cooling) in a Polyether ether ketone (PEEK) substrate. The cross section of the reaction channel had a depth of 100 µm and a width of 500 µm, and the PEEK wall had a thickness of 250 µm. Thanks to a calibration, they were able to estimate temperature profiles between 30 and 50 °C at different flow velocities and time intervals, by IR imaging of the PEEK wall external surface. The reported accuracy of their temperature measurements was about 1 K with a spatial resolution of 200 µm/pixel.

2.2. Measurement of Liquid Surface Temperature

IR techniques have also been widely used in evaporation studies. In some cases, IR measurements allow for the direct mapping of the temperature of the gas-liquid interface and not the wall temperature as in the previously mentioned studies. Buffone and Sefiane [9] used IR imaging in capillaries with a diameter ranging from 600 to 1630 µm. They observed the signal from the open end of the capillary and were able to map the temperature of the meniscus surface in the case of evaporation of volatile liquids: ethanol, methanol, acetone or pentane. The thermal sensitivity and spatial resolution of their setup were 20 mK at 30 °C and 30 µm, respectively. With the strong simplifying assumption of the same emissivity for all liquids, they were nevertheless able to compare the temperature distribution along the meniscus of different volatile liquids for various temperature gradients.

3. Applicability of IRT to Gas Microflows and Current Limitations

The IR imaging technique has been widely employed in gases for leak detection [10][11] and to measure temperature. For example, Safitri and Mannan [12] have detected the leak and studied temperature by directly measuring the IR signal of methane gas, which is the major constituent in liquefied natural gas (LNG). The aim of their study was to determine the concentration and temperature of LNG vapor plumes generated by spills of LNG on concrete and water (in an open environment). Initial experiments were carried out to study the emissivity of methane in the range of 110–300 K. Subsequently, the temperature of methane gas was measured in this range. The uncertainty of the measurements has not been explicitly reported, but the authors emphasized the fact that the main uncertainty is linked to the poor knowledge of gas emissivity.
The main issue for direct gas IRT is the signal dependency on the gas emissivity, which itself is a function of temperature and is not always known with great accuracy. Moreover, only certain gases can emit energy in the infrared spectrum, but this emission is restricted to a very narrow spectral band, especially at room temperature [3]. In addition, temperature differences in a gas induce a change in the local density, and consequently changes the apparent emissivity of gases, and this phenomenon is amplified inside microscale systems.
In experiments, the IR camera is usually placed at a distance from the source whose temperature is of interest. Certain gases, such as CO2 and H2O, absorb IR wavelengths. The presence of these gases in the measurement path alters the IR signal, resulting in an error in the temperature estimation. Also, the concentration of H2O vapor in the atmosphere is a function of local relative humidity. However, other gases, such as N2, O2 and Ar do not absorb in the IR range, therefore having these gases in the measurement path would make the temperature measurements more reliable [13][14].
Currently, there have been very few attempts to measure temperature inside gas microflows using IRT, and researchers mainly focus on the wall radiation itself. For example, Kumar et al. [15] have developed thin Indium Tin Oxide (ITO) coated sapphire sensors with the objective of non-intrusive surface temperature measurements involving gas microflows. They were able to quantify the apparent emissivity of an ITO layer in the temperature range of 5 to 75 °C.
Researchers have reported temperature and spatial resolutions of about 10 mK [10] and 31 µm [16]. A temporal resolution of 20 ms is quite easy to achieve [16]. If, in principle, the IR technique can be employed to measure the temperature of gases, the implementation of the technique is not straightforward, since some of the key parameters, such as emissivity, are functions of temperature. In addition, for confined gas flows, the close presence of the walls that emit their own radiation considerably limits the possibility of IRT of the gas itself. These limitations have strongly restricted the usage of IR techniques to directly measure temperature in gases, particularly in internal gas microflows.

This entry is adapted from the peer-reviewed paper 10.3390/mi13111819


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