The particular benefits of microfluidic systems, in terms of heat mass transfer enhancement, require conducting local flow diagnostics, especially when unsteady properties of the microflow can play a critical role at the reaction interface, as currently observed in the fields of bioengineering and chemical engineering. The present paper focuses on unsteady confined flows within microsystems characterized by various geometries of crossing channels and exhibiting high surface-to-volume ratios. An experimental analysis of the signal measured at microsensors embedded to the wall of microsystems is discussed. In the objective of performing flow diagnostics, including regime identification and wall flow structure recognition, two methods for electrochemical signal processing are investigated and compared within an experimental network of crossing minichannels. One method is based on the use of a transfer function, while the other, the so-called Sobolik solution (Sobolik et al. in Coll Czech Chem Commun 52:913928, 1987), consists of finding a direct solution to the mass balance equation. Sobolik's method has been selected given its ability to provide a description, over a wide range of Reynolds numbers (317\Re\3,535), for all wall shear rate fluctuations, as well as for the associated mixing scales in the power spectra density (PSD).
Unsteady flows in milli-and microsystems : analysis of wall shear rate fluctuations