Microscale flow sensors

Shear stress measurement plays an important role for characterization and control of both macro-and microscale flows. There are various existing techniques suitable for either steady state or instantaneous nature of the flow. The rapid development of MEMS manufacturing technology has immensely contributed to improvement in spatial and temporal resolution of shear stress sensors. Therefore, turbulent flow control has become feasible for many practical apphcations. However, the cahbration and implementation issues of shear stress measurements are not completely estabhshed. Therefore, the implementation procedure for shear stress measurement is problem specific and requires complete understanding of the measurement principle. The existing shear stress measurement technologies have not been implemented over a wide range of microscale flow problems. Therefore, future research should concentrate on critical issues relevant to shear stress measurement in various microflow applications. This wfll contribute towards development of matured shear stress measurement techniques. 

In microfiuidic devices the measurement of the wall shear stress is challenging in general. For a direct measurement, methods commonly used such as oil film interferometry or liquid crystal coatings cannot be applied since already the thin layer of oil alters the fluids behavior at the wall. Shear stress sensors, like micro-piUars, surface mounted and elevated hot wires, and Preston tubes, might be applicable in macroscopic flows but not for microscale flow investigations. The first reason is their size, which is often larger than the whole microfiuidic channel or structure on the surface. Furthermore, in some cases like the studies on endothelial cells mentioned above, the wall is a biological tissue that cannot be altered or replaced. Secondly, the sensors would... 

Microscale Flow Visualization Particle Image Velocimetry (PIV) Shear Stress Sensors... 

Costin, C.D., Synovec, R.E., A microscale-molecular weight sensor probing molecular diffusion between adjacent laminar flows by refractive index gradient detection. Anal. Chem. 2002, 74, 4558 1565. 

To build an efficient, high-quality microscale fuel cell, microfabrication techniques need to be combined with appropriate materials such as Nation based membrane electrode assemblies (MEAs). These techniques must be able to produce three-dimensional structures, allow reactant and product flow into and out of the device, process appropriate materials, and should be of low cost. Fortimately, traditional thin film techniques can be modified for microscale fuel cell fabrication, while maintaining their advantages of surface preparation, sensor integration, and finishing or packaging. In addition, other techniques are also available and are discussed in the following sections. 

Because the fabrication process is such an essential part of microfluidics, an overview of the principles underlying the microfabrication technology is presented. Pressure, flow, and temperature measurements are essential variables for characterizing fluid motion in any system. An important goal is the design and construction of self-contained microfluidic systems. Because of their small size, incorporation of pressure, flow, and temperature sensors directly on the microfluid system chip is highly desirable. There are relatively few examples where microfluidic systems have been constructed with these on-board sensors. There have been so many microsensor developments in recent years that it is only a matter of time before such systems will appear. Small-scale actuators to provide either open- or closed-loop control of the flow in microchannels are needed and these efforts are addressed. While experimental work on fluid flow itself in microscale structures is rather sparse, some results will be presented that emphasize the similarity and/or differences between macroscopic and microscopic flow of liquids. Although there are not many applications of... 

Tlie technological requiiements for designing and constructing microfluidic devices for micro- and nano-technology ai e expanding. A microfluidic platform for the constraction of microscale components and autonomous systems consists of a combination of liquid-phase photopolynierization, lithography, and laminar flow to generate channels, valves, actuators, sensors, and systems. " ... 

Cardiovascular disease, namely, coronary artery disease, remains the leading cause of death in the developed nations. Over the last few years, MEMS sensors have advanced the understanding of blood flow, namely, fluid shear stress, in arterial circulation. Fluid shear stress is defined as the frictional force acting tangentially on the surface of a blood vessel wall. Furthermore, the measurement of wall shear stress is important to study the durability of prosthetic valves and to monitor thrombosis or blood clots in cardiopulmonary bypass machines, artificial hearts, and left ventricular assist devices. Luminal shear stress measurement predicts the development of atherosclerotic plaque in patients at risk for acute heart attacks. In this context, the application of microscale hot-wire anemometry bridges fluid mechanics of blood flow with vascular biology. 

It is not practical to measure pressures inside microchan-nels and nanochannels by use of conventional sensors, since it is very difficult or impossible to implement these sensors in microsystems without disturbing the flow fleld. Therefore, some novel methods are required for pressure measurement on the microscale. 

Flow control systems are critical components of most of the energy systems involving fluid flow and heat transfer. These systems are essential for performance optimization of both macroscale and microscale devices. Micropumps, microvalves, microshear stress sensors, and microflow sensors are integral components of flow control systems. Capillary micropump, MHD micropump, thermocapillary micropump, and electrokinetic micropump have been presented in earlier chapters. The present chapter reports various microactuators and shear stress sensors for flow control systems. More details on microvalves and microflow sensors can be found in other references (Nguyen and Wereley, 2006). 

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