Thermal characterisation of μL volumes using a thin film thermocouple based sensor

A microfabricated sensor capable of measuring the thermal conductivity of small samples is presented. Thin film thermocouples and a resistive heater were fabricated on a polyimide substrate, to form a hot-wire-like sensor. Microfabrication allowed the manufacture of small sensors, with current devices formed with 20 mu m line widths and 75 mu m separations between the heater and sensors. A thin layer of polyimide is used to encapsulate the device, without degrading thermal sensitivity. This gives the devices the advantages of flexibility, robustness, electrical isolation, chemical resistance and biocompatibility. The operation of the devices was demonstrated by measuring the thermal conductivity of 100 mu L droplets of glycerin/water mixtures. Additionally, 2-propanol/water mixtures were tested to derive the dependence of the thermal conductivity on the concentration of the mixture, as this is not available in the literature. The devices were subsequently used to monitor the time varying thermal properties of smaller volumes of non-liquid samples. Wetted paper was used, due to its thickness (similar to 100 mu m) and relatively slow change in thermal conductivity as it dries. It was found that the thermal conductivity decreases as the paper dries out and that this change was detected by the devices. Furthermore, it was demonstrated that the theoretical and experimental drying times of the sheet of paper investigated exhibit a similar dependence on the air temperature. This provided clear evidence for the device's ability to monitor the thermal properties of small volumes. (C) 2011 Elsevier B.V. All rights reserved.