Measuring Thermal Conductivity in a Microfluidic Device With the Transient Thermal Offset (TTO) Method

Measurements of thermal conductivity on microliter-sized samples can be of great value in applications where the sample fluid is costly or scarcely available. Such measurements can be used for a broad range of purposes such as quality control and bioanalytical applications. Currently available methods for measuring the thermal conductivity of small liquid samples are often not suited for high-throughput testing due to the complexity of the sensor hardware, or the complexity of the required data processing. In this study, a novel sensor device and sensing method are presented that require only one simple planar resistive sensing structure to be incorporated in a microchannel. The working principle of the so-called Transient Thermal Offset (TTO) method is demonstrated with numerical simulations, as well as by practical experiments on various water/ethanol mixtures using an in-house designed prototype sensor device. The developed device is able to determine the thermal conductivity of water/ethanol mixtures with volumes less than $3~\mu $ l with an accuracy of 0.5%. The standard deviation on the experimental measurements is less than 0.009 W/mK. The setup enables rapid testing of small amounts of static liquid samples at high-throughput, as well as long-time monitoring of changes in thermal conductivity of liquids inside a microchannel. The purposeful sensor design enables further miniaturization that would allow testing even smaller sample volumes.