Laser-based measurement of liquid temperature or concentration at a solid–liquid interface

Abstract

Abstract This work presents a real-time, non-contact, laser-based thermoreflectance technique to measure changes in temperature or concentration of stationary or flowing liquids at a transparent solid–liquid interface, e.g., a glass window. Variations in temperature or concentration result in a change in refractive indices of the liquid, which, in turn, alter the reflectivity at the interface. A 3 mW semiconductor laser diode serves as the light source, and a silicon photodiode monitors the intensity variations of the reflected laser beam. The temperature of three liquids, water, ethanol, and 1-propanol, are measured with very good agreement found between the laser technique and a calibrated thermistor. The concentration of a methanol–propanol solution is successfully measured as well. The maximum uncertainty is 0.6°C for the temperature measurement and 0.2% for the concentration measurement, respectively. The presented experimental configuration is simple, inexpensive and reliable. Additionally very high spatial and temporal resolution are possible: the beam spot size can be readily reduced to ∼20 μm or less, and a temporal resolution of ∼1 μs or less can be achieved with a high-speed data acquisition system. Thus, temperature or concentration changes in a flowing liquid in small-scale devices such as microelectro-mechanical-systems (MEMS) and microfluidic structures, and the systems with fast temporal variation, e.g., rapid solidification and fast mixing, can be effectively measured.