Development of a thermal sensor for probing small volumes

Abstract

This thesis presents the design, fabrication and characterisation of a thermal micro-sensor capable of operating on small scale samples. It was initially characterised by measuring the thermal conductivity of 100 μl liquid droplets and thin materials (sheets of paper) before being applied to the measurement of the water content of plant leaves. The device consists of a micro-heater and two thin-film thermocouples formed on a flexible polyimide substrate and used microfabrication techniques to precisely pattern the sensing elements.
The heater was used to induce a temperature gradient within a sample in contact with the device, which was recorded as a difference in temperature (ΔT) between the two thermocouples. Various experiments have demonstrated that ∆T was dependent on the total thermal resistance (consisting of the bulk thermal conductivity and thermal contact resistance) of the sample under inspection. The device’s sensitivity to bulk thermal conductivity was shown by recording the variation in ∆T for 100 μl droplets of glycerine/water mixtures. Different compositions of the mixture served as thermal conductivity standards. The measurement of mixtures of propanol/water further demonstrated that the device could be used to indirectly monitor the composition of small volume binary solutions by measurement of the thermal conductivity.
By monitoring the thermal conductivity of wetted paper, it was shown that the device was sensitive to the increase in the total thermal resistance as the paper dried out. Furthermore, theoretical and experimental drying times were in agreement and exhibited a similar dependence on the air temperature. This provided clear evidence of the device’s ability to monitor thermal conductivity of small samples.
The final element of this work was the real-time monitoring of the water content of plant leaves. The device was clamped to an abscised leaf which was allowed to dry over a period of 6 hours. A comparison between the weight of the leaf and ∆T measurements showed a linear dependence. It was found that the changes in thermal properties were dominated by the water content of the leaf. The device was subsequently shown to be sensitive to changes in the water content of the leaves of plants subjected to water stress conditions, demonstrating its ability to monitor the real-time water content in-situ.