Micromachined strain gauges for the determination of liquid flow friction coefficients in microchannels

In this research program, we have performed and tested cupro-nickel (Cu–Ni) strain gauges micromachined on different sorts of silicon nitride (Si 3 N 4 ) membranes. The design of the gauges obeys an electrical Wheatstone bridge configuration. We have found a good agreement between the expected electromechanical response of the bridge and the experimental signals. The results have displayed sensitivity to static pressure ranging from 50 to 100 µV V −1 bar −1 as a function of the thickness and of the diameter of the membranes. This is part of a study devoted to determining liquid flow friction coefficients in silicon–Pyrex microchannels. Preliminary attempts (Reynolds number up to 300) made using global pressure measurements and with very simple local pressure probes are discussed. Further experiments using Cu–Ni strain gauges are described. Their micromachining, characterization and integration along silicon microchannels are presented. These sensors permitted us to perform the first local and reliable pressure drop measurements in a 7.5 µm deep microchannel. The results are in good agreement with the classical laminar theory for a Reynolds number ranging from 0.2 to 3.