Review of Optical Thermometry Techniques for Flows at the Microscale towards Their Applicability to Gas Microflows.

Thermometry techniques have been widely developed during the last decades to analyze thermal properties of various fluid flows. Following the increasing interest for microfluidic applications, most of these techniques have been adapted to the microscale and some new experimental approaches have emerged. In the last years, the need for a detailed experimental analysis of gaseous microflows has drastically grown due to a variety of exciting new applications. Unfortunately, thermometry is not yet well developed for analyzing gas flows at the microscale. Thus, the present review aims at analyzing the main currently available thermometry techniques adapted to microflows. Following a rapid presentation and classification of these techniques, the review is focused on optical techniques, which are the most suited for application at microscale. Their presentation is followed by a discussion about their applicability to gas microflows, especially in confined conditions, and the current challenges to be overcome are presented. A special place is dedicated to Raman and molecular tagging thermometry techniques due to their high potential and low intrusiveness.

Optofluidic Formaldehyde Sensing: Towards On-Chip Integration.

Formaldehyde (HCHO), a chemical compound used in the fabrication process of a broad range of household products, is present indoors as an airborne pollutant due to its high volatility caused by its low boiling point ( T = - 19 °C). Miniaturization of analytical systems towards palm-held devices has the potential to provide more efficient and more sensitive tools for real-time monitoring of this hazardous air pollutant. This work presents the initial steps and results of the prototyping process towards on-chip integration of HCHO sensing, based on the Hantzsch reaction coupled to the fluorescence optical sensing methodology. This challenge was divided into two individually addressed problems: (1) efficient airborne HCHO trapping into a microfluidic context and (2) 3,5-diacetyl-1,4-dihydrolutidine (DDL) molecular sensing in low interrogation volumes. Part (2) was addressed in this paper by proposing, fabricating, and testing a fluorescence detection system based on an ultra-low light Complementary metal-oxide-semiconductor (CMOS) image sensor. Two three-layer fluidic cell configurations (quartz-SU-8-quartz and silicon-SU-8-quartz) were tested, with both possessing a 3.5 µL interrogation volume. Finally, the CMOS-based fluorescence system proved the capability to detect an initial 10 µg/L formaldehyde concentration fully derivatized into DDL for both the quartz and silicon fluidic cells, but with a higher signal-to-noise ratio (SNR) for the silicon fluidic cell ( S N R s i l i c o n = 6.1 ) when compared to the quartz fluidic cell ( S N R q u a r t z = 4.9 ). The signal intensity enhancement in the silicon fluidic cell was mainly due to the silicon absorption coefficient at the excitation wavelength,   a ( λ a b s = 420   nm ) = 5 × 10 4   cm - 1 , which is approximately five times higher than the absorption coefficient at the fluorescence emission wavelength, a ( λ e m = 515   nm ) = 9.25 × 10 3   cm - 1 .

Velocity Measurements in Channel Gas Flows in the Slip Regime by means of Molecular Tagging Velocimetry.

Direct measurements of the slip velocity in rarefied gas flows produced by local thermodynamic non-equilibrium at the wall represent crucial information for the validation of existing theoretical and numerical models. In this work, molecular tagging velocimetry (MTV) by direct phosphorescence is applied to argon and helium flows at low pressures in a 1-mm deep channel. MTV has provided accurate measurements of the molecular displacement of the gas at average pressures of the order of 1 kPa. To the best of our knowledge, this work reports the very first flow visualizations of a gas in a confined domain and in the slip flow regime, with Knudsen numbers up to 0.014. MTV is cross-validated with mass flowrate measurements by the constant volume technique. The two diagnostic methods are applied simultaneously, and the measurements in terms of average velocity at the test section are in good agreement. Moreover, preliminary results of the slip velocity at the wall are computed from the MTV data by means of a reconstruction method.

Editorial for the Special Issue on Gas Flows in Microsystems.

The last two decades have witnessed a rapid development of microelectromechanical systems (MEMS) involving gas microflows in various technical fields [...].

Design Guidelines for Thermally Driven Micropumps of Different Architectures Based on Target Applications via Kinetic Modeling and Simulations.

The manufacturing process and architecture of three Knudsen type micropumps are discussed and the associated flow performance characteristics are investigated. The proposed fabrication process, based on the deposition of successive dry film photoresist layers with low thermal conductivity, is easy to implement, adaptive to specific applications, cost-effective, and significantly improves thermal management. Three target application designs, requiring high mass flow rates (pump A), high pressure differences (pump B), and relatively high mass flow rates and pressure differences (pump C), are proposed. Computations are performed based on kinetic modeling via the infinite capillary theory, taking into account all foreseen manufacturing and operation constraints. The performance characteristics of the three pump designs in terms of geometry (number of parallel microchannels per stage and number of stages) and inlet pressure are obtained. It is found that pumps A and B operate more efficiently at pressures higher than 5 kPa and lower than 20 kPa, respectively, while the optimum operation range of pump C is at inlet pressures between 1 kPa and 20 kPa. In all cases, it is advisable to have the maximum number of stages as well as of parallel microchannels per stage that can be technologically realized.

Sub-ppb Level Detection of BTEX Gaseous Mixtures with a Compact Prototype GC Equipped with a Preconcentration Unit.

In this work, a compact gas chromatograph prototype for near real-time benzene, toluene, ethylbenzene and xylenes (BTEX) detection at sub-ppb levels has been developed. The system is composed of an aluminium preconcentrator (PC) filled with Basolite C300, a 20 m long Rxi-624 capillary column and a photoionization detector. The performance of the device has been evaluated in terms of adsorption capacity, linearity and sensitivity. Initially, PC breakthrough time for an equimolar 1 ppm BTEX mixture has been determined showing a remarkable capacity of the adsorbent to quantitatively trap BTEX even at high concentrations. Then, a highly linear relationship between sample volume and peak area has been obtained for all compounds by injecting 100-ppb samples with volumes ranging from 5⁻80 mL. Linear plots were also observed when calibration was conducted in the range 0⁻100 ppb using a 20 mL sampling volume implying a total analysis time of 19 min. Corresponding detection limits of 0.20, 0.26, 0.49, 0.80 and 1.70 ppb have been determined for benzene, toluene, ethylbenzene, m/p-xylenes and o-xylene, respectively. These experimental results highlight the potential applications of our device to monitor indoor or outdoor air quality.

Slip length measurement of gas flow.

In this paper, we present a review of the most important techniques used to measure the slip length of gas flow on isothermal surfaces. First, we present the famous Millikan experiment and then the rotating cylinder and spinning rotor gauge methods. Then, we describe the gas flow rate experiment, which is the most widely used technique to probe a confined gas and measure the slip. Finally, we present a promising technique using an atomic force microscope introduced recently to study the behavior of nanoscale confined gas.