Precise flow metrology has an increasing demand in many microfluidic related applications. At the scale and scope of interests, Capillary number instead of Reynold number defines the flow characteristics. The interactions between fluid medium and flow channel surface or the surface tension, cavitation, dissolution, and others play critical roles in microfluidic flow metrology. Conventional flow measurement approaches are not sufficient for solving these issues. This chapter will review the currently available products on the market, their microfluidic flow sensing technologies, the technologies with research and development, the major factors impacting flow metrology, and the prospective sensing approaches for future microfluidic flow sensing.

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Microfluidic Flow Sensing Approaches

Determination of blood viscosity requires consistent measurement of blood flow rates, which leads to measurement errors and presents several issues when there are continuous changes in hematocrit changes. Instead of blood viscosity, a coflowing channel as a pressure sensor is adopted to quantify the dynamic flow of blood. Information on blood (i.e., hematocrit, flow rate, and viscosity) is not provided in advance. Using a discrete circuit model for the coflowing streams, the analytical expressions for four properties (i.e., pressure, shear stress, and two types of work) are then derived to quantify the flow of the test fluid. The analytical expressions are validated through numerical simulations. To demonstrate the method, the four properties are obtained using the present method by varying the flow patterns (i.e., constant flow rate or sinusoidal flow rate) as well as test fluids (i.e., glycerin solutions and blood). Thereafter, the present method is applied to quantify the dynamic flows of RBC aggregation-enhanced blood with a peristaltic pump, where any information regarding the blood is not specific. The experimental results indicate that the present method can quantify dynamic blood flow consistently, where hematocrit changes continuously over time.

Quantitative Monitoring of Dynamic Blood Flows Using Coflowing Laminar Streams in a Sensorless Approach

In this study, a microfluidic cantilever flow sensor was designed and manufactured to monitor liquid flow rate within the range of 100-1000 µl/min. System simulation was also performed to determine the influential optimal parameters and compare the results with experimental data. A flowmeter was constructed as a curved cantilever with dimensions of 6.9 × 0.5 × 0.6 mm3 and a microchannel carved with a CO2 laser inside the cantilever beam. The fabrication substance was Polydimethylsiloxane. Different flow rates were injected using a syringe pump to test the performance of the flowmeter. Vertical displacement of the cantilever was measured in each flowrate using a digital microscope. According to the results, the full-scale overall device accuracy was up to ± 1.39%, and the response time of the sensor was measured to be 6.3 s. The microchip sensitivity was 0.126 µm/(µl/min) in the range of measured flow rates. The sensor could also be utilized multiple times with an acceptable error value. The experimental data obtained by the constructed microchip had a linear trend (R2 = 0.995) and were of good consistency with simulation results. Furthermore, according to the experimental and the simulation data, the initially curved cantilever structure had a higher bending and sensitivity level than a perfectly straight cantilever construction. 

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A suspended polymeric microfluidic sensor for liquid flow rate measurement in microchannels

Micromachined thermal flow sensors on the market are primarily manufactured with the calorimetric sensing principle. The success has been in limited industries such as automotive, medical, and gas process control. Applications in some emerging and abrupt applications are hindered due to technical challenges. This paper reviews the current progress with micromachined devices based on the less popular thermal time-of-flight sensing technology: its theory, design of the micromachining process, control schemes, and applications. Thermal time-of-flight sensing could effectively solve some key technical hurdles that the calorimetric sensing approach has. It also offers fluidic property-independent data acquisition, multiparameter measurement, and the possibility for self-calibration. This technology may have a significant perspective on future development.

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Micromachined Thermal Time-of-Flight Flow Sensors and Their Applications

Embedded sensors in microfluidic systems play a big role in different chemical and biological analyses. Versatile metal electrodes in microfluidic chips have been designed to detect the physical and chemical parameters of the fluid, but in situ monitoring by flexible and transparent embedded sensors still remains a significant challenge. Here, such a challenge is tackled for the first time, by simple fabrication of multiapplication sensing patches into a microfluidic chip. The sensing patches are made with polydimethylsiloxane (PDMS) and fluorescent dye. Different strategies of assembling sensing patches could be used to realize both local and dynamic sensing for multiscale detection. In Strategy 1, one sensing patch is utilized to measure the temperature of microheaters below 100 °C and its detecting resolution is 1.25 °C. In Strategy 2, four sensing patches are placed in four channels to monitor the real-time temperature change (23 °C–50 °C) in different microchannels. In Strategy 3, two sensing patches are fabricated in a microchannel, which has a high signal resolution to monitor the flow rate of less than $10 ~\mu \text{L}$ /min. The simple fabrication process and different combinatorial strategies facilitate the development of miniaturization and functionalization of microfluidic sensors, so as to realize the monitoring of more parameters and play a significant role in the fields of biological and chemical analysis.

Multiscale Integrated Temperature/Flow Velocity Sensor Patches for Microfluidic Applications

The design, fabrication, operation, calibration, and performance of a microfluidic flow meter utilizing a micromachined (MEMS) thermal time-of-flight sensing chip are presented. The MEMS sensing chip integrates multiple sensing elements (thermistors) on a silicon substrate. This sensing chip works on the principle of thermal excitation with a modulated power source from the microheater while the responses of the sensing elements at both upstream and downstream of the modulated thermal source are processed for both the time differences and the amplitudes of the heat transfer in the microfluidic flow. Unlike most of the current flow meter products based on the thermal sensing principle that only offer the calorimetric mass flow rates, this flow meter can measure not only the mass flow rate but also the flow media properties. Experimental results for water and isopropyl alcohol are discussed, which demonstrate the capability and performance of the novel microfluidic flow meter.

A microfluidic flow meter with micromachined thermal sensing elements.

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Rong Gan

Papers

A microfluidic flow meter with micromachined thermal sensing elements.

Repeatability of the master meter method in calibrating compressed hydrogen dispenser