Several other thermal flow meters exist and function in a similar way [3]. flow microfluidic sensor rate elveflow overview For the fluidic mixing process with miscible fluids, this dual thermal time-of-flight sensing approach can provide a more desirable measurement than the other thermal sensing approaches. A thermal time-of-flight sensor can indeed achieve these conditions with multiple sensing elements.

flow sensor mass labmate microfluidic elements After calibration in DI water and subsequent verification, the same sensor was kept at the same microfluidic channel at null flow condition with a 5 Vdc power applied for 48hours. Therefore, if the sensor only has a microheater and a sensing element pair, the measurement will still be dependent on the flow medium properties. flow sensors microfluidic low calorimetric elveflow One of the selections of the channel is a fused silica tube. Thedual-phase or multi-phase flow for the immiscible fluids would involve various liquidliquid, gasliquid, liquidgasliquid, and supercritical fluid flows beyond the capabilities of the conventional flow sensing approaches. A fluid is drawn in by the capillary forces and will thus spontaneously start moving [2]. The Standard Flow Meter has a screen and interface buttons which displays the flow rate, andallows for stand-alone control. Additional digitalsensors will have to be integrated into the microfluidic device for diagnostic quality data acquisition. Therefore, the channel will need to be thin enough and have good thermal conductivity for heat transfer effectiveness. The membrane is about 1.5 to 3mm in diameter and 100m in thickness, strong enough to hold the fluidic pressure inside the microfluidic channel. Only one sensing element is placed downstream. As the two air bubbles (one big and one small) pass through the channel sequentially, the fluidic properties that the sensor senses will be drastically different from those of the pure liquid. The dependence of the microfluids pressure loss on the dynamic viscosity also requires a temperature sensor at the proximity for the needed compensation. Todays microfluidics is yet the well-established one for implementation but excellent academic approaches and science and technology tools [6, 7, 8, 9, 10, 11]. If the massive deployment of such devices were realized, the foreseeable heavy burdens of the medical systems for the aged societies worldwide would be significantly alleviated, and many lives of human beings could be saved. The capillary number then would be much more important than the Reynolds number [80]. Compared to the gaseous fluids, liquid has a much large heat capacitance making the sensing element resistance-related temperature effects less pronounced. Although the report did not speculate the reasons for the deviations, this phenomenon could be a direct reflection of the water interactions with the microfluidic channel walls. For this chapters limited space, only continuous flow sensing technologies are discussed with applicable pulsed flow features. The required manufacture process makes these sensors very costly without being able to be volume produced. The mechanical deflection can be read out with an optical microscope or photodiode. In another approach, the measurement of the magnetic impedance of a hair microfluidic flow sensor offers the ultra-low-power option [79]. They also often require a transparent microfluidic channel, which would not be readily available in real applications. In an alternative optical sensing approach, [77] a collimated light beam was employed to excite the surface plasmon resonance at a gold film on top of a polymethyl methacrylate (PMMA) microfluidic channel. The comparison of such standards among different European national metrology institutes indicated an uncertainty (k=2) ranging from 0.05 to 6% for the flow rate ranges of 17nl/min to 167ml/min. This prevents the reduction in footprint and cost. The tube is usually made of thermally conductive materials such as stainless steel or fused silica. For the gaseous sensors, the membrane often has openings that balance the surfaces fluidic pressure against the membrane deformation. However, in practical applications, the channels are often opaque. In the classic fluidic dynamics, the Moody chart indicates that at laminar flow, the friction factor is inversely proportional to Reynolds number where only viscosity of the fluid plays the role and diffusion is normally not in consideration. In recent years, 3D printing, precision micro-injection, laser processing, hot embossing [20, 21, 22, 23], and other alternative tools also greatly enriched the variety of microfluidic devices. For the gasliquid mixture, cavitation discussed in the previous section or bubbles will very much likely be formed depending on how dissolvable the gas into the liquid will be. The deviation was further reduced by running the flow at the full scale for another 30minutes (Test D). Cavitation is often known as a detrimental phenomenon in high-speed flows that leads to mechanical damages at the flow path. Thermal flow sensors have been applied to small flow measurement for both gas and liquid before the microfluidic concept emerged.

The thermal time-of-flight sensing concept can be traced back to the late 1940s [48] and has been an interest in many subsequent research works [49, 50, 51]. A stripped single-mode optical fiber was positioned across a microfluidic channel and aligned with a multi-mode fiber receiver. To this end, thermal time-of-flight sensing technology offers much of the competitive advantages. In microfluidics, cavitation inception is via the diffusion of dissolved gas into the available nuclei. The sensitivity of these sensors also requires meaningful pressure or critical mass to activate the deformation of the cantilever or diaphragm. However, to gauge the conventional infusion applications, a sensor with a fast response time of fewer than 3msec while having a large dynamic range of at least 4000:1 will be needed to meet the requirements for control of total dosage within 5% deviations. From the expanded insert, one sees that the delivery is actually with a pulsed dosage having a wide spectrum of speeds, and the nominal speed is achieved via the adjustment of the time intervals between any of the two pulsed doses. In a microscale optical flow sensor report, [75] an optical fiber structure was fabricated in the form of a drag force cantilever to measure the microfluidic flow. While the system level products enable various applications, the lack of a miniaturized, standalone, performance dramatizing, and cost-effective device would not maintain the expected or envisioned phenomenal growth. The heat transfer needed for the measurement provided by the sensor is achieved via thermal diffusion. The second component of the flow sensor is the microwave resonator, designed into an open-ended half-wavelength ring resonator with a microstrip structure on a high-performance microwave substrate made of a 35m copper layer on top and bottom surfaces. The sensor is placed at the outer wall of a thermally conductive fine quartz glass tube by machining the tube surface into a smooth flat. The flow rate is then calculated based on the spread of heat, which is directly related to the flow rate. With ever-improving micromachining device fabrication technologies, the microfluidics once was only viable on a 2 wafer, and now 8 and even 12 wafers are being routinely produced. This article aims at presenting a short review of the existing techniques for flow control and flow rate measurement in microfluidics. Therefore, it is of interest to have additional measurement approaches that can alert in situwhen the cavitation is present in microfluidics. One is the microfluidic channel with the membrane that was micromachined with PDMS soft lithography and replica molding. These sensors can monitor gas or oil flows without any specific calibration. Simultaneously, it is thin enough for the sensitivity of the resonator function needed for the measurements. Many research works on sensors have been dedicated to the biomedical and chemical sensing development based on electrochemical, optical, mass, or magnetic sensing principles. Even with the miscible fluids, the microbubbles would likely present in all cases. Some efforts were also made to measure the flowrate with the SAW devices. The changes of the tube oscillation in time and space are a direct measure of the mass flow. The demonstrated Coriolis sensor could measure liquid mass flow, density, and temperature (if a temperature sensor is integrated) simultaneously. As PhD students, we found it difficult to access the research we needed, so we decided to create a new Open Access publisher that levels the playing field for scientists across the world. In a mass flow meter operating on the Coriolis principle the fluid flows on a vibrating channel. The measurement scheme of flowrate with these alternative thermal sensing designs could also be classified into the above three thermal sensing principles. *Address all correspondence to: liji@Siargo.com. But commercialization of many of those is still in question. Brief introduction to this section that descibes Open Access especially from an IntechOpen perspective, Want to get in touch? TheECO Flow Meter comes with an aqueous calibration, and optional software for calibrating the meterfor different liquid types is available. These package approaches are also similar to the traditional capillary thermal mass flow sensors, where the hot wires are winded onto the surface of a special stainless tube. These two temperature sensors can be simple resistors of identical resistance values or identical thermal-piles. The SAW sensors can be independent of the fluidic properties; however, they require a much larger footprint, and temperature compensation is also complicated compared to thermal sensing approaches. The user is able to have the meter signal average for datasmoothing if desired, and the user can select how frequently the flow rate value is recorded in the The next sections will describe some of the main technologies used for flow measurement. The reported data achieved a 50nL/min detection limit and about 10:1 dynamic range. However, many of the proposed biosensors or chemical sensors are very specific, and most are research-orientated, as being determined by the catalytic or affinity properties of the biological recognition agent in a particular study and the sensor itself requires a sophisticated electronic system for readout or analysis. The heat transfer was from a microheater with a constant heat diffusion at a fixed glass wall area. More than twenty different physical measurement principles are commercially available on the market for flow metrology. Figure 1 is the graphic illustration of these three measurement principles for the typical micromachined thermal flow sensors on a silicon substrate. The green-colored materials need to have good thermal conductivity while excellent surface passivation for reliability. We'll assume you're ok with this, but you can opt-out if you wish. By observing the heat distribution over time, it is able to deduce the fluid velocity and thus the flow rate. Some selected researches micromachined flow sensing technologies are discussed below. Therefore, a primary standard or a reference defined by an international norm governs the manufacture of a flow sensing product with specific sensing technology. The microwave flow sensor is consisting of two critical components. Our team is growing all the time, so were always on the lookout for smart people who want to help us reshape the world of scientific publishing. Live flow monitoring can be achieved using flow sensors. The hot wire uses a resistor as heater and sensing element. In order to make the best choice, it is important to consider the following elements: Fluid volume displacement uses mechanical parts to directly displace a certain volume of fluid. Optical or image processing would help understand the physical or even chemical process, but it would not help improve the flow measurement accuracy. Therefore, it could also be a type of differential pressure sensing. We are a community of more than 103,000 authors and editors from 3,291 institutions spanning 160 countries, including Nobel Prize winners and some of the worlds most-cited researchers. Metering the microfluidic flow is critical for many microfluidic applications requiring precise control of the desired microfluidic process or handling. In addition, PC software and LabVIEW VI are included with the meter,allowing for computer control as well. The membrane is frequently made with silicon nitride or silicon nitride and oxide combination. The sensors microheater was modulated with a sine wave, and the phase-shifts at the sensing elements were recorded for the flowrate calibration. We have seen that there is are a variety of flow rate control solutions when it comes to flow control and flow measurement. All of these control techniques have different advantages. When the fluid flows through the pillars, the flow will force the pillars to bend, resulting in the change of the magnetic field sensed by the GMI layer and output the signals that can be correlated to the flowrate. In another report using the optical approach for flow sensing, miniaturized fluorescence sensing is attempted for micro molecular tagging velocimetry in microfluidics [76], but these methods are not cost-effective and yet to reach the small footprint. The Coriolis force acting on the moving fluid will affect the frequency, phase shift or amplitude of the initial vibration proportionally to the mass flow rate. Therefore, the commercially available approach [41] for the package is to have the sensor placed outside the channel with the sensors surface close to the outer channel wall. However, for high volume applications, a faster closed-loop calibration would be preferred. The red dots are from the peristaltic pump having a large dispersion of the actual flow speed, and the blue dots are from the high precision syringe pump. Due to the size restriction, such a sensor does not favor being placed inside the microfluidic channel. Microfluidic dissolution phenomena impose big challenges in metering the flow for a desired metrological accuracy, either with immiscible or miscible fluids. However, in a few reports on the long-term stability of the commercially available calorimetric flow sensors for microfluidics, it was reported that the measurement accuracy tended to have a time-dependence. The sensor chip was fixed to a fine tube with a machined flat surface in the product package. One advantage of the Coriolis sensing approach is that the fluid density can be simultaneously determined from the oscillation frequency of the measuring tube. The microfluidic peristaltic pump uses amechanical rotorto squeeze a flexible tube containing the fluid resulting inalternative compressions and relaxations that will draw in the liquid and result in flow. Several key components, including microfluidic channels, microvalves, micropumps, needles, mixers, and sensors, are considered the necessary ones for the desired microfluidic chip or system. The conventional flow sensors might be the first commercially available standalone sensing products for microfluidics. Fortunately, microfluidics growth is parallel with the significant advancement in the MEMS and LSI/VLSI IC industry. The fluidic flow will cause the temperature redistribution inside the microfluidic channel, which alters the refractive index above the metal film. The formed flow sensor was placed inside the microfluidic channel. Paper microfluidics is an emerging field that uses paper as a support for fluidic experiments. However, in an ideally integrated microfluidic system, there will be valves and other actuators. However, it will normally require dual transducers placed in opposite directions or at a certain angle with respect to a reflector. Figure 3 shows the response of a thermal time-of-flight sensor used to detect the bubbles inside the microfluidic channel. There are a variety of approaches to realize data acquisition. Many different microfluidic flow sensor technologies have been studied and developed. Because of the heat diffusion, control the heat for the low flowrate measurement would be very challenging, resulting in a small dynamic range and large measurement errors (full-scale error rate) towards the low detection limit. In a similar manner, we can also use small tubes in order to draw the liquid through channels. The ECO Flow Meter offers the same performance as the Standard model, but as it does not havea display screen and interface buttons, it can only be controlled through a computer. A gas such as air dissolution in water can decrease nucleation temperature, making the enlargement of the bubble nuclei of water resulting in cooling [91]. This flow rate control solution is one of the most widely used in microfluidics. Despite the advantages, a commercially available thermal time-of-flight flow meter is not seen until the past decade [42]. For the microfluidic applications, the microheater is driven with a modulated microheater, the constant heating spot in the flow channel is therefore eliminated. The microscopic structure formed by the paper fibers created a network of small capillary channels. Optical flow sensing is attractive to the microfluidic application for its non-invasive and high accuracy features. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. These relatively complicated components and the substrate make the process compatibility with the electronics a dilemma. 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. Publishing on IntechOpen allows authors to earn citations and find new collaborators, meaning more people see your work not only from your own field of study, but from other related fields too. Most MEMS foundries have the necessary equipment for manufacturing such sensors, which allows a very favorable cost and makes it possible for high volume applications. Comparing the peristaltic pump performance and a precise syringe pump can be found in Figure 5, the right plot, which is the polar measurements by a thermal time-of-flight sensor at a set point of 20mL/hr. To perform effective experiments in microfluidics, one needs to master the different flow control technologies available to use the most suitable way to control microfluidic flows. In practice, many of the devices serving drug infusion are utilizing peristaltic pumps, which have much lower accuracy than the precise syringe pumps [93]. The sensor outputs flow velocity as well as fluidic mass flowrate and the additional data of the fluidic properties, making the thermal time-of-flight technology an ideal approach for the desired microfluidic flow measurement applications. 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