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

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 w indow. Variations in temperature or concentration result in a change in ref ractive indices of the liquid, which, in turn, alter the reflectivity at th e interface. A 3 mW semiconductor laser diode serves as the light source, a nd a silicon photodiode monitors the intensity variations of the reflected laser beam. The temperature of three liquids, water, ethanol, and 1-propano l, are measured with very good agreement found between the laser technique and a calibrated thermistor. The concentration of a methanol-propanol solut ion is successfully measured as well. The maximum uncertainty is 0.6 degree sC for the temperature measurement and 0.2% for the concentration measureme nt, respectively. The presented experimental configuration is simple, inexp ensive and reliable. Additionally very high spatial and temporal resolution are possible: the beam spot size can be readily reduced to similar to 20 m um or less, and a temporal resolution of similar to 1 mus or less can be ac hieved with a high-speed data acquisition system. Thus, temperature or conc entration changes in a flowing liquid in small-scale devices such as microe lectro-mechanical-systems (MEMS) and microfluidic structures, and the syste ms with fast temporal variation, e.g., rapid solidification and fast mixing , can be effectively measured. (C) 2000 Elsevier Science Inc. All rights re served.