A critical review of the measurement techniques for the analysis of gas microflows through microchannels

A large deviation in the published experimental data on the dynamic and thermal behavior of microflows has been observed with respect to the classical theory but, from a chronological analysis of these experimental results, it can be realized how the deviations between the behavior of fluids through microchannels with respect to the large-sized channels are decreasing. Some of the observed inconsistencies in the data were originated from the experimental method used for the investigation of convective microflows. This fact highlights the need of a development of specific measurement techniques for the microfluidics field. In this work, we explore and categorize different approaches found in literature for measuring microflow characteristics, especially for gas flows, and the geometry of the microchannels pointing out the advantages and disadvantages inherent to each experimental technique. Starting from the operative definition of friction factor, the main parameters that must be checked in an experimental work in order to characterize the flow are reviewed. A discussion based on uncertainty analysis will be presented in order to individuate the main operative parameters that one must be able to measure accurately in order to determine pressure drop and convective heat transfer coefficient in the microchannels with a low level of uncertainty. In the paper each measurement technique is critically analysed to evidence the important issues which may have been overlooked in previous researches. The main goal of this study is to give a summary of experimental procedure and a useful guideline for experimental research in microfluidics.