Flow Microsensor Research Articles

A liquid–metal-based microscale calorimetric in-chip flow sensor for flow rate measuring

Abstract This work proposes a liquid–metal-based calorimetric micro-flow sensor within a polydimethylsiloxane (PDMS) chip. It can measure the flow rate of fluid in microscale channels, with a range as low as several microliters per minute. This in-chip sensor is proposed to solve the issue of detecting the flow rate in microfluidic systems. To make the sensor compatible with PDMS microfluidic chips, low-melting-point gallium-based alloy and bismuth-based (bi-based) alloy are used to make the micro heater and bi-metal thermocouples, for these alloys can be easily injected into a PDMS chip to form electrodes. To minimize heat resistance (or temperature difference) between fluid and the detecting ends of thermocouples, these ends are directly exposed to liquid in the flow channel with the help of a special reversible bonding technology. Thermocouples are connected in series to improve the sensor’s response. A novel method to bond and electrically connect the sensor to a print circuit board is also elaborated. Since the calorimetric flow sensor is sensitive to heating power, fluid temperature and environment cooling, a dimensionless parameter less independent of these factors is deduced from heat transfer theory, and this idea is used in result processing to offset the bad effect. Experiments with pure water show that this sensor can be used to detect flow rates, with a resolution up to 4 µl min−1 mV−1 and a range of 12 µl min−1 in this case, and that at different heating powers, the thermal potential results vary significantly whereas the dimensionless results nearly keep the same. Present work indicates that this sensor has the potential to be integrated into a PDMS microfluidic system and to provide accurate and stable results if a dimensionless method is used in data processing.

Bioinspired Artificial Hair Sensors for Flight-by-Feel of Unmanned Aerial Vehicles: A Review

Flight-by-feel is an emerging approach to flight control that uses distributed arrays of pressure, strain, and flow sensors to guide aircraft. Among these, hair-type flow sensors have received the least attention yet hold some advantages over conventional sensors. This paper reviews hair-like flow microsensors developed since 2013, focusing on developments in design, construction, and application. Hair-like flow sensors can be found in artificial cochleae, submersible navigation, terrestrial robots, and, rarely but increasingly, on aircraft. In this survey, we categorize hair-like flow sensors into three types (long whisker-like hairs, ultrasensitive microscale hairs, and short trichoid-like hairs), and primarily cover sensors that may be suitable for use on aircraft. The recent progress in flow-based flight control using distributed sensing is also discussed, along with the optimization of sensor placement and the potential for flight-by-feel in sixth-generation military and civilian aircraft designs. This survey aims to provide a consolidated account of the history and state-of-the-art of artificial hair-cell flow sensors, motivate consideration of flight-by-feel as a viable flight control paradigm, and define avenues for future research. As engineering and biological science continue to converge, we hope that researchers in both fields find this survey an inspirational and useful resource.

Study on a differential pressure microflow sensor on microfluidic chip

Based on the principle of differential pressure flow measurement, graphene film was used to measure the differential pressure in the micro channel of microfluidic chip and finally to obtain the micro flow rates. The relationship between differential pressure and flow rate in micro channel was studied and validated by CFD simulation. The fiber F-P cavity using graphene film was fabricated as the pressure sensitive structure, and the relationship between the measured pressure and the optical power output from the pressure sensitive structures was studied. The mechanism model used to measure the micro flow rates in the micro channel of microfluidic chip was established, and a sample of micro flow sensor was fabricated. The relationship between various quantities in the process of converting flow rate to optical power was verified through experiments, and the mechanism model established was verified. The experimental results demonstrate that fabricated sample could achieve accurate measurement of micro flow rate in the micro channel of microfluidic chip.

Investigation on the Effective Measures for Improving the Performance of Calorimetric Microflow Sensor.

The performance of the calorimetric microflow sensor is closely related to the thermal sensing part design, including structure parameter, heater temperature, and operation environment. In this paper, several measures to enhance the performance of the calorimetric microflow sensor were proposed and further verified by numerical simulations. The results demonstrate that it is more favorable to reduce the negative impact of flow separation as the space between detectors and heater is set to be 1.6 μm so as to improve the accuracy of the sensor. With an appropriate gap, the front arranged obstacle of the upstream detector can effectively widen the measure range of the sensor, benefiting from the decrease in upstream viscous dissipation. Compared to a cantilever structure, the resonances can be effectively suppressed when the heater and detectors are designed as bridge structures. In particular, the maximum amplitude of the bridge structure is only 0.022 μm at 70 sccm, which is 53% lower than that of the cantilever structure. The optimized sensor widens the range by 14.3% and significantly increases the sensitivity at high flow rates. Moreover, the feasibility of the improved measures is also illustrated via the consistency of the trend between the simulation results and experimental ones.

High-performance integrated chip-level thermoelectric device for power generation and microflow detection

Miniaturized thermoelectric devices (TED) possess great benefits in energy converting and sensing. By employing improved microfabrication technology, we succeed made an ultra-highly integrated TED with enhanced interfacial and structural qualities at a high filling factor of 61 %. Our micro-TED (2 cm × 2 cm×6 µm, 10,082 TE pairs) achieved an open-circuit voltage up to 2.6 V, an output power of 1.2 mW (temperature difference of 62.8 K). The micro-TED delivered a large temperature gradient up to ∼600 K/mm with input current, achieving a net cooling temperature of 3.6 K with a 3 µm thick thermoelectric layer. As a novel microflow sensor, micro-TED can distinguish ultra-slow cold/hot airflows down to 4 mm/s. The ultra-thin integrated micro-TED can find multi-featured applications in ultra-low-grade thermal energy harvesting, localized chip-to-chip cooling, and hypersensitive sensing.

The Characterisation of Differential Pressure Based Flow Sensor in a Microfluidic Environment

Flows in a microfluidic environment differ in their flow parameter changes compared to macro flows and should be handled carefully. Recently microfluidic innovations have been drawn into much consideration in biomedical, chemical synthesis, and cooling industries. There exists a need for a flow rate sensor for measuring tiny fluid flows in developing microfluidic Lab-on-chip products, Organ-on-chip, and other products for biological, chemical, and other flow analysis. Working with microfluidic products mainly revolves around flow measurement. The control of these frameworks is just conceivable with sensors that estimate flow rate. In this work, we understand the dynamics of microfluidic flow and try to determine its influence on sensor behavior. The paper presents the construction of flow-sensing techniques for microflows. The design of the microchannel opted for the study is also discussed. A microfluidic flow rate sensing technique is put forward using the physical principle of fluid dynamics; based on pressure in a rectangular microchannel. For rectangular microchannel, the variation in pressure had an impact on the flow rate measurement of the microflow sensor.

A Review on the Role of Microflow Parameter Measurements for Microfluidics Applications

Microfluidics has risen to a new zone of exploration because of its application in numerous fields. The integration of microfluidics and sensor technology bridges gaps in heat transfer areas, the medical field, and the chemical industry at the microscale flow level. This paper reviews the latest work conducted in microfluidics with the help of microflow parameter measurements in microfluidic applications, microflow sensor inventions, novel microflow pathway design, and an assessment of the keyway of fluid behavior in microchannels. The emphasis is on highlighting a significant part of recent research on developing microfluidics applications using the previously explored microflow characteristic measurements. The details of heat transfer, blending, and sorting, along with different medical applications, including drug delivery, inferred that heat transfer is the most explored application domain. Comparing newly evolving microflow sensors will make the sensor selection easy for the user’s required microflow conditions. The effects of microchannel geometry and channel wall parameters on different microflow characteristic measurements are identified. This study will enhance the understanding of the performance of microflow systems by providing new flexibility in microfluidics. The study of microflow parameter measurements is reviewed in more depth, making its way for future microfluidic application developments.

A Proton Battery Stack Real-Time Monitor with a Flexible Six-in-One Microsensor.

A proton battery is a hybrid battery produced by combining a hydrogen fuel cell and a battery system in an attempt to obtain the advantages of both systems. As the battery life of a single proton battery is not good, the proton battery stack is developed by connecting in parallel, which can greatly improve the battery life of proton batteries. In order to obtain important information about the proton battery stack in real time, a flexible six-in-one microsensor is embedded in the proton battery stack. This study has successfully developed a health diagnostic tool for a proton battery stack using micro-electro-mechanical systems (MEMS) technology. This study also focused on the innovatively developed hydrogen microsensor, and integrated the voltage, current, temperature, humidity, and flow microsensors, as previously developed by our laboratory, to complete the flexible six-in-one microsensor. Six important internal physical parameters were simultaneously measured during the entire operation of the proton battery stack. It also established a complete database and monitor system in real time to detect the internal health status of the proton cell stack and observe if there were problems, such as water accumulation, aging, or failure, in order to understand the changes and effects of the various physical quantities of long-term operation. The study found that the proton batteries exhibited significant differences in the hydrogen absorb rates and hydrogen release rates. The ceramic circuit board used in the original sensor is replaced by a flexible board to improve problems such as peeling and breaking.

Application of Monitoring Module Three-in-One Microsensor to Real-Time Microscopic Monitoring of Polarizer Sheet in Roll-to-Roll Process

The Roll-to-Roll (R2R) process refers to a high-efficiency, low-cost, continuous production method. The roll material used for processing is a flexible plastic or metal film. In many R2R processes, polarizing films are high-precision products with a high output value. In the production of conventional polarizers, product inspection will only be carried out after the production of the polarizing film is completed. The principal raw material of a polarizer sheet is a hydrophilic polymer, the properties of which may be influenced by water vapor, which degrades its quality. Whether or not the product is impacted can be ascertained by means of a quality inspection, but it must be performed after the process is finished. However, it is already too late when a defective product is detected: the production cost is increased, the schedule is influenced and the delivery date is delayed. The focus of this research was on environmental monitoring of the production drying process oven, but the commercial all-in-one sensors currently on the market cannot tolerate the temperature of the factory’s high-temperature oven. In particular, a commercial flow sensor is rarely suited to high-temperature applications. Some are expensive and cannot be widely distributed. Therefore, this study aimed to develop an integrated sensor to measure the internal environment of the drying process oven for the real-time monitoring of a polarizer sheet in the Roll-to-Roll (R2R) process. This study used micro-electro-mechanical systems (MEMS) technology to make a flexible three-in-one (temperature, humidity and flow) micro-sensor. We monitored the temperature, humidity and flow uniformity in a laboratory oven to simulate the environment of the actual factory oven, with the aim to provide data to confirm whether or not a polarizer sheet has dried. Our system can be monitored instantly by Arduino, or even Raspberry Pi 3, to realize the flexible micro-sensor layout and field verification, in order to optimize the R2R process and to enhance the yield and performance of the polarizer sheet process.

Silicon monolithic microflow sensors: a review

Flow measurement is an essential requirement in medical, industrial, automotive and environmental applications, Thanks to the rapid development of microelectromechanical system (MEMS) micro-fabrication techniques, silicon-based microflow sensor chips used as the key component of microfluidic control systems have attracted particular attentions due to the miniaturized chip-size, high precision, low power consumption, rapid response and low-cost batch-fabrication capability. Up to present, many different types of silicon microflow sensors have been proposed and developed. This paper provides a review on recently completed works on design, fabrication and properties of the MEMS-based silicon monolithic microflow sensors. Some technical challenges and application outlooks are also discussed in the paper.

Flexible 6-in-1 Microsensor for Real-Time Microscopic Monitoring of Proton Battery.

According to the comparison between a proton battery and a proton exchange membrane fuel cell (PEMFC), the PEMFC requires oxygen and hydrogen for generating electricity, so a hydrogen tank is required, leading to larger volume of PEMFC. The proton battery can store hydrogen in the carbon layer, combined with the oxygen in the air to form water to generate electricity; thus, the battery cost and the space for a hydrogen tank can be reduced a lot, and it is used more extensively. As the proton battery is a new research area, multiple important physical quantities inside the proton battery should be further understood and monitored so as to enhance the performance of battery. The proton battery has the potential for practical applications, as well as water electrolysis, proton storage and discharge functions, and it can be produced without expensive metals. Therefore, in this study, we use micro-electro-mechanical systems (MEMS) technology to develop a diagnostic tool for the proton battery based on the developed microhydrogen sensor, integrated with the voltage, current, temperature, humidity and flow microsensors developed by this laboratory to complete a flexible integrated 6-in-1 microsensor, which is embedded in the proton battery to measure internal important physical parameters simultaneously so that the reaction condition in the proton battery can be mastered more accurately. In addition, the interaction of physical quantities of the proton battery are discussed so as to enhance the proton battery’s performance.

Time-of-Flight Flow Microsensor using Free-Standing Microfilaments

In this work we present the processing conditions and the characterization results of a time-of-flight (TOF) flow microsensor implemented using polysilicon microfilaments. Numerical simulation (ANSYS®/FLOTRAN®) and experimental results for analysis with air flow and nitrogen are compared considering different heater to sensor distances and flows. A good agreement was observed between experimental and simulated results. However, experimental TOF values resulted ever consistently higher than the simulated ones due to delays introduced by the circuitry and the data acquisition system and also in the thermal response of the microfilament. Analysis of the heater microfilament reveal that temperatures of the order of 100 °C (that is low enough to not affect the gas flow) can be obtained with a power dissipation of tens of mW. The time-of-flight measurements show that the proposed structure is suitable for the detection of low volumetric flows (tens of SCCM) and presents a response time of the order of milliseconds.

A Fast Response and CMOS Compatible Micro Gas Flow Sensor With Thermal Feedback

This letter presents a CMOS compatible micro flow sensor (MFS) that operates in a thermal feedback (TF) mode. The proposed MFS contains four parallel polysilicon resistors on the micro bridges. The gas flow induced temperature difference between two outlying thermistors is minimized and balanced through the negative thermal feedback, while the power distribution on two inlying microheaters is automatically adjusted. Accordingly, the measured voltage difference between two inlying microheaters is used as the sensor output signal. For the N2 gas flow, the proposed MFS works in TF mode achieve a prominent sensitivity of 1.74V/(m/s) and a bidirectional detection range of −6 ∼ 6 m/s, which shows an almost 2X enhanced flow range as compared to the counterpart works in a constant voltage mode. Moreover, the rise time of MFS under a suddenly generated gas flow is experimentally determined as 3.1 ms (error ±0.13 ms). The measured rise time is 43% faster than that of the micro flow sensor without thermal feedback, which proves the successful development of a fast response MFS.

Design, optimization and numerical simulation of a MicroFlow sensor in the realistic model of human aorta

Among all arterial diseases, aneurysm and atherosclerosis are of great importance. In these diseases the cross-section area of the artery and therefore the blood flow velocity changes. Therefore, it is a good idea to use a micro flow sensor for measuring the blood flow velocity to diagnosing these diseases. In this study, design, geometric optimization and numerical simulation of a hotfilm microsensor in the realistic model of human's aorta are investigated. A normal aorta geometry is extracted from CT Angiography images, and after applying oscillating boundary conditions on the inlet and outlet arteries, blood flow parameters are investigated by computational fluid dynamics (CFD) simulation. After designing the microsensor, the effects of its location in different aorta regions on the blood flow characteristics are numerically investigated. The results show that in the presence of microsensor, streamlines patterns almost remain unchanged while the maximum blood flow velocity in the aorta cross section where the microsensor is located, increases up to 10%. It is also found that the secondary flow weakens when the microsensor enters the artery causing a reduction in velocity measurement error. Furthermore, because of the presence of a catheter, the pressure drop increases up to 768 Pa. Results show that less than 30% of the arterial cross-sectional area where the microsensor is located, senses an increase in temperature.

Sensitive and reliable thermal micro-flow sensor for a drug infusion system

We developed a sensitive and reliable thermal micro-flow sensor (TμFS) to measure extremely low flow rates for a drug infusion system. The TμFS exploits a calorimetric principle. Various temperature differences upstream and downstream of a heated area were monitored by three temperature sensors to enable measurement of both cooling and heating effects at the multiple temperature-sensing areas. The flow rate was measured by subtraction or summation of the temperature differences between selected locations upstream and downstream. The temperature differences between heat loss and diffusion increased sensitivity of flow rate detection in range of 0–100 mL/h. The TμFS takes non-invasive and non-intrusive measurements by monitoring thermal variations on the outer surface of a silicone IV tube. The TμFS could detect flow rates as low as ∼0.1 mL/h, with uncertainty <5%. The micro-flow sensor was accurate at a range of flow rates, initial liquid temperature, and tube inner diameters. In addition, temperatures variations in the TμFS for flow rates agreed with numerical simulation.

Design and implementation of a passive micro flow sensor based on diamagnetic levitation

This study presents a prototype of a micro-flow sensor based on magnetic levitation for the use in microfluidic systems. Accurate assessment of flow rates is crucial in the microfluidic system design. Micro-sensors utilizing levitation constitute an important topic in this regard. Diamagnetic levitation is an efficient technique, which is useful in applications with low power consumption and eliminates friction. With the proposed approach, zero mechanical contact in a microfluidic channel can be achieved. A sensor capable of accurately measuring flow rates was designed in this study. The corresponding flow rate range was between 1000 μL/min and 7000 μL/min. Levitation was accomplished with pyrolytic graphite and a ring magnet (NdFeB) acting as a lifter. The displacement of the micro-magnet in the micro channel in longitudinal direction was monitored via a microscope-camera system and was measured via a laser sensor above the lifter-magnet. A commercial analysis software (COMSOL Multiphysics Version 5.3 CPU License No: 17076072) was used for dynamic analysis and validation of experimental results. The flow rates were obtained using the data from the laser sensor via an exclusively coded C# program. The developed sensor prototype, which has the advantages of simple structure, small size and low cost, is a substantial candidate for the use in microfluidic devices requiring high accuracy.

A Flexible 5-In-1 Microsensor for Internal Microscopic Diagnosis of Vanadium Redox Flow Battery Charging Process.

Multiple important physical parameters in the vanadium redox flow battery are difficult to measure accurately, and the multiple important physical parameters (e.g., temperature, flow, voltage, current, pressure, and electrolyte concentration) are correlated with each other; all of them have a critical influence on the performance and life of vanadium redox flow battery. In terms of the feed of fuel to vanadium redox flow battery, the pump conveys electrolytes from the outside to inside for reaction. As the performance of vanadium redox flow battery can be tested only by an external machine—after which, the speed of pump is adjusted to control the flow velocity of electrolyte—the optimum performance cannot be obtained. There is a demand for internal real-time microscopic diagnosis of vanadium redox flow batteries, and this study uses micro-electro-mechanical systems (MEMS) technology to develop a flexible five-in-one (temperature, flow, voltage, current, and pressure) microsensor, which is embedded in vanadium redox flow battery, for real-time sensing. Its advantages include: (1) Small size and the simultaneous measurement of five important physical quantities; (2) elastic measurement position and accurate embedding; and (3) high accuracy, sensitivity, and quick response time. The flexible five-in-one microsensor embedded in the vanadium redox flow battery can instantly monitor the changes in different physical quantities in the vanadium redox flow battery during charging; as such, optimum operating parameters can be found out so that performance and life can be enhancec.

Micro-flow sensor for continuous resin fluidity monitoring between fibers

The resin micro-flow between fibers influences bubble evacuation and fiber impregnation during the processing of fiber reinforced polymer (FRP) composites. The trapped bubbles and unimpregnated regions within a laminate will develop into voids after the processing, which will impair the structural performance severely. In this study, a new sensing principle of the step-changing difference of capillary pressure was proposed for the micro-flow measurement between reinforced fibers in FRP composites. A highly sensitive and calibration-free micro-flow sensor was designed by simply introducing an optical fiber with super-infiltration polyimide coating along the reinforced fibers. The FRP composite around the optical fiber was regarded as part of the sensing structure. The key resin flow states between fibers were first detected continuously during composite processing.

Development of a flow microsensor for selective detection of nitric oxide in the presence of hydrogen peroxide

The detection of reactive oxygen and nitrogen species is of utmost importance in several pathological situations. Indeed, these reactive species are biomarkers of oxidative stress and their real-time monitoring is crucial to adapt medical treatments. We report here on the electrochemical detection of nitric oxide (NO) in the presence of hydrogen peroxide (H2O2). The detection was performed by using different sensing microdevices involving either static solutions in wells or flowing solutions in microfluidic channels. Furthermore, an original strategy was proposed to further enhance the selectivity of NO detection at Pt/poly(eugenol) modified platinum electrodes by designing a dual-electrode microfluidic device based on a pre-electrolysis of interfering species including H2O2 at an upstream electrode prior to NO detection.

Numerical investigations on the effect of fin shape and surface roughness on hydrothermal characteristics of slip flows in microchannels with pin fins

Accurate modelling of convective heat transfer in micro/nano gas flows is critical for many applications such as temperature/mass flow micro-sensors and mixing/separation analysis of gases in micro-systems. This study numerically investigates the effect of pin fin shape and wall roughness on heat transfer and flow field of gas flows in rough microchannels in the presence of the second order slip boundary condition. The hydrothermal characteristics of flows were obtained for a smooth microchannel under slip boundary conditions for the constant wall temperature (330 K) and constant wall heat flux (10 kW/m2) boundary conditions. Afterwards, four types of pin fin shapes (rectangular, diamond, oblong, and elliptic) and two types of surface roughness (regular and random rough elements) were taken into account. According to the obtained results, although velocity slip raises Nusselt number, temperature jump tends to reduce it. It was shown that the generated recirculating flows between the roughness elements reduce Nusselt number and increase friction factor. Furthermore; it was found that the effect of pin fin shape diminishes with surface roughness.