Single element thermal sensor for measuring thermal conductivity and flow rate inside a microchannel

Abstract The increasing development of continuous-flow applications in the field of microfluidics generates demand for in-line monitoring methods. The thermal conductivity (κ) of a liquid has been proven to be a valuable measurand for quality control, process monitoring, and analytical testing. However, most available methods for measuring κ of microliter-sized samples are limited for use on stagnant samples. In this work, a novel method and associated prototype device for measuring κ under flow conditions is presented. The so-called Transient Thermal Offset (TTO) method requires only a single metal resistive structure that is excitated with direct current (DC) pulses. To demonstrate the working, proof-of-principle experiments are performed on liquids with various κ under different flow rates. The results show that, after calibration, the presented microfluidic device can be used for accurately measuring κ of liquids under flow, as well as for determining the flow rate of liquids with a known κ. Within the explored ranges, both parameters can be determined with an average error of approximately 2.6%. The results confirm that, also under flow conditions, uncertainties concerning probing depth are eliminated with the TTO method.